linux-kernel - Re: [PATCH] mm/slub: use unchecked percpu access within preemptible sections

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Message-ID: <54C0601D.5050006@oracle.com>
Date:	Wed, 21 Jan 2015 21:27:41 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
CC:	linux-kernel@...r.kernel.org, cl@...ux.com, brouer@...hat.com,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] mm/slub: use unchecked percpu access within preemptible
 sections

On 01/21/2015 08:59 PM, Joonsoo Kim wrote:
> On Tue, Jan 20, 2015 at 11:31:43PM -0500, Sasha Levin wrote:
>> > Commit "mm/slub: optimize alloc/free fastpath by removing preemption on/off"
>> > has added access to percpu memory while the code is preemptible.
>> > 
>> > While those accesses are okay, this creates a huge amount of warnings from
>> > the code that checks for that.
>> > 
>> > Signed-off-by: Sasha Levin <sasha.levin@...cle.com>
> Hello,
> 
> I already sent the patch to fix this issue and it is in mmotm, but,
> not be released yet.
> 
> https://lkml.org/lkml/2015/1/19/17

The patch you sent out still has the issue. The one I sent goes on top of
it.

+	do {
+		tid = this_cpu_read(s->cpu_slab->tid); <=== checked percpu access
+		c = raw_cpu_ptr(s->cpu_slab);
+	} while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid));


Thanks,
Sasha

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