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New pthread rwlock that is more scalable.

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This replaces the pthread rwlock with a new implementation that uses a
more scalable algorithm (primarily through not using a critical section
anymore to make state changes).  The fast path for rdlock acquisition and
release is now basically a single atomic read-modify write or CAS and a few
branches.  See nptl/pthread_rwlock_common.c for details.

	* nptl/DESIGN-rwlock.txt: Remove.
	* nptl/lowlevelrwlock.sym: Remove.
	* nptl/Makefile: Add new tests.
	* nptl/pthread_rwlock_common.c: New file.  Contains the new rwlock.
	* nptl/pthreadP.h (PTHREAD_RWLOCK_PREFER_READER_P): Remove.
	(PTHREAD_RWLOCK_WRPHASE, PTHREAD_RWLOCK_WRLOCKED,
	PTHREAD_RWLOCK_RWAITING, PTHREAD_RWLOCK_READER_SHIFT,
	PTHREAD_RWLOCK_READER_OVERFLOW, PTHREAD_RWLOCK_WRHANDOVER,
	PTHREAD_RWLOCK_FUTEX_USED): New.
	* nptl/pthread_rwlock_init.c (__pthread_rwlock_init): Adapt to new
	implementation.
	* nptl/pthread_rwlock_rdlock.c (__pthread_rwlock_rdlock_slow): Remove.
	(__pthread_rwlock_rdlock): Adapt.
	* nptl/pthread_rwlock_timedrdlock.c
	(pthread_rwlock_timedrdlock): Adapt.
	* nptl/pthread_rwlock_timedwrlock.c
	(pthread_rwlock_timedwrlock): Adapt.
	* nptl/pthread_rwlock_trywrlock.c (pthread_rwlock_trywrlock): Adapt.
	* nptl/pthread_rwlock_tryrdlock.c (pthread_rwlock_tryrdlock): Adapt.
	* nptl/pthread_rwlock_unlock.c (pthread_rwlock_unlock): Adapt.
	* nptl/pthread_rwlock_wrlock.c (__pthread_rwlock_wrlock_slow): Remove.
	(__pthread_rwlock_wrlock): Adapt.
	* nptl/tst-rwlock10.c: Adapt.
	* nptl/tst-rwlock11.c: Adapt.
	* nptl/tst-rwlock17.c: New file.
	* nptl/tst-rwlock18.c: New file.
	* nptl/tst-rwlock19.c: New file.
	* nptl/tst-rwlock2b.c: New file.
	* nptl/tst-rwlock8.c: Adapt.
	* nptl/tst-rwlock9.c: Adapt.
	* sysdeps/aarch64/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/arm/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/hppa/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/ia64/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/m68k/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/microblaze/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/mips/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/nios2/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/s390/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/sh/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/sparc/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/tile/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/unix/sysv/linux/alpha/bits/pthreadtypes.h
	(pthread_rwlock_t): Adapt.
	* sysdeps/unix/sysv/linux/powerpc/bits/pthreadtypes.h
	(pthread_rwlock_t): Adapt.
	* sysdeps/x86/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* nptl/nptl-printers.py (): Adapt.
	* nptl/nptl_lock_constants.pysym: Adapt.
	* nptl/test-rwlock-printers.py: Adapt.
	* nptl/test-rwlockattr-printers.c: Adapt.
	* nptl/test-rwlockattr-printers.py: Adapt.
This commit is contained in:
Torvald Riegel
2014-05-22 16:00:12 +02:00
parent fbb31e20bc
commit cc25c8b4c1
42 changed files with 1548 additions and 915 deletions
Download Patch File Download Diff File Expand all files Collapse all files

105
nptl/pthread_rwlock_tryrdlock.c
View File

@@ -18,56 +18,95 @@
#include <errno.h>
#include "pthreadP.h"
#include <lowlevellock.h>
#include <futex-internal.h>
#include <elide.h>
#include <atomic.h>
#include <stdbool.h>
#include "pthread_rwlock_common.c"
/* See pthread_rwlock_common.c for an overview. */
int
__pthread_rwlock_tryrdlock (pthread_rwlock_t *rwlock)
{
int result = EBUSY;
bool wake = false;
int futex_shared =
rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED;
if (ELIDE_TRYLOCK (rwlock->__data.__rwelision,
rwlock->__data.__lock == 0
&& rwlock->__data.__nr_readers == 0
&& rwlock->__data.__writer, 0))
return 0;
lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
if (rwlock->__data.__writer == 0
&& (rwlock->__data.__nr_writers_queued == 0
|| PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
/* For tryrdlock, we could speculate that we will succeed and go ahead and
register as a reader. However, if we misspeculate, we have to do the
same steps as a timed-out rdlock, which will increase contention.
Therefore, there is a trade-off between being able to use a combinable
read-modify-write operation and a CAS loop as used below; we pick the
latter because it simplifies the code, and should perform better when
tryrdlock is used in cases where writers are infrequent.
Because POSIX does not require a failed trylock to "synchronize memory",
relaxed MO is sufficient here and on the failure path of the CAS
below. */
unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
unsigned int rnew;
do
{
if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
{
--rwlock->__data.__nr_readers;
result = EAGAIN;
/* If we are in a read phase, try to acquire unless there is a
primary writer and we prefer writers and there will be no
recursive read locks. */
if (((r & PTHREAD_RWLOCK_WRLOCKED) != 0)
&& (rwlock->__data.__flags
== PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP))
return EBUSY;
rnew = r + (1 << PTHREAD_RWLOCK_READER_SHIFT);
}
else
{
result = 0;
/* See pthread_rwlock_rdlock. */
if (rwlock->__data.__nr_readers == 1
&& rwlock->__data.__nr_readers_queued > 0
&& rwlock->__data.__nr_writers_queued > 0)
/* If there is a writer that has acquired the lock and we are in
a write phase, fail. */
if ((r & PTHREAD_RWLOCK_WRLOCKED) != 0)
return EBUSY;
else
{
++rwlock->__data.__readers_wakeup;
wake = true;
/* If we do not care about potentially waiting writers, just
try to acquire. */
rnew = (r + (1 << PTHREAD_RWLOCK_READER_SHIFT))
^ PTHREAD_RWLOCK_WRPHASE;
}
}
/* If we could have caused an overflow or take effect during an
overflow, we just can / need to return EAGAIN. There is no need to
have actually modified the number of readers because we could have
done that and cleaned up immediately. */
if (rnew >= PTHREAD_RWLOCK_READER_OVERFLOW)
return EAGAIN;
}
/* If the CAS fails, we retry; this prevents that tryrdlock fails spuriously
(i.e., fails to acquire the lock although there is no writer), which is
fine for C++14 but not currently allowed by POSIX.
However, because tryrdlock must not appear to block, we should avoid
starving this CAS loop due to constant changes to __readers:
While normal rdlock readers that won't be able to acquire will just block
(and we expect timeouts on timedrdlock to be longer than one retry of the
CAS loop), we can have concurrently failing tryrdlock calls due to
readers or writers that acquire and release in the meantime. Using
randomized exponential back-off to make a live-lock unlikely should be
sufficient.
TODO Back-off.
Acquire MO so we synchronize with prior writers. */
while (!atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers,
&r, rnew));
if ((r & PTHREAD_RWLOCK_WRPHASE) != 0)
{
/* Same as in __pthread_rwlock_rdlock_full:
We started the read phase, so we are also responsible for
updating the write-phase futex. Relaxed MO is sufficient.
Note that there can be no other reader that we have to wake
because all other readers will see the read phase started by us
(or they will try to start it themselves); if a writer started
the read phase, we cannot have started it. Furthermore, we
cannot discard a PTHREAD_RWLOCK_FUTEX_USED flag because we will
overwrite the value set by the most recent writer (or the readers
before it in case of explicit hand-over) and we know that there
are no waiting readers. */
atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 0);
}
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
return 0;
if (wake)
futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX, futex_shared);
return result;
}
strong_alias (__pthread_rwlock_tryrdlock, pthread_rwlock_tryrdlock)