Re: [PATCH] ipc/sem.c: fix update sem_otime when calling sem_op insemaphore initialization

[Manfred Spraul]

This is a multi-part message in MIME format.On 09/22/2013 05:14 PM, Jia He wrote:
>   Hi Manfred
>
> On Sun, 22 Sep 2013 12:42:05 +0200 from manfred@xxxxxxxxxxxxxxxx wrote:
> > Hi all,
> >
> > On 09/22/2013 10:26 AM, Mike Galbraith wrote:
> > > On Sun, 2013-09-22 at 10:17 +0200, Mike Galbraith wrote:
> > > > On Sun, 2013-09-22 at 10:11 +0800, Jia He wrote:
> > > > > In commit 0a2b9d4c,the update of semaphore's sem_otime(last semop time)
> > > > > was removed because he wanted to move setting sem->sem_otime to one
> > > > > place. But after that, the initial semop() will not set the otime
> > > > > because its sem_op value is 0(in semtimedop,will not change
> > > > > otime if alter == 1).
> > > > >
> > > > > the error case:
> > > > > process_a(server)       process_b(client)
> > > > > semget()
> > > > > semctl(SETVAL)
> > > > > semop()
> > > > >                           semget()
> > > > >                           setctl(IP_STAT)
> > > > >                           for(;;) {               <--not successful here
> > > > >                             check until sem_otime > 0
> > > > >                           }
> > Good catch:
> > Since commit 0a2b9d4c, wait-for-zero semops do not update sem_otime anymore.
> >
> > Let's reverse that part of my commit and move the update of sem_otime back
> > into perform_atomic_semop().
> >
> > Jia: If perform_atomic_semop() updates sem_otime, then the update in
> > do_smart_update() is not necessary anymore.
> > Thus the whole logic with passing arround "semop_completed" can be removed, too.
> > Are you interested in writing that patch?
> Not all perform_atomic_semop() can cover the points of do_smart_update()
> after I move the parts of updating otime.
> Eg. in semctl_setval/exit_sem/etc.    That is, it seems I need to write some
> other codes to update sem_otime outside perform_atomic_semop(). That
> still violate your original goal---let one function do all the update otime
> things.
No. The original goal was an optimization:
The common case is one semop that increases a semaphore value - and that 
allows another semop that is sleeping to proceed.
Before, this caused two get_seconds() calls.
The current (buggy) code avoids the 2nd call.

> IMO, what if just remove the condition alter in sys_semtimedop:
> -        if (alter && error == 0)
> +       if (error == 0)
>              do_smart_update(sma, sops, nsops, 1, &tasks);
> In old codes, it would set the otime even when sem_op == 0
do_smart_update() can be expensive - thus it shouldn't be called if we 
didn't change anything.

Attached is a proposed patch - fully untested. It is intended to be 
applied on top of Jia's patch.

_If_ the performance degradation is too large, then the alternative 
would be to set sem_otime directly in semtimedop() for successfull 
non-alter operations.

--
    Manfred