linux-kernel - Re: [PATCH] sched: Avoid that __wait_on_bit

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Message-ID: <20160811173651.GA31803@redhat.com>
Date:	Thu, 11 Aug 2016 19:36:52 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Bart Van Assche <bart.vanassche@...disk.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Neil Brown <neilb@...e.de>,
	Michael Shaver <jmshaver@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Avoid that __wait_on_bit_lock() hangs

Hi Bart,

On 08/10, Bart Van Assche wrote:
>
> That's an excellent catch. With your previous patch and this patch applied I
> can't reproduce the hang in truncate_inode_pages_range() anymore.

Great, thanks.

I'll send another debugging patch tomorrow, I was a bit busy today. The next
step is obvious, we need to know the caller.

But just in case, this doesn't necessarily mean that the usage of
__ClearPageLocked() is actually buggy, we don't really know this so far...

And I can't understand another oddity. Your test-case hangs in kill_bdev()
path which sleeps with bdev->bd_openers == 0 under bdev->bd_mutex so it can't
be re-opened. However, since your change in abort_exclusive_wait() helped,
there should be the readers sleeping in lock_killable() and thus bd_openers
can't be zero.

Nevermind, I don't understand this code even remotely, we will see later
who should be asked.

> I still
> see some other wait_on_page_bit() hangs after an I/O error has occurred.
> However, the hangs that I still see are related to waiting on buffer head
> state changes and not on the PG_locked page flag.

I don't know if this is right or not... lets discuss this later.

Thanks!

Oleg.