linux-kernel - Re: [PATCH -mm] slub: update cpu after new

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Message-ID: <Pine.LNX.4.64.0704251642450.22530@blonde.wat.veritas.com>
Date:	Wed, 25 Apr 2007 16:49:36 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Christoph Lameter <clameter@....com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm] slub: update cpu after new_slab()

On Wed, 25 Apr 2007, Christoph Lameter wrote:
> On Wed, 25 Apr 2007, Hugh Dickins wrote:
> 
> > SLUB gave me a NULL pointer dereference in slab_alloc(), in the
> > slab_lock(page) of its "Current cpuslab is acceptable" block: cpu
> > 1 had been looking at cpu_slab[2], which then went NULL beneath it.
> > Since new_slab() may reenable interrupts and sleep (when __GFP_WAIT),
> > we may get rescheduled on a different cpu: so need to reevaluate it.
> 
> Right. local_irq_save does not switch off preemption as I thought.

Strange comment.  Preemption is not possible while IRQs are disabled,
but new_slab() rightly reenables them within itself in the __GFP_WAIT
case, since it's going off to do a page allocation and may need to wait.

(And actually, this kernel was a CONFIG_PREEMPT_NONE=y one.)

> 
> Acked-by: Christoph Lameter <clameter@....com>

Thanks,
Hugh
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