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Subject: Re: [PATCH] synchronize_irq needs a barrier
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reply-To: benh@kernel.crashing.org
To: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, akpm@linux-foundation.org,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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Date: Fri, 19 Oct 2007 14:58:30 +1000
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On Fri, 2007-10-19 at 12:48 +0800, Herbert Xu wrote:

> [IRQ]: Fix synchronize_irq races with IRQ handler
> 
> As it is some callers of synchronize_irq rely on memory barriers
> to provide synchronisation against the IRQ handlers.  For example,
> the tg3 driver does
> 
> 	tp->irq_sync = 1;
> 	smp_mb();
> 	synchronize_irq();
> 
> and then in the IRQ handler:
> 
> 	if (!tp->irq_sync)
> 		netif_rx_schedule(dev, &tp->napi);
> 
> Unfortunately memory barriers only work well when they come in
> pairs.  Because we don't actually have memory barriers on the
> IRQ path, the memory barrier before the synchronize_irq() doesn't
> actually protect us.
> 
> In particular, synchronize_irq() may return followed by the
> result of netif_rx_schedule being made visible.
> 
> This patch (mostly written by Linus) fixes this by using spin
> locks instead of memory barries on the synchronize_irq() path.
> 
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Good for me.

Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

Cheers,
Ben.