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/* POSIX reader--writer lock: core parts.
Copyright (C) 2016-2018 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */
#include <errno.h>
#include <sysdep.h>
#include <pthread.h>
#include <pthreadP.h>
#include <sys/time.h>
#include <stap-probe.h>
#include <atomic.h>
#include <futex-internal.h>
/* A reader--writer lock that fulfills the POSIX requirements (but operations
on this lock are not necessarily full barriers, as one may interpret the
POSIX requirement about "synchronizing memory"). All critical sections are
in a total order, writers synchronize with prior writers and readers, and
readers synchronize with prior writers.
A thread is allowed to acquire a read lock recursively (i.e., have rdlock
critical sections that overlap in sequenced-before) unless the kind of the
rwlock is set to PTHREAD_RWLOCK_PREFER_WRITERS_NONRECURSIVE_NP.
This lock is built so that workloads of mostly readers can be executed with
low runtime overheads. This matches that the default kind of the lock is
PTHREAD_RWLOCK_PREFER_READER_NP. Acquiring a read lock requires a single
atomic addition if the lock is or was previously acquired by other
readers; releasing the lock is a single CAS if there are no concurrent
writers.
Workloads consisting of mostly writers are of secondary importance.
An uncontended write lock acquisition is as fast as for a normal
exclusive mutex but writer contention is somewhat more costly due to
keeping track of the exact number of writers. If the rwlock kind requests
writers to be preferred (i.e., PTHREAD_RWLOCK_PREFER_WRITERS_NP or the
no-recursive-readers variant of it), then writer--to--writer lock ownership
hand-over is fairly fast and bypasses lock acquisition attempts by readers.
The costs of lock ownership transfer between readers and writers vary. If
the program asserts that there are no recursive readers and writers are
preferred, then write lock acquisition attempts will block subsequent read
lock acquisition attempts, so that new incoming readers do not prolong a
phase in which readers have acquired the lock.
The main components of the rwlock are a writer-only lock that allows only
one of the concurrent writers to be the primary writer, and a
single-writer-multiple-readers lock that decides between read phases, in
which readers have acquired the rwlock, and write phases in which a primary
writer or a sequence of different primary writers have acquired the rwlock.
The single-writer-multiple-readers lock is the central piece of state
describing the rwlock and is encoded in the __readers field (see below for
a detailed explanation):
State WP WL R RW Notes
---------------------------
#1 0 0 0 0 Lock is idle (and in a read phase).
#2 0 0 >0 0 Readers have acquired the lock.
#3 0 1 0 0 Lock is not acquired; a writer will try to start a
write phase.
#4 0 1 >0 0 Readers have acquired the lock; a writer is waiting
and explicit hand-over to the writer is required.
#4a 0 1 >0 1 Same as #4 except that there are further readers
waiting because the writer is to be preferred.
#5 1 0 0 0 Lock is idle (and in a write phase).
#6 1 0 >0 0 Write phase; readers will try to start a read phase
(requires explicit hand-over to all readers that
do not start the read phase).
#7 1 1 0 0 Lock is acquired by a writer.
#8 1 1 >0 0 Lock acquired by a writer and readers are waiting;
explicit hand-over to the readers is required.
WP (PTHREAD_RWLOCK_WRPHASE) is true if the lock is in a write phase, so
potentially acquired by a primary writer.
WL (PTHREAD_RWLOCK_WRLOCKED) is true if there is a primary writer (i.e.,
the thread that was able to set this bit from false to true).
R (all bits in __readers except the number of least-significant bits
denoted in PTHREAD_RWLOCK_READER_SHIFT) is the number of readers that have
or are trying to acquired the lock. There may be more readers waiting if
writers are preferred and there will be no recursive readers, in which
case RW (PTHREAD_RWLOCK_RWAITING) is true in state #4a.
We want to block using futexes but using __readers as a futex word directly
is not a good solution. First, we want to wait on different conditions
such as waiting for a phase change vs. waiting for the primary writer to
release the writer-only lock. Second, the number of readers could change
frequently, which would make it likely that a writer's futex_wait fails
frequently too because the expected value does not match the value of
__readers anymore.
Therefore, we split out the futex words into the __wrphase_futex and
__writers_futex fields. The former tracks the value of the WP bit and is
changed after changing WP by the thread that changes WP. However, because
of the POSIX requirements regarding mutex/rwlock destruction (i.e., that
destroying a rwlock is allowed as soon as no thread has acquired or will
acquire the lock), we have to be careful and hand over lock ownership (via
a phase change) carefully to those threads waiting. Specifically, we must
prevent a situation in which we are not quite sure whether we still have
to unblock another thread through a change to memory (executing a
futex_wake on a former futex word that is now used for something else is
fine).
The scheme we use for __wrphase_futex is that waiting threads that may
use the futex word to block now all have to use the futex word to block; it
is not allowed to take the short-cut and spin-wait on __readers because
then the waking thread cannot just make one final change to memory to
unblock all potentially waiting threads. If, for example, a reader
increments R in states #7 or #8, it has to then block until __wrphase_futex
is 0 and it can confirm that the value of 0 was stored by the primary
writer; in turn, the primary writer has to change to a read phase too when
releasing WL (i.e., to state #2), and it must change __wrphase_futex to 0
as the next step. This ensures that the waiting reader will not be able to
acquire, release, and then destroy the lock concurrently with the pending
futex unblock operations by the former primary writer. This scheme is
called explicit hand-over in what follows.
Note that waiting threads can cancel waiting only if explicit hand-over has
not yet started (e.g., if __readers is still in states #7 or #8 in the
example above).
Writers determine the primary writer through WL. Blocking using futexes
is performed using __writers_futex as a futex word; primary writers will
enable waiting on this futex by setting it to 1 after they acquired the WL
bit and will disable waiting by setting it to 0 before they release WL.
This leaves small windows where blocking using futexes is not possible
although a primary writer exists, but in turn decreases complexity of the
writer--writer synchronization and does not affect correctness.
If writers are preferred, writers can hand over WL directly to other
waiting writers that registered by incrementing __writers: If the primary
writer can CAS __writers from a non-zero value to the same value with the
PTHREAD_RWLOCK_WRHANDOVER bit set, it effectively transfers WL ownership
to one of the registered waiting writers and does not reset WL; in turn,
a registered writer that can clear PTHREAD_RWLOCK_WRHANDOVER using a CAS
then takes over WL. Note that registered waiting writers can cancel
waiting by decrementing __writers, but the last writer to unregister must
become the primary writer if PTHREAD_RWLOCK_WRHANDOVER is set.
Also note that adding another state/bit to signal potential writer--writer
contention (e.g., as done in the normal mutex algorithm) would not be
helpful because we would have to conservatively assume that there is in
fact no other writer, and wake up readers too.
To avoid having to call futex_wake when no thread uses __wrphase_futex or
__writers_futex, threads will set the PTHREAD_RWLOCK_FUTEX_USED bit in the
respective futex words before waiting on it (using a CAS so it will only be
set if in a state in which waiting would be possible). In the case of
__writers_futex, we wake only one thread but several threads may share
PTHREAD_RWLOCK_FUTEX_USED, so we must assume that there are still others.
This is similar to what we do in pthread_mutex_lock. We do not need to
do this for __wrphase_futex because there, we always wake all waiting
threads.
Blocking in the state #4a simply uses __readers as futex word. This
simplifies the algorithm but suffers from some of the drawbacks discussed
before, though not to the same extent because R can only decrease in this
state, so the number of potentially failing futex_wait attempts will be
bounded. All threads moving from state #4a to another state must wake
up threads blocked on the __readers futex.
The ordering invariants that we have to take care of in the implementation
are primarily those necessary for a reader--writer lock; this is rather
straightforward and happens during write/read phase switching (potentially
through explicit hand-over), and between writers through synchronization
involving the PTHREAD_RWLOCK_WRLOCKED or PTHREAD_RWLOCK_WRHANDOVER bits.
Additionally, we need to take care that modifications of __writers_futex
and __wrphase_futex (e.g., by otherwise unordered readers) take place in
the writer critical sections or read/write phases, respectively, and that
explicit hand-over observes stores from the previous phase. How this is
done is explained in more detail in comments in the code.
Many of the accesses to the futex words just need relaxed MO. This is
possible because we essentially drive both the core rwlock synchronization
and the futex synchronization in parallel. For example, an unlock will
unlock the rwlock and take part in the futex synchronization (using
PTHREAD_RWLOCK_FUTEX_USED, see above); even if they are not tightly
ordered in some way, the futex synchronization ensures that there are no
lost wake-ups, and woken threads will then eventually see the most recent
state of the rwlock. IOW, waiting threads will always be woken up, while
not being able to wait using futexes (which can happen) is harmless; in
turn, this means that waiting threads don't need special ordering wrt.
waking threads.
The futex synchronization consists of the three-state futex word:
(1) cannot block on it, (2) can block on it, and (3) there might be a
thread blocked on it (i.e., with PTHREAD_RWLOCK_FUTEX_USED set).
Relaxed-MO atomic read-modify-write operations are sufficient to maintain
this (e.g., using a CAS to go from (2) to (3) but not from (1) to (3)),
but we need ordering of the futex word modifications by the waking threads
so that they collectively make correct state changes between (1)-(3).
The futex-internal synchronization (i.e., the conceptual critical sections
around futex operations in the kernel) then ensures that even an
unconstrained load (i.e., relaxed MO) inside of futex_wait will not lead to
lost wake-ups because either the waiting thread will see the change from
(3) to (1) when a futex_wake came first, or this futex_wake will wake this
waiting thread because the waiting thread came first.
POSIX allows but does not require rwlock acquisitions to be a cancellation
point. We do not support cancellation.
TODO We do not try to elide any read or write lock acquisitions currently.
While this would be possible, it is unclear whether HTM performance is
currently predictable enough and our runtime tuning is good enough at
deciding when to use elision so that enabling it would lead to consistently
better performance. */
static int
__pthread_rwlock_get_private (pthread_rwlock_t *rwlock)
{
return rwlock->__data.__shared != 0 ? FUTEX_SHARED : FUTEX_PRIVATE;
}
static __always_inline void
__pthread_rwlock_rdunlock (pthread_rwlock_t *rwlock)
{
int private = __pthread_rwlock_get_private (rwlock);
/* We decrease the number of readers, and if we are the last reader and
there is a primary writer, we start a write phase. We use a CAS to
make this atomic so that it is clear whether we must hand over ownership
explicitly. */
unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
unsigned int rnew;
for (;;)
{
rnew = r - (1 << PTHREAD_RWLOCK_READER_SHIFT);
/* If we are the last reader, we also need to unblock any readers
that are waiting for a writer to go first (PTHREAD_RWLOCK_RWAITING)
so that they can register while the writer is active. */
if ((rnew >> PTHREAD_RWLOCK_READER_SHIFT) == 0)
{
if ((rnew & PTHREAD_RWLOCK_WRLOCKED) != 0)
rnew |= PTHREAD_RWLOCK_WRPHASE;
rnew &= ~(unsigned int) PTHREAD_RWLOCK_RWAITING;
}
/* We need release MO here for three reasons. First, so that we
synchronize with subsequent writers. Second, we might have been the
first reader and set __wrphase_futex to 0, so we need to synchronize
with the last reader that will set it to 1 (note that we will always
change __readers before the last reader, or we are the last reader).
Third, a writer that takes part in explicit hand-over needs to see
the first reader's store to __wrphase_futex (or a later value) if
the writer observes that a write phase has been started. */
if (atomic_compare_exchange_weak_release (&rwlock->__data.__readers,
&r, rnew))
break;
/* TODO Back-off. */
}
if ((rnew & PTHREAD_RWLOCK_WRPHASE) != 0)
{
/* We need to do explicit hand-over. We need the acquire MO fence so
that our modification of _wrphase_futex happens after a store by
another reader that started a read phase. Relaxed MO is sufficient
for the modification of __wrphase_futex because it is just used
to delay acquisition by a writer until all threads are unblocked
irrespective of whether they are looking at __readers or
__wrphase_futex; any other synchronizes-with relations that are
necessary are established through __readers. */
atomic_thread_fence_acquire ();
if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 1)
& PTHREAD_RWLOCK_FUTEX_USED) != 0)
futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private);
}
/* Also wake up waiting readers if we did reset the RWAITING flag. */
if ((r & PTHREAD_RWLOCK_RWAITING) != (rnew & PTHREAD_RWLOCK_RWAITING))
futex_wake (&rwlock->__data.__readers, INT_MAX, private);
}
static __always_inline int
__pthread_rwlock_rdlock_full (pthread_rwlock_t *rwlock,
const struct timespec *abstime)
{
unsigned int r;
/* Make sure we are not holding the rwlock as a writer. This is a deadlock
situation we recognize and report. */
if (__glibc_unlikely (atomic_load_relaxed (&rwlock->__data.__cur_writer)
== THREAD_GETMEM (THREAD_SELF, tid)))
return EDEADLK;
/* If we prefer writers, recursive rdlock is disallowed, we are in a read
phase, and there are other readers present, we try to wait without
extending the read phase. We will be unblocked by either one of the
other active readers, or if the writer gives up WRLOCKED (e.g., on
timeout).
If there are no other readers, we simply race with any existing primary
writer; it would have been a race anyway, and changing the odds slightly
will likely not make a big difference. */
if (rwlock->__data.__flags == PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP)
{
r = atomic_load_relaxed (&rwlock->__data.__readers);
while (((r & PTHREAD_RWLOCK_WRPHASE) == 0)
&& ((r & PTHREAD_RWLOCK_WRLOCKED) != 0)
&& ((r >> PTHREAD_RWLOCK_READER_SHIFT) > 0))
{
/* TODO Spin first. */
/* Try setting the flag signaling that we are waiting without having
incremented the number of readers. Relaxed MO is fine because
this is just about waiting for a state change in __readers. */
if (atomic_compare_exchange_weak_relaxed
(&rwlock->__data.__readers, &r, r | PTHREAD_RWLOCK_RWAITING))
{
/* Wait for as long as the flag is set. An ABA situation is
harmless because the flag is just about the state of
__readers, and all threads set the flag under the same
conditions. */
while ((atomic_load_relaxed (&rwlock->__data.__readers)
& PTHREAD_RWLOCK_RWAITING) != 0)
{
int private = __pthread_rwlock_get_private (rwlock);
int err = futex_abstimed_wait (&rwlock->__data.__readers,
r, abstime, private);
/* We ignore EAGAIN and EINTR. On time-outs, we can just
return because we don't need to clean up anything. */
if (err == ETIMEDOUT)
return err;
}
/* It makes sense to not break out of the outer loop here
because we might be in the same situation again. */
}
else
{
/* TODO Back-off. */
}
}
}
/* Register as a reader, using an add-and-fetch so that R can be used as
expected value for future operations. Acquire MO so we synchronize with
prior writers as well as the last reader of the previous read phase (see
below). */
r = atomic_fetch_add_acquire (&rwlock->__data.__readers,
(1 << PTHREAD_RWLOCK_READER_SHIFT)) + (1 << PTHREAD_RWLOCK_READER_SHIFT);
/* Check whether there is an overflow in the number of readers. We assume
that the total number of threads is less than half the maximum number
of readers that we have bits for in __readers (i.e., with 32-bit int and
PTHREAD_RWLOCK_READER_SHIFT of 3, we assume there are less than
1 << (32-3-1) concurrent threads).
If there is an overflow, we use a CAS to try to decrement the number of
readers if there still is an overflow situation. If so, we return
EAGAIN; if not, we are not a thread causing an overflow situation, and so
we just continue. Using a fetch-add instead of the CAS isn't possible
because other readers might release the lock concurrently, which could
make us the last reader and thus responsible for handing ownership over
to writers (which requires a CAS too to make the decrement and ownership
transfer indivisible). */
while (__glibc_unlikely (r >= PTHREAD_RWLOCK_READER_OVERFLOW))
{
/* Relaxed MO is okay because we just want to undo our registration and
cannot have changed the rwlock state substantially if the CAS
succeeds. */
if (atomic_compare_exchange_weak_relaxed (&rwlock->__data.__readers, &r,
r - (1 << PTHREAD_RWLOCK_READER_SHIFT)))
return EAGAIN;
}
/* We have registered as a reader, so if we are in a read phase, we have
acquired a read lock. This is also the reader--reader fast-path.
Even if there is a primary writer, we just return. If writers are to
be preferred and we are the only active reader, we could try to enter a
write phase to let the writer proceed. This would be okay because we
cannot have acquired the lock previously as a reader (which could result
in deadlock if we would wait for the primary writer to run). However,
this seems to be a corner case and handling it specially not be worth the
complexity. */
if (__glibc_likely ((r & PTHREAD_RWLOCK_WRPHASE) == 0))
return 0;
/* Otherwise, if we were in a write phase (states #6 or #8), we must wait
for explicit hand-over of the read phase; the only exception is if we
can start a read phase if there is no primary writer currently. */
while (((r & PTHREAD_RWLOCK_WRPHASE) != 0)
&& ((r & PTHREAD_RWLOCK_WRLOCKED) == 0))
{
/* Try to enter a read phase: If the CAS below succeeds, we have
ownership; if it fails, we will simply retry and reassess the
situation.
Acquire MO so we synchronize with prior writers. */
if (atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers, &r,
r ^ PTHREAD_RWLOCK_WRPHASE))
{
/* We started the read phase, so we are also responsible for
updating the write-phase futex. Relaxed MO is sufficient.
We have to do the same steps as a writer would when handing
over the read phase to us because other readers cannot
distinguish between us and the writer; this includes
explicit hand-over and potentially having to wake other readers
(but we can pretend to do the setting and unsetting of WRLOCKED
atomically, and thus can skip this step). */
if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 0)
& PTHREAD_RWLOCK_FUTEX_USED) != 0)
{
int private = __pthread_rwlock_get_private (rwlock);
futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private);
}
return 0;
}
else
{
/* TODO Back off before retrying. Also see above. */
}
}
/* We were in a write phase but did not install the read phase. We cannot
distinguish between a writer and another reader starting the read phase,
so we must wait for explicit hand-over via __wrphase_futex.
However, __wrphase_futex might not have been set to 1 yet (either
because explicit hand-over to the writer is still ongoing, or because
the writer has started the write phase but has not yet updated
__wrphase_futex). The least recent value of __wrphase_futex we can
read from here is the modification of the last read phase (because
we synchronize with the last reader in this read phase through
__readers; see the use of acquire MO on the fetch_add above).
Therefore, if we observe a value of 0 for __wrphase_futex, we need
to subsequently check that __readers now indicates a read phase; we
need to use acquire MO for this so that if we observe a read phase,
we will also see the modification of __wrphase_futex by the previous
writer. We then need to load __wrphase_futex again and continue to
wait if it is not 0, so that we do not skip explicit hand-over.
Relaxed MO is sufficient for the load from __wrphase_futex because
we just use it as an indicator for when we can proceed; we use
__readers and the acquire MO accesses to it to eventually read from
the proper stores to __wrphase_futex. */
unsigned int wpf;
bool ready = false;
for (;;)
{
while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex))
| PTHREAD_RWLOCK_FUTEX_USED) == (1 | PTHREAD_RWLOCK_FUTEX_USED))
{
int private = __pthread_rwlock_get_private (rwlock);
if (((wpf & PTHREAD_RWLOCK_FUTEX_USED) == 0)
&& !atomic_compare_exchange_weak_relaxed
(&rwlock->__data.__wrphase_futex,
&wpf, wpf | PTHREAD_RWLOCK_FUTEX_USED))
continue;
int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex,
1 | PTHREAD_RWLOCK_FUTEX_USED, abstime, private);
if (err == ETIMEDOUT)
{
/* If we timed out, we need to unregister. If no read phase
has been installed while we waited, we can just decrement
the number of readers. Otherwise, we just acquire the
lock, which is allowed because we give no precise timing
guarantees, and because the timeout is only required to
be in effect if we would have had to wait for other
threads (e.g., if futex_wait would time-out immediately
because the given absolute time is in the past). */
r = atomic_load_relaxed (&rwlock->__data.__readers);
while ((r & PTHREAD_RWLOCK_WRPHASE) != 0)
{
/* We don't need to make anything else visible to
others besides unregistering, so relaxed MO is
sufficient. */
if (atomic_compare_exchange_weak_relaxed
(&rwlock->__data.__readers, &r,
r - (1 << PTHREAD_RWLOCK_READER_SHIFT)))
return ETIMEDOUT;
/* TODO Back-off. */
}
/* Use the acquire MO fence to mirror the steps taken in the
non-timeout case. Note that the read can happen both
in the atomic_load above as well as in the failure case
of the CAS operation. */
atomic_thread_fence_acquire ();
/* We still need to wait for explicit hand-over, but we must
not use futex_wait anymore because we would just time out
in this case and thus make the spin-waiting we need
unnecessarily expensive. */
while ((atomic_load_relaxed (&rwlock->__data.__wrphase_futex)
| PTHREAD_RWLOCK_FUTEX_USED)
== (1 | PTHREAD_RWLOCK_FUTEX_USED))
{
/* TODO Back-off? */
}
ready = true;
break;
}
/* If we got interrupted (EINTR) or the futex word does not have the
expected value (EAGAIN), retry. */
}
if (ready)
/* See below. */
break;
/* We need acquire MO here so that we synchronize with the lock
release of the writer, and so that we observe a recent value of
__wrphase_futex (see below). */
if ((atomic_load_acquire (&rwlock->__data.__readers)
& PTHREAD_RWLOCK_WRPHASE) == 0)
/* We are in a read phase now, so the least recent modification of
__wrphase_futex we can read from is the store by the writer
with value 1. Thus, only now we can assume that if we observe
a value of 0, explicit hand-over is finished. Retry the loop
above one more time. */
ready = true;
}
return 0;
}
static __always_inline void
__pthread_rwlock_wrunlock (pthread_rwlock_t *rwlock)
{
int private = __pthread_rwlock_get_private (rwlock);
atomic_store_relaxed (&rwlock->__data.__cur_writer, 0);
/* Disable waiting by writers. We will wake up after we decided how to
proceed. */
bool wake_writers = ((atomic_exchange_relaxed
(&rwlock->__data.__writers_futex, 0) & PTHREAD_RWLOCK_FUTEX_USED) != 0);
if (rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP)
{
/* First, try to hand over to another writer. */
unsigned int w = atomic_load_relaxed (&rwlock->__data.__writers);
while (w != 0)
{
/* Release MO so that another writer that gets WRLOCKED from us will
synchronize with us and thus can take over our view of
__readers (including, for example, whether we are in a write
phase or not). */
if (atomic_compare_exchange_weak_release (&rwlock->__data.__writers,
&w, w | PTHREAD_RWLOCK_WRHANDOVER))
/* Another writer will take over. */
goto done;
/* TODO Back-off. */
}
}
/* We have done everything we needed to do to prefer writers, so now we
either hand over explicitly to readers if there are any, or we simply
stay in a write phase. See pthread_rwlock_rdunlock for more details. */
unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
/* Release MO so that subsequent readers or writers synchronize with us. */
while (!atomic_compare_exchange_weak_release
(&rwlock->__data.__readers, &r, (r ^ PTHREAD_RWLOCK_WRLOCKED)
^ ((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0 ? 0
: PTHREAD_RWLOCK_WRPHASE)))
{
/* TODO Back-off. */
}
if ((r >> PTHREAD_RWLOCK_READER_SHIFT) != 0)
{
/* We must hand over explicitly through __wrphase_futex. Relaxed MO is
sufficient because it is just used to delay acquisition by a writer;
any other synchronizes-with relations that are necessary are
established through __readers. */
if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 0)
& PTHREAD_RWLOCK_FUTEX_USED) != 0)
futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private);
}
done:
/* We released WRLOCKED in some way, so wake a writer. */
if (wake_writers)
futex_wake (&rwlock->__data.__writers_futex, 1, private);
}
static __always_inline int
__pthread_rwlock_wrlock_full (pthread_rwlock_t *rwlock,
const struct timespec *abstime)
{
/* Make sure we are not holding the rwlock as a writer. This is a deadlock
situation we recognize and report. */
if (__glibc_unlikely (atomic_load_relaxed (&rwlock->__data.__cur_writer)
== THREAD_GETMEM (THREAD_SELF, tid)))
return EDEADLK;
/* First we try to acquire the role of primary writer by setting WRLOCKED;
if it was set before, there already is a primary writer. Acquire MO so
that we synchronize with previous primary writers.
We do not try to change to a write phase right away using a fetch_or
because we would have to reset it again and wake readers if there are
readers present (some readers could try to acquire the lock more than
once, so setting a write phase in the middle of this could cause
deadlock). Changing to a write phase eagerly would only speed up the
transition from a read phase to a write phase in the uncontended case,
but it would slow down the contended case if readers are preferred (which
is the default).
We could try to CAS from a state with no readers to a write phase, but
this could be less scalable if readers arrive and leave frequently. */
bool may_share_futex_used_flag = false;
unsigned int r = atomic_fetch_or_acquire (&rwlock->__data.__readers,
PTHREAD_RWLOCK_WRLOCKED);
if (__glibc_unlikely ((r & PTHREAD_RWLOCK_WRLOCKED) != 0))
{
/* There is another primary writer. */
bool prefer_writer =
(rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP);
if (prefer_writer)
{
/* We register as a waiting writer, so that we can make use of
writer--writer hand-over. Relaxed MO is fine because we just
want to register. We assume that the maximum number of threads
is less than the capacity in __writers. */
atomic_fetch_add_relaxed (&rwlock->__data.__writers, 1);
}
for (;;)
{
/* TODO Spin until WRLOCKED is 0 before trying the CAS below.
But pay attention to not delay trying writer--writer hand-over
for too long (which we must try eventually anyway). */
if ((r & PTHREAD_RWLOCK_WRLOCKED) == 0)
{
/* Try to become the primary writer or retry. Acquire MO as in
the fetch_or above. */
if (atomic_compare_exchange_weak_acquire
(&rwlock->__data.__readers, &r,
r | PTHREAD_RWLOCK_WRLOCKED))
{
if (prefer_writer)
{
/* Unregister as a waiting writer. Note that because we
acquired WRLOCKED, WRHANDOVER will not be set.
Acquire MO on the CAS above ensures that
unregistering happens after the previous writer;
this sorts the accesses to __writers by all
primary writers in a useful way (e.g., any other
primary writer acquiring after us or getting it from
us through WRHANDOVER will see both our changes to
__writers).
??? Perhaps this is not strictly necessary for
reasons we do not yet know of. */
atomic_fetch_add_relaxed (&rwlock->__data.__writers,
-1);
}
break;
}
/* Retry if the CAS fails (r will have been updated). */
continue;
}
/* If writer--writer hand-over is available, try to become the
primary writer this way by grabbing the WRHANDOVER token. If we
succeed, we own WRLOCKED. */
if (prefer_writer)
{
unsigned int w = atomic_load_relaxed
(&rwlock->__data.__writers);
if ((w & PTHREAD_RWLOCK_WRHANDOVER) != 0)
{
/* Acquire MO is required here so that we synchronize with
the writer that handed over WRLOCKED. We also need this
for the reload of __readers below because our view of
__readers must be at least as recent as the view of the
writer that handed over WRLOCKED; we must avoid an ABA
through WRHANDOVER, which could, for example, lead to us
assuming we are still in a write phase when in fact we
are not. */
if (atomic_compare_exchange_weak_acquire
(&rwlock->__data.__writers,
&w, (w - PTHREAD_RWLOCK_WRHANDOVER - 1)))
{
/* Reload so our view is consistent with the view of
the previous owner of WRLOCKED. See above. */
r = atomic_load_relaxed (&rwlock->__data.__readers);
break;
}
/* We do not need to reload __readers here. We should try
to perform writer--writer hand-over if possible; if it
is not possible anymore, we will reload __readers
elsewhere in this loop. */
continue;
}
}
/* We did not acquire WRLOCKED nor were able to use writer--writer
hand-over, so we block on __writers_futex. */
int private = __pthread_rwlock_get_private (rwlock);
unsigned int wf = atomic_load_relaxed
(&rwlock->__data.__writers_futex);
if (((wf & ~(unsigned int) PTHREAD_RWLOCK_FUTEX_USED) != 1)
|| ((wf != (1 | PTHREAD_RWLOCK_FUTEX_USED))
&& !atomic_compare_exchange_weak_relaxed
(&rwlock->__data.__writers_futex, &wf,
1 | PTHREAD_RWLOCK_FUTEX_USED)))
{
/* If we cannot block on __writers_futex because there is no
primary writer, or we cannot set PTHREAD_RWLOCK_FUTEX_USED,
we retry. We must reload __readers here in case we cannot
block on __writers_futex so that we can become the primary
writer and are not stuck in a loop that just continuously
fails to block on __writers_futex. */
r = atomic_load_relaxed (&rwlock->__data.__readers);
continue;
}
/* We set the flag that signals that the futex is used, or we could
have set it if we had been faster than other waiters. As a
result, we may share the flag with an unknown number of other
writers. Therefore, we must keep this flag set when we acquire
the lock. We do not need to do this when we do not reach this
point here because then we are not part of the group that may
share the flag, and another writer will wake one of the writers
in this group. */
may_share_futex_used_flag = true;
int err = futex_abstimed_wait (&rwlock->__data.__writers_futex,
1 | PTHREAD_RWLOCK_FUTEX_USED, abstime, private);
if (err == ETIMEDOUT)
{
if (prefer_writer)
{
/* We need to unregister as a waiting writer. If we are the
last writer and writer--writer hand-over is available,
we must make use of it because nobody else will reset
WRLOCKED otherwise. (If we use it, we simply pretend
that this happened before the timeout; see
pthread_rwlock_rdlock_full for the full reasoning.)
Also see the similar code above. */
unsigned int w = atomic_load_relaxed
(&rwlock->__data.__writers);
while (!atomic_compare_exchange_weak_acquire
(&rwlock->__data.__writers, &w,
(w == PTHREAD_RWLOCK_WRHANDOVER + 1 ? 0 : w - 1)))
{
/* TODO Back-off. */
}
if (w == PTHREAD_RWLOCK_WRHANDOVER + 1)
{
/* We must continue as primary writer. See above. */
r = atomic_load_relaxed (&rwlock->__data.__readers);
break;
}
}
/* We cleaned up and cannot have stolen another waiting writer's
futex wake-up, so just return. */
return ETIMEDOUT;
}
/* If we got interrupted (EINTR) or the futex word does not have the
expected value (EAGAIN), retry after reloading __readers. */
r = atomic_load_relaxed (&rwlock->__data.__readers);
}
/* Our snapshot of __readers is up-to-date at this point because we
either set WRLOCKED using a CAS (and update r accordingly below,
which was used as expected value for the CAS) or got WRLOCKED from
another writer whose snapshot of __readers we inherit. */
r |= PTHREAD_RWLOCK_WRLOCKED;
}
/* We are the primary writer; enable blocking on __writers_futex. Relaxed
MO is sufficient for futex words; acquire MO on the previous
modifications of __readers ensures that this store happens after the
store of value 0 by the previous primary writer. */
atomic_store_relaxed (&rwlock->__data.__writers_futex,
1 | (may_share_futex_used_flag ? PTHREAD_RWLOCK_FUTEX_USED : 0));
/* If we are in a write phase, we have acquired the lock. */
if ((r & PTHREAD_RWLOCK_WRPHASE) != 0)
goto done;
/* If we are in a read phase and there are no readers, try to start a write
phase. */
while (((r & PTHREAD_RWLOCK_WRPHASE) == 0)
&& ((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0))
{
/* Acquire MO so that we synchronize with prior writers and do
not interfere with their updates to __writers_futex, as well
as regarding prior readers and their updates to __wrphase_futex,
respectively. */
if (atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers,
&r, r | PTHREAD_RWLOCK_WRPHASE))
{
/* We have started a write phase, so need to enable readers to wait.
See the similar case in __pthread_rwlock_rdlock_full. Unlike in
that similar case, we are the (only) primary writer and so do
not need to wake another writer. */
atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1);
goto done;
}
/* TODO Back-off. */
}
/* We became the primary writer in a read phase and there were readers when
we did (because of the previous loop). Thus, we have to wait for
explicit hand-over from one of these readers.
We basically do the same steps as for the similar case in
__pthread_rwlock_rdlock_full, except that we additionally might try
to directly hand over to another writer and need to wake up
other writers or waiting readers (i.e., PTHREAD_RWLOCK_RWAITING). */
unsigned int wpf;
bool ready = false;
for (;;)
{
while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex))
| PTHREAD_RWLOCK_FUTEX_USED) == PTHREAD_RWLOCK_FUTEX_USED)
{
int private = __pthread_rwlock_get_private (rwlock);
if (((wpf & PTHREAD_RWLOCK_FUTEX_USED) == 0)
&& !atomic_compare_exchange_weak_relaxed
(&rwlock->__data.__wrphase_futex, &wpf,
PTHREAD_RWLOCK_FUTEX_USED))
continue;
int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex,
PTHREAD_RWLOCK_FUTEX_USED, abstime, private);
if (err == ETIMEDOUT)
{
if (rwlock->__data.__flags
!= PTHREAD_RWLOCK_PREFER_READER_NP)
{
/* We try writer--writer hand-over. */
unsigned int w = atomic_load_relaxed
(&rwlock->__data.__writers);
if (w != 0)
{
/* We are about to hand over WRLOCKED, so we must
release __writers_futex too; otherwise, we'd have
a pending store, which could at least prevent
other threads from waiting using the futex
because it could interleave with the stores
by subsequent writers. In turn, this means that
we have to clean up when we do not hand over
WRLOCKED.
Release MO so that another writer that gets
WRLOCKED from us can take over our view of
__readers. */
unsigned int wf = atomic_exchange_relaxed
(&rwlock->__data.__writers_futex, 0);
while (w != 0)
{
if (atomic_compare_exchange_weak_release
(&rwlock->__data.__writers, &w,
w | PTHREAD_RWLOCK_WRHANDOVER))
{
/* Wake other writers. */
if ((wf & PTHREAD_RWLOCK_FUTEX_USED) != 0)
futex_wake (&rwlock->__data.__writers_futex,
1, private);
return ETIMEDOUT;
}
/* TODO Back-off. */
}
/* We still own WRLOCKED and someone else might set
a write phase concurrently, so enable waiting
again. Make sure we don't loose the flag that
signals whether there are threads waiting on
this futex. */
atomic_store_relaxed
(&rwlock->__data.__writers_futex, wf);
}
}
/* If we timed out and we are not in a write phase, we can
just stop being a primary writer. Otherwise, we just
acquire the lock. */
r = atomic_load_relaxed (&rwlock->__data.__readers);
if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
{
/* We are about to release WRLOCKED, so we must release
__writers_futex too; see the handling of
writer--writer hand-over above. */
unsigned int wf = atomic_exchange_relaxed
(&rwlock->__data.__writers_futex, 0);
while ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
{
/* While we don't need to make anything from a
caller's critical section visible to other
threads, we need to ensure that our changes to
__writers_futex are properly ordered.
Therefore, use release MO to synchronize with
subsequent primary writers. Also wake up any
waiting readers as they are waiting because of
us. */
if (atomic_compare_exchange_weak_release
(&rwlock->__data.__readers, &r,
(r ^ PTHREAD_RWLOCK_WRLOCKED)
& ~(unsigned int) PTHREAD_RWLOCK_RWAITING))
{
/* Wake other writers. */
if ((wf & PTHREAD_RWLOCK_FUTEX_USED) != 0)
futex_wake (&rwlock->__data.__writers_futex,
1, private);
/* Wake waiting readers. */
if ((r & PTHREAD_RWLOCK_RWAITING) != 0)
futex_wake (&rwlock->__data.__readers,
INT_MAX, private);
return ETIMEDOUT;
}
}
/* We still own WRLOCKED and someone else might set a
write phase concurrently, so enable waiting again.
Make sure we don't loose the flag that signals
whether there are threads waiting on this futex. */
atomic_store_relaxed (&rwlock->__data.__writers_futex, wf);
}
/* Use the acquire MO fence to mirror the steps taken in the
non-timeout case. Note that the read can happen both
in the atomic_load above as well as in the failure case
of the CAS operation. */
atomic_thread_fence_acquire ();
/* We still need to wait for explicit hand-over, but we must
not use futex_wait anymore. */
while ((atomic_load_relaxed
(&rwlock->__data.__wrphase_futex)
| PTHREAD_RWLOCK_FUTEX_USED)
== PTHREAD_RWLOCK_FUTEX_USED)
{
/* TODO Back-off. */
}
ready = true;
break;
}
/* If we got interrupted (EINTR) or the futex word does not have
the expected value (EAGAIN), retry. */
}
/* See pthread_rwlock_rdlock_full. */
if (ready)
break;
if ((atomic_load_acquire (&rwlock->__data.__readers)
& PTHREAD_RWLOCK_WRPHASE) != 0)
ready = true;
}
done:
atomic_store_relaxed (&rwlock->__data.__cur_writer,
THREAD_GETMEM (THREAD_SELF, tid));
return 0;
}