linux-kernel - Re: [PATCH] mm: slub: Ensure that slab

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Date:	Tue, 8 Mar 2016 14:40:52 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Vineet Gupta <vgupta@...opsys.com>
cc:	linux-mm@...ck.org, Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Noam Camus <noamc@...hip.com>, stable@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-snps-arc@...ts.infradead.org
Subject: Re: [PATCH] mm: slub: Ensure that slab_unlock() is atomic

On Tue, 8 Mar 2016, Vineet Gupta wrote:

> # set the bit
> 80543b8e:	ld_s       r2,[r13,0] <--- (A) Finds PG_locked is set
> 80543b90:	or         r3,r2,1    <--- (B) other core unlocks right here
> 80543b94:	st_s       r3,[r13,0] <--- (C) sets PG_locked (overwrites unlock)

Duh. Guess you  need to take the spinlock also in the arch specific
implementation of __bit_spin_unlock(). This is certainly not the only case
in which we use the __ op to unlock.

You need a true atomic op or you need to take the "spinlock" in all
cases where you modify the bit. If you take the lock in __bit_spin_unlock
then the race cannot happen.

> Are you convinced now !

Yes, please fix your arch specific code.