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

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Message-ID: <alpine.LFD.0.999.0709241243530.3579@woody.linux-foundation.org>
Date:	Mon, 24 Sep 2007 13:04:56 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Badari Pulavarty <pbadari@...il.com>
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, 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);
 		}
-
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