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

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Date:	Mon, 9 Sep 2013 18:09:42 +0300
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	devel@...verdev.osuosl.org, Minchan Kim <minchan@...nel.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] staging: zram: minimize `slot_free_lock' usage (v2)

On (09/09/13 17:52), Dan Carpenter wrote:
> On Mon, Sep 09, 2013 at 05:42:59PM +0300, Sergey Senozhatsky wrote:
> > > 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.
> > 
> > atomic pointer test, which is either NULL or !NULL.
> > 
> 
> That's not how concurency works.  Atomic types are complicated than
> that.  Anyway, the zram maintainers don't need me to explain that to
> them so I'll let them take over from here.
> 

yes, I understand that. but can't we check slot_free_rq pointer
(32 or 64 bit read) w/o locking to just decide if we must:
-- call handle_pending_slot_free()
-- take the slot_free_lock
-- check slot_free_rq again (this time under the slot_free_lock) and perform
   slot_free_rq operations while it is !NULL.

	-ss

> regards,
> dan carpenter
> 
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