linux-kernel - Re: [PATCH] Memory shortage can result in inconsistent flocks state

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Date:	Thu, 13 Sep 2007 10:04:16 +0400
From:	Pavel Emelyanov <xemul@...nvz.org>
To:	"J. Bruce Fields" <bfields@...ldses.org>
CC:	Trond Myklebust <trond.myklebust@....uio.no>,
	Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	devel@...nvz.org
Subject: Re: [PATCH] Memory shortage can result in inconsistent flocks state

J. Bruce Fields wrote:
> On Tue, Sep 11, 2007 at 04:38:13PM +0400, Pavel Emelyanov wrote:
>> This is a known feature that such "re-locking" is not atomic,
>> but in the racy case the file should stay locked (although by
>> some other process), but in this case the file will be unlocked.
> 
> That's a little subtle (I assume you've never seen this actually
> happen?), but it makes sense to me.

Well, this situation is hard to notice since usually programs
try to finish up when some error is returned from the kernel,
but I do believe that this could happen in one of the openvz
kernels since we limit the kernel memory usage for "containers"
and thus -ENOMEM is a common error.

>> The proposal is to prepare the lock in advance keeping no chance
>> to fail in the future code.
> 
> And the patch certainly looks correct.
> 
> I can add it to my (trivial) lock patches, if that's helpful--it'll
> get folded into the branch -mm pulls from and I can pass it along to
> Linus for 2.6.24.

Thanks.

> What I don't have that I wish I did is good regression tests for the
> flock or lease code (for posix locks I've been using connectathon,
> though that misses some important things too).
> 
> --b.
> 

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