linux-kernel - Re: nfs: infinite loop in fcntl(F

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Message-ID: <20080411191910.GB16965@fieldses.org>
Date:	Fri, 11 Apr 2008 15:19:10 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	trond.myklebust@....uio.no, eshel@...aden.ibm.com, neilb@...e.de,
	akpm@...ux-foundation.org, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: nfs: infinite loop in fcntl(F_SETLKW)

On Fri, Apr 11, 2008 at 09:12:23PM +0200, Miklos Szeredi wrote:
> > > OK. So the correct fix here should really be applied to fcntl_setlk().
> > > There is absolutely no reason why we should be looping at all if the
> > > filesystem has a ->lock() method.
> > > 
> > > In fact, this looping behaviour was introduced recently in commit
> > > 7723ec9777d9832849b76475b1a21a2872a40d20.
> > 
> > Apologies, that was indeed a behavioral change introduced in a commit
> > that claimed just to be shuffling code around.
> 
> Yeah, that patch looks totally wrong.  It's not generally a good idea
> to do a loop where the exit condition depends on something you don't
> control.  And error values from filesystem methods are typically like
> that.  For example with fuse, the error code could come from an
> unprivileged userspace process.
> 
> I didn't realize this aspect of the bug previously, because I
> concentrated on the lockd inconsistency.
> 
> Btw, why hasn't this work been posted on -fsdevel prior to merging
> into mainline?

http://marc.info/?l=linux-fsdevel&m=117581629326194&w=2

--b.

> 
> > 
> > > Marc, Bruce, why was this
> > > done, and how are filesystems now supposed to behave?
> > > 
> > 
> > The assumption must have been that -EAGAIN could only mean that we
> > needed to keep blocking, and hence was a nonsensical return from a
> > filesystem lock method that waited itself for the lock to become
> > available--such a method would return 0, -EINTR (or some more exotic
> > error), or continue waiting.
> 
> EAGAIN for a blocking lock is nonsensical, so my original patch could
> still make sense.  But that's no longer a regression, and not all that
> important.
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