Re: [PATCH] synchronize_irq needs a barrier

Nick Piggin [off-list ref] wrote:

quoted

First of all let's agree on some basic assumptions:

* A pair of spin lock/unlock subsumes the effect of a full mb.

Not unless you mean a pair of spin lock/unlock as in
2 spin lock/unlock pairs (4 operations).

*X = 10;
spin_lock(&lock);
/* *Y speculatively loaded here */
/* store to *X leaves CPU store queue here */
spin_unlock(&lock);
y = *Y;

Good point.

Although in this case we're still safe because in the worst
cases:

CPU0				CPU1
irq_sync = 1
synchronize_irq
	spin lock
	load IRQ_INPROGRESS
irq_sync sync is visible
	spin unlock
				spin lock
					load irq_sync
	while (IRQ_INPROGRESS)
		wait
	return
				set IRQ_INPROGRESS
				spin unlock
				tg3_msi
					ack IRQ
					if (irq_sync)
						return
				spin lock
				clear IRQ_INPROGRESS
				spin unlock

------------------------------------------------------------

CPU0				CPU1
				spin lock
					load irq_sync
irq_sync = 1
synchronize_irq
				set IRQ_INPROGRESS
				spin unlock
	spin lock
	load IRQ_INPROGRESS
irq_sync sync is visible
	spin unlock
	while (IRQ_INPROGRESS)
		wait
				tg3_msi
					ack IRQ
					if (irq_sync)
						return
					do work
				spin lock
				clear IRQ_INPROGRESS
				spin unlock
	return

So because we're using the same lock on both sides, it does
do the right thing even without the memory barrier.

Cheers,
-- 
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Email: Herbert Xu ~{PmV>HI~} [off-list ref]
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