Subject: Re: [PATCH] synchronize_irq needs a barrier

From: Benjamin Herrenschmidt <[email-masked]>
Date: Fri, 19 Oct 2007 14:58:30 +1000

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 <[email-masked]>

Good for me.

Acked-by: Benjamin Herrenschmidt <[email-masked]>

Cheers,
Ben.