linux-ext4 - Re: 2.6.23-rc6: hanging ext3 dbench tests

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Date:	Mon, 24 Sep 2007 17:28:46 -0700
From:	Badari Pulavarty <pbadari@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andy Whitcroft <apw@...dowen.org>, sct@...hat.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	adilger@...sterfs.com, ext4 <linux-ext4@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>, mel@....ul.ie
Subject: Re: 2.6.23-rc6: hanging ext3 dbench tests

On Mon, 2007-09-24 at 13:04 -0700, Linus Torvalds wrote:
> 
> On Mon, 24 Sep 2007, Badari Pulavarty wrote:
> > 
> > Whats happening on my machine is ..
> > 
> > dbench forks of 4 children and sends them a signal to start the work.
> > 3 out of 4 children gets the signal and does the work. One of the child
> > never gets the signal so, it waits forever in pause(). So, parent waits
> > for a longtime to kill it.
> 
> Since this *seems* to have nothing to do with the filesystem, and since it 
> *seems* to have been introduced between -rc3 and -rc4, I did
> 
> 	gitk v2.6.23-rc3..v2.6.23-rc4 -- kernel/
> 
> to see what has changed. One of the commits was signal-related, and that 
> one doesn't look like it could possibly matter.
> 
> The rest were scheduler-related, which doesn't surprise me. In fact, even 
> before I looked, my reaction to your bug report was "That sounds like an 
> application race condition".
> 
> Applications shouldn't use "pause()" for waiting for a signal. It's a 
> fundamentally racy interface - the signal could have happened just 
> *before* calling pause. So it's almost always a bug to use pause(), and 
> any users should be fixed to use "sigsuspend()" instead, which can 
> atomically (and correctly) pause for a signal while the process has masked 
> it outside of the system call.
> 
> Now, I took a look at the dbench sources, and I have to say that the race 
> looks *very* unlikely (there's quite a small window in which it does
> 
> 	children[i].status = getpid();
> 	** race window here **
> 	pause();
> 
> and it would require *just* the right timing so that the parent doesn't 
> end up doing the "sleep(1)" (which would make the window even less likely 
> to be hit), but there does seem to be a race condition there. And it 
> *could* be that you just happen to hit it on your hw setup.
> 
> So before you do anything else, does this patch (TOTALLY UNTESTED! DONE 
> ENTIRELY LOOKING AT THE SOURCE! IT MAY RAPE ALL YOUR PETS, AND CALL YOU 
> BAD NAMES!) make any difference?
> 
> (patch against unmodified dbench-2.0)
> 
> 		Linus
> 
> ---
> diff --git a/dbench.c b/dbench.c
> index ccf5624..4be5712 100644
> --- a/dbench.c
> +++ b/dbench.c
> @@ -91,10 +91,15 @@ static double create_procs(int nprocs, void (*fn)(struct child_struct * ))
>  
>  	for (i=0;i<nprocs;i++) {
>  		if (fork() == 0) {
> +			sigset_t old, blocked;
> +
> +			sigemptyset(&blocked);
> +			sigaddset(&blocked, SIGCONT);
> +			sigprocmask(SIG_BLOCK, &blocked, &old);
>  			setbuffer(stdout, NULL, 0);
>  			nb_setup(&children[i]);
>  			children[i].status = getpid();
> -			pause();
> +			sigsuspend(&old);
>  			fn(&children[i]);
>  			_exit(0);
>  		}

With the modified dbench, I couldn't reproduce the problem so far.
I will let it run through the night (just to be sure). 

For now, we can treat it as a tool/App issue :)

Thanks,
Badari

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