linux-kernel - Re: [PATCH 1/7] zram: fix race between reset and flushing pending work

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Message-ID: <20140114001555.GV1992@bbox>
Date:	Tue, 14 Jan 2014 09:15:55 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, Nitin Gupta <ngupta@...are.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH 1/7] zram: fix race between reset and flushing pending
 work

On Mon, Jan 13, 2014 at 03:55:27PM -0800, Andrew Morton wrote:
> On Mon, 13 Jan 2014 20:18:56 +0900 Minchan Kim <minchan@...nel.org> wrote:
> 
> > Dan and Sergey reported that there is a racy between reset and
> > flushing of pending work so that it could make oops by freeing
> > zram->meta in reset while zram_slot_free can access zram->meta
> > if new request is adding during the race window.
> > 
> > This patch moves flush after taking init_lock so it prevents
> > new request so that it closes the race.
> > 
> > ..
> >
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -553,14 +553,14 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
> >  	size_t index;
> >  	struct zram_meta *meta;
> >  
> > -	flush_work(&zram->free_work);
> > -
> >  	down_write(&zram->init_lock);
> >  	if (!zram->init_done) {
> >  		up_write(&zram->init_lock);
> >  		return;
> >  	}
> >  
> > +	flush_work(&zram->free_work);
> > +
> >  	meta = zram->meta;
> >  	zram->init_done = 0;
> 
> This makes zram.lock nest inside zram.init_lock, which afaict is new
> behaviour.

Originally, it was nested so it's not new. :)
Look at zram_make_request which hold init_lock and then zram_bvec_rw
hold zram->lock.

> 
> That all seems OK and logical - has it been well tested with lockdep?

Yeb.

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-- 
Kind regards,
Minchan Kim
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