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/* pthread_rwlock emulation (version without exponential back-off)
*
* Copyright (C) 2022-2025 Willy Tarreau <w@1wt.eu>
*
* Permission is hereby granted, free of charge, to any person obtaining
* a copy of this software and associated documentation files (the
* "Software"), to deal in the Software without restriction, including
* without limitation the rights to use, copy, modify, merge, publish,
* distribute, sublicense, and/or sell copies of the Software, and to
* permit persons to whom the Software is furnished to do so, subject to
* the following conditions:
*
* The above copyright notice and this permission notice shall be
* included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
* OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
* HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
* WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
* OTHER DEALINGS IN THE SOFTWARE.
*/
/* Pthread rwlock emulation using plocks (to avoid expensive futexes).
*
* These are a direct mapping on Progressive Locks, with the exception that
* since there's a common unlock operation in pthreads, we need to know if
* we need to unlock for reads or writes, so we set the topmost bit to 1 when
* a write lock is acquired to indicate that a write unlock needs to be
* performed. This only divides by two the maximum number of threads that
* may be supported compared to the default plock implementation, which is
* generally OK. In order to ease integration into existing code, the storage
* here is the provided pthread_rwlock_t cast as a unsigned long. It is
* expected to be zero when unlocked so that code that would forget to
* call pthread_rwlock_init() after a calloc() and that would happen to work
* by pure luck would continue to work.
*
* This variant does NOT use exponential backoff as it was found to
* significantly reduce performance on some platforms when the application
* makes excessive use of pthread_rwlocks but contention remains low (which
* is often the case when using rwlocks). If this is needed in the future,
* please do not do it in this file and provide it in an alternate one
* instead so that users can choose the one they want.
*
* It is recommended to link this code statically into the target executable
* to make sure that the redefined symbols have precedence over the ones
* provided by an external shared pthread library. It has no other dependency
* beyond pthread and plock.h, so the build process is trivial:
*
* $ cc -O2 -c pth_rwl.c -pthread
* # link the resulting .o into the final executable
*
* This source file (and its required dependencies) may be directly copied into
* the target project as long as its license is compatible with this one (which
* should generally be the case).
*/
#include <pthread.h>
#include "plock.h"
int pthread_rwlock_init(pthread_rwlock_t *restrict rwlock, const pthread_rwlockattr_t *restrict attr)
{
unsigned long *lock = (unsigned long *)rwlock;
*lock = 0;
return 0;
}
int pthread_rwlock_destroy(pthread_rwlock_t *rwlock)
{
unsigned long *lock = (unsigned long *)rwlock;
*lock = 0;
return 0;
}
int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock)
{
pl_lorw_rdlock((unsigned long *)rwlock);
return 0;
}
int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock)
{
return !!pl_cmpxchg((unsigned long *)rwlock, 0, PLOCK_LORW_SHR_BASE);
}
int pthread_rwlock_timedrdlock(pthread_rwlock_t *restrict rwlock, const struct timespec *restrict abstime)
{
return pthread_rwlock_tryrdlock(rwlock);
}
int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock)
{
pl_lorw_wrlock((unsigned long *)rwlock);
return 0;
}
int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock)
{
return !!pl_cmpxchg((unsigned long *)rwlock, 0, PLOCK_LORW_EXC_BASE);
}
int pthread_rwlock_timedwrlock(pthread_rwlock_t *restrict rwlock, const struct timespec *restrict abstime)
{
return pthread_rwlock_trywrlock(rwlock);
}
int pthread_rwlock_unlock(pthread_rwlock_t *rwlock)
{
pl_lorw_unlock((unsigned long *)rwlock);
return 0;
}
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