On Mon, Feb 21, 2022 at 05:56:16PM +0800, Longpeng(Mike) via wrote: > POSIX specifies an absolute time for sem_timedwait(), it would be > affected if the system time is changing, but there is not a relative > time or monotonic clock version of sem_timedwait, so we cannot gain > from POSIX semaphore anymore.
It doesn't appear in any man pages on my systems, but there is a new-ish API sem_clockwait() that accepts a choice of clock as a parameter. This is apparently a proposed POSIX standard API introduced in glibc 2.30, along with several others: https://sourceware.org/legacy-ml/libc-announce/2019/msg00001.html [quote] * Add new POSIX-proposed pthread_cond_clockwait, pthread_mutex_clocklock, pthread_rwlock_clockrdlock, pthread_rwlock_clockwrlock and sem_clockwait functions. These behave similarly to their "timed" equivalents, but also accept a clockid_t parameter to determine which clock their timeout should be measured against. All functions allow waiting against CLOCK_MONOTONIC and CLOCK_REALTIME. The decision of which clock to be used is made at the time of the wait (unlike with pthread_condattr_setclock, which requires the clock choice at initialization time). [/quote] > diff --git a/include/qemu/thread-posix.h b/include/qemu/thread-posix.h > index b792e6e..5466608 100644 > --- a/include/qemu/thread-posix.h > +++ b/include/qemu/thread-posix.h > @@ -27,13 +27,9 @@ struct QemuCond { > }; > > struct QemuSemaphore { > -#ifndef CONFIG_SEM_TIMEDWAIT > pthread_mutex_t lock; > pthread_cond_t cond; > unsigned int count; > -#else > - sem_t sem; > -#endif > bool initialized; > }; As a point of history, the original code only used sem_t. The pthreads based fallback was introduced by Paolo in commit c166cb72f1676855816340666c3b618beef4b976 Author: Paolo Bonzini <pbonz...@redhat.com> Date: Fri Nov 2 15:43:21 2012 +0100 semaphore: implement fallback counting semaphores with mutex+condvar OpenBSD and Darwin do not have sem_timedwait. Implement a fallback for them. Signed-off-by: Paolo Bonzini <pbonz...@redhat.com> Signed-off-by: Anthony Liguori <aligu...@us.ibm.com> I'm going to assume this fallback impl is less efficient than the native sem_t impl as the reason for leaving the original impl, or maybe Paolo just want to risk accidental bugs by removing the existing usage ? Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
