linux-kernel - Re: [patch] drivers/zram: Don't disable preemption in zcomp_stream

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Date:   Mon, 17 Oct 2016 18:19:00 +0200
From:   Mike Galbraith <umgwanakikbuti@...il.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [patch] drivers/zram: Don't disable preemption in
 zcomp_stream_get/put()

On Mon, 2016-10-17 at 16:24 +0200, Sebastian Andrzej Siewior wrote:
> On 2016-10-16 05:14:22 [+0200], Mike Galbraith wrote:
> > 
> > In v4.7, the driver switched to percpu compression streams, disabling
> > preemption (get/put_cpu_ptr()).  Use get/put_cpu_light() instead.
> 
> I am not convinced that this will work. Nothing prevents
> zram_bvec_write() to be reentrant on the same CPU what I can tell from
> browsing over the code and since it uses zstrm->buffer for compression
> it can go wrong. Also I don't know if crypto's tfm handler can be used
> in parallel for any ops (it usually does not work for crypto).
> 
> I suggest a local lock or a good reason why the this patch works.

I used a local lock first, but lockdep was unhappy with it.  Ok, back
to the drawing board.  Seems to work, but...

	-Mike