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SubjectRe: [PATCH] Revert "workqueue: re-add lockdep dependencies for flushing"
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On Mon, 2018-10-22 at 22:28 +0200, Johannes Berg wrote:
> The lockdep report even more or less tells you what's going on. Perhaps
> we need to find a way to make lockdep not print "lock()" but "start()"
> or "flush()" for work items ... but if you read it this way, you see:
>
> CPU0 CPU1
>
> lock(i_mutex_key)
> start(dio->complete_work)
> lock(i_mutex_key)
> flush(wq dio/...)
>
> which is *clearly* a problem.

Your patch made lockdep report that the direct I/O code deadlocks although it
clearly doesn't deadlock. So where do you think the bug is? In the direct I/O
code or in your patch?

The code in the column with label "CPU0" is code called by do_blockdev_direct_IO().
From the body of that function:

/* will be released by direct_io_worker */
inode_lock(inode);

I think that is sufficient evidence that the direct I/O code is fine and that
your patch caused lockdep to produce an incorrect deadlock report.

Bart.

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