Re: [RFC 1/2] sem-posix: remove the posix semaphore support

[Daniel P . Berrangé]

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 <pbonzini@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 <pbonzini@redhat.com>
    Signed-off-by: Anthony Liguori <aliguori@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
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