linux-ext4 - Re: RFC: hold i_rwsem until aio completes

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Date:   Wed, 15 Jan 2020 07:54:06 +0100
From:   Christoph Hellwig <hch@....de>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@....de>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-ext4@...r.kernel.org, cluster-devel@...hat.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: RFC: hold i_rwsem until aio completes

On Tue, Jan 14, 2020 at 10:47:07AM -0800, Matthew Wilcox wrote:
> It would be helpful if we could also use the same lockdep logic
> for PageLocked.  Again, it's a case where returning to userspace with
> PageLock held is fine, because we're expecting an interrupt to come in
> and drop the lock for us.

Yes, this is a very typical pattern for I/O.  Besides the page and
buffer head bit locks it also applies to the semaphore in the xfs_buf
structure and probably various other places that currently used hand
crafted or legacy locking primitives to escape lockdep.

> Perhaps the right answer is, from lockdep's point of view, to mark the
> lock as being released at the point where we submit the I/O.  Then
> in the completion path release the lock without telling lockdep we
> released it.

That is similar to what the fsfreeze code does, but I don't think it
is very optimal, as misses to track any dependencies after I/O
submission, and at least some of the completions paths do take
locks.  But it might be a start.