Re: [URGENT PATCH] ext4: fix potential deadlock in ext4_evict_inode()

On Fri, Aug 26, 2011 at 1:22 PM, Ted Ts'o [off-list ref] wrote:

On Fri, Aug 26, 2011 at 09:58:45AM -0700, Jiaying Zhang wrote:

quoted

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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