Re: [PATCH] synchronize_irq needs a barrier
- From: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
- Date: Thu, 18 Oct 2007 12:57:15 +1000
In general, I tend to think that for this function to make any sense
(that is, to synchronize anything at all), it needs a barrier or you are
just making a decision based on a totally random value of desc->status
since it can have been re-ordered, speculatively loaded, pre-fetched,
whatever'ed... :-).
Take a real life example:
drivers/message/fusion/mptbase.c
/* Disable interrupts! */
CHIPREG_WRITE32(&ioc->chip->IntMask, 0xFFFFFFFF);
ioc->active = 0;
synchronize_irq(pdev->irq);
And we aren't in a spinlock here.
That's just a random example grepped.... I think I see a few more. Then,
some drivers like tg3 actually do an smp_mb() before calling
synchronize_irq(). But then, some don't.
I think trying to have all drivers be correct here is asking for
trouble, we'd rather have synchronize_irq() be uber-safe. It's not like
it was used in hot path anyway.
Ben.
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