linux-kernel - Re: [PATCH] bcache: prevent crash on changing writeback

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Message-ID: <53F5A648.6060904@canonical.com>
Date:	Thu, 21 Aug 2014 09:56:56 +0200
From:	Stefan Bader <stefan.bader@...onical.com>
To:	Slava Pestov <sp@...era.io>
CC:	linux-kernel@...r.kernel.org, linux-bcache@...r.kernel.org,
	Kent Overstreet <kmo@...erainc.com>
Subject: Re: [PATCH] bcache: prevent crash on changing writeback_running

On 19.08.2014 20:03, Slava Pestov wrote:
> Thanks, this is now in our development branch.

Thanks. :) This kind of brings up another question: how are stable patches
handled for bcache. Someone handling them centrally like for the network stack
or tagging for stable and if that missed, individual proposals?

I am asking because it looks like for all kernels between 3.13..3.16 (inclusive)
the set of

a664d0f05a2ec02c8f042db536d84d15d6e19e81
  bcache: fix crash on shutdown in passthrough mode
9e5c353510b26500bd6b8309823ac9ef2837b761
  bcache: fix uninterruptible sleep in writeback thread

plus the one I did would be useful. Right now I only tested with 3.13 with those
and at least avoid the lockdep warnings and crashes I saw before. I only see an
unexplainable load of 1 when idle but having one bcache device configured in a
VM (Xen or KVM). Since writeback should be fixed, maybe gc...

-Stefan
> 
> On Tue, Aug 19, 2014 at 6:01 AM, Stefan Bader
> <stefan.bader@...onical.com> wrote:
>> commit a664d0f05a2ec02c8f042db536d84d15d6e19e81
>>     bcache: fix crash on shutdown in passthrough mode
>>
>> added a safeguard in the shutdown case. At least while not being
>> attached it is also possible to trigger a kernel bug by writing into
>> writeback_running. This change  adds the same check before trying to
>> wake up the thread for that case.
>>
>> Signed-off-by: Stefan Bader <stefan.bader@...onical.com>
>> ---
>>  drivers/md/bcache/writeback.h | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/md/bcache/writeback.h b/drivers/md/bcache/writeback.h
>> index 0a9dab1..073a042 100644
>> --- a/drivers/md/bcache/writeback.h
>> +++ b/drivers/md/bcache/writeback.h
>> @@ -63,7 +63,8 @@ static inline bool should_writeback(struct cached_dev *dc, struct bio *bio,
>>
>>  static inline void bch_writeback_queue(struct cached_dev *dc)
>>  {
>> -       wake_up_process(dc->writeback_thread);
>> +       if (!IS_ERR_OR_NULL(dc->writeback_thread))
>> +               wake_up_process(dc->writeback_thread);
>>  }
>>
>>  static inline void bch_writeback_add(struct cached_dev *dc)
>> --
>> 1.9.1
>>
>> --
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