linux-ext4 - Re: [URGENT PATCH] ext4: fix potential deadlock in ext4_evict

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Message-ID: <CAFgt=MA+YokAz4Nzd+OFk7XTDYXMbjXv8G_oXFieDMDr-u4RAg@mail.gmail.com>
Date:	Fri, 26 Aug 2011 22:17:52 -0700
From:	Jiaying Zhang <jiayingz@...gle.com>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	Tao Ma <tm@....ma>, Dave Chinner <david@...morbit.com>,
	linux-ext4@...r.kernel.org
Subject: Re: [URGENT PATCH] ext4: fix potential deadlock in ext4_evict_inode()

On Fri, Aug 26, 2011 at 1:22 PM, Ted Ts'o <tytso@....edu> wrote:
> On Fri, Aug 26, 2011 at 09:58:45AM -0700, Jiaying Zhang wrote:
>> Now thinking about an alternative approach to resolve the deadlock
>> mentioned above, maybe we can use mutex_trylock() in
>> ext4_end_io_work() and if we can't grab the mutex lock for an inode,
>> just requeue the work to the end of workqueue?
>
> Good idea!  That should speed up work queue processing in general, I
> think.
>
> The downside is that inodes that currently locked might take longer to
> complete.  In the case of fsync() we'll just force the I/O completion
> to happen in the context of the fsync'ing process, so I don't think it
> should be a problem in practice I think.
>
Ted,

I am working on a patch to use mutex_trylock() in ext4_end_io_work()
so that we can fix the described deadlock without needing to call
mutex_lock() and  ext4_flush_completed_IO() in ext4_evict_inode().
I run into some problem while testing it. Given that the deadlock my
original patch intended to fix only exists with dioread_nolock enabled
but the lockdep issue happens in all of cases, I think we should roll
that part back as you have planned. I am going to send a separate patch
later to fix the deadlock issue once I resolved the problem found in
my test.

Jiaying

>                                        - Ted
>
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