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

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Message-ID: <Pine.LNX.4.64.0704251703360.24086@blonde.wat.veritas.com>
Date:	Wed, 25 Apr 2007 17:13:25 +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:
> 
> > > 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.
> 
> Yes I expected local_irq_save to increase the preempt count and then 
> local_irq_enable to simply enable interrupts without affecting the preempt 
> count. Thus the process should stay on the same processor.
> 
> Never thought it would be possible to move to a different processor in mid 
> flight.

But, surely you wouldn't have expected it to stay on the processor
throughout the waiting page allocation??  I think you're misremembering
your expectations, and this was just a simple, understandable, oversight.

Quite a serious one, though: it got caught in my case by the NULL
dereference, but it's probably been switching cpu there much more
often - one cpu diddling with what's private to another, with
unpredictable results.

Hugh
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