linux-kernel - Re: [rfc][patch] futex: restartable futex

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Message-Id: <1173394952.24738.1070.camel@localhost.localdomain>
Date:	Fri, 09 Mar 2007 00:02:31 +0100
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ulrich Drepper <drepper@...hat.com>
Subject: Re: [rfc][patch] futex: restartable futex_wait?

On Thu, 2007-03-08 at 18:29 +0100, Ingo Molnar wrote:
> * Nick Piggin <npiggin@...e.de> wrote:
> 
> > Hi Ingo,
> > 
> > I'm seeing an LTP test fail for ltp test sigaction_16_24. Basically, 
> > it tests whether the SA_RESTART flag works for the sem_wait operation.

Not sure, whether the testcase is correct or not. See below

> > I see sem_wait is implemented with futex_wait, so I wonder whether we 
> > can make it restartable? Am I going about it the right way? (Seems to 
> > fix the testcase here).
> 
> i think that's quite right. I'm wondering why this never came up before? 
> But your fix is not complete i think:
> 
> > +             restart->arg2 = time;
> > +             return -ERESTART_RESTARTBLOCK;
> > +     }
> 
> 'time' here is relative, so the restarted syscall will do a /full/ wait 
> again.
> 
> maybe we should rather convert futex timed-waits to hrtimers? Thomas?

The problem is that the original API is based on relative time and
therefor can not be changed. 

sem_wait returns -EINTR to the application when it is interrupted, while
pthread_mutex_lock does not.

http://www.opengroup.org/onlinepubs/009695399/functions/sem_wait.html

http://www.opengroup.org/onlinepubs/009695399/functions/pthread_mutex_lock.html

We need to create a seperate op for the futex - just like the pi_futex
and use absolute time there too. 

	tglx


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