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Message-ID: <56DFC604.6070407@synopsys.com>
Date: Wed, 9 Mar 2016 12:13:16 +0530
From: Vineet Gupta <Vineet.Gupta1@...opsys.com>
To: Christoph Lameter <cl@...ux.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>, <linux-parisc@...r.kernel>,
"Peter Zijlstra" <peterz@...radead.org>,
"James E.J. Bottomley" <jejb@...isc-linux.org>,
Helge Deller <deller@....de>,
"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [PATCH] mm: slub: Ensure that slab_unlock() is atomic
+CC linux-arch, parisc folks, PeterZ
On Wednesday 09 March 2016 02:10 AM, Christoph Lameter wrote:
> 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.
__bit_spin_lock() by definition is *not* required to be atomic, bit_spin_lock() is
- so I don't think we need a spinlock there.
There is clearly a problem in slub code that it is pairing a test_and_set_bit()
with a __clear_bit(). Latter can obviously clobber former if they are not a single
instruction each unlike x86 or they use llock/scond kind of instructions where the
interim store from other core is detected and causes a retry of whole llock/scond
sequence.
BTW ARC is not the only arch which suffers from this - other arches potentially
also are. AFAIK PARISC also doesn't have atomic r-m-w and also uses a set of
external hashed spinlocks to protect the r-m-w sequences.
https://lkml.org/lkml/2014/6/1/178
So there also we have the same race because the outer spin lock is not taken for
slab_unlock() -> __bit_spin_lock() -> __clear_bit.
Arguably I can fix the ARC !LLSC variant of test_and_set_bit() to not set the bit
unconditionally but only if it was clear (PARISC does the same). That would be a
slight micro-optimization as we won't need another snoop transaction to make line
writable and that would also elide this problem, but I think there is a
fundamental problem here in slub which is pairing atomic and non atomic ops - for
performance reasons. It doesn't work on all arches and/or configurations.
> You need a true atomic op or you need to take the "spinlock" in all
> cases where you modify the bit.
No we don't in __bit_spin_lock and we already do in bit_spin_lock.
> If you take the lock in __bit_spin_unlock
> then the race cannot happen.
Of course it won't but that means we penalize all non atomic callers of the API
with a superfluous spinlock which is not require din first place given the
definition of API.
>> Are you convinced now !
>
> Yes, please fix your arch specific code.
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