linux-kernel - Re: [PATCH] synchronize

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Message-Id: <E1IiiX6-0002V0-00@gondolin.me.apana.org.au>
Date:	Fri, 19 Oct 2007 11:28:20 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	nickpiggin@...oo.com.au (Nick Piggin)
Cc:	herbert@...dor.apana.org.au, torvalds@...ux-foundation.org,
	benh@...nel.crashing.org, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	mingo@...e.hu, tglx@...utronix.de
Subject: Re: [PATCH] synchronize_irq needs a barrier

Nick Piggin <nickpiggin@...oo.com.au> wrote:
> 
>> 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,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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