linux-kernel - Re: [PATCH 1/2] staging: zram: minimize `slot_free

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Message-ID: <20130910232725.GC2450@swordfish>
Date:	Wed, 11 Sep 2013 02:27:25 +0300
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	Jerome Marchand <jmarchan@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	driverdev-devel@...uxdriverproject.org,
	Minchan Kim <minchan@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] staging: zram: minimize `slot_free_lock' usage (v2)

On (09/10/13 17:34), Sergey Senozhatsky wrote:
[..]
> > 
> > Now I think we can drop the call to handle_pending_slot_free() in
> > zram_bvec_rw() altogether. As long as the write lock is held when
> > handle_pending_slot_free() is called, there is no race. It's no different
> > from any write request and the current code handles R/W concurrency
> > already.
> 
> Yes, I think that can work. 
> 
> To summarize, there should be 3 patches:
> 1) handle_pending_slot_free() in zram_bvec_rw() (as suggested by Jerome Marchand)
> 2) handle_pending_slot_free() race with reset (found by Dan Carpenter)
> 3) drop init_done and use init_done()
> 
> I'll prepare a patches later today.

I've sent two patches:
 staging: zram: fix handle_pending_slot_free() and zram_reset_device() race
 staging: zram: remove init_done from zram struct (v3)

Cc'd driverdev-devel@...uxdriverproject.org as suggested by Dan.

please discard any previous patches and sorry for the noise.

Thanks,
	-ss

> 
> > Jerome
> > 
> > > 
> > >>
> > >> 1) You haven't given us any performance numbers so it's not clear if the
> > >>    locking is even a problem.
> > >>
> > >> 2) The v2 patch introduces an obvious deadlock in zram_slot_free()
> > >>    because now we take the rw_lock twice.  Fix your testing to catch
> > >>    this kind of bug next time.
> > >>
> > >> 3) Explain why it is safe to test zram->slot_free_rq when we are not
> > >>    holding the lock.  I think it is unsafe.  I don't want to even think
> > >>    about it without the numbers.
> > >>
> > >> regards,
> > >> dan carpenter
> > >>
> > > 
> > > --
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> > 
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