[IRQ]: Fix synchronize_irq races with IRQ handler

From: Herbert Xu
Date: Mon Oct 22 2007 - 23:27:23 EST


On Mon, Oct 22, 2007 at 07:10:05AM +1000, Benjamin Herrenschmidt wrote:
>
> Hrm... not on yet. Herbert, care to resend, looks like it fell down the
> wrong hole in Linus mailbox :-)

Thanks for the reminder Ben. Here it is again:

[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@xxxxxxxxxxxxxxxxxxx>
Acked-by: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>

Cheers,
--
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
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--
diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index 80eab7a..1f31422 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -29,12 +29,28 @@
void synchronize_irq(unsigned int irq)
{
struct irq_desc *desc = irq_desc + irq;
+ unsigned int status;

if (irq >= NR_IRQS)
return;

- while (desc->status & IRQ_INPROGRESS)
- cpu_relax();
+ do {
+ unsigned long flags;
+
+ /*
+ * Wait until we're out of the critical section. This might
+ * give the wrong answer due to the lack of memory barriers.
+ */
+ while (desc->status & IRQ_INPROGRESS)
+ cpu_relax();
+
+ /* Ok, that indicated we're done: double-check carefully. */
+ spin_lock_irqsave(&desc->lock, flags);
+ status = desc->status;
+ spin_unlock_irqrestore(&desc->lock, flags);
+
+ /* Oops, that failed? */
+ } while (status & IRQ_INPROGRESS);
}
EXPORT_SYMBOL(synchronize_irq);

-
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