linux-kernel - Re: [PATCH] mm/slub: fix lockups on PREEMPT && !SMP kernels

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Message-ID: <alpine.DEB.2.11.1503131126550.13899@gentwo.org>
Date:	Fri, 13 Mar 2015 11:29:23 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Mark Rutland <mark.rutland@....com>
cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	David Rientjes <rientjes@...gle.com>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Steve Capper <steve.capper@...aro.org>
Subject: Re: [PATCH] mm/slub: fix lockups on PREEMPT && !SMP kernels

On Fri, 13 Mar 2015, Mark Rutland wrote:

>  	 */
> -	do {
> -		tid = this_cpu_read(s->cpu_slab->tid);
> -		c = raw_cpu_ptr(s->cpu_slab);
> -	} while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid));
> +	c = raw_cpu_ptr(s->cpu_slab);
> +	tid = READ_ONCE(c->tid);
>

Ok that works for the !SMP case. What about SMP and PREEMPT now?

And yes code like this was deemed safe for years and the race condition is
very subtle and difficult to trigger (also given that PREEMPT is rarely
used these days).

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