linux-kernel - Re: [PATCH] synchronize

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Date:	Fri, 19 Oct 2007 14:26:54 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	akpm@...ux-foundation.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linuxppc-dev@...abs.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] synchronize_irq needs a barrier


> The whole lock/set IRQ_INPROGRESS/unlock path can then only happen
> before the locked section above, in which case we see and wait nicely
> and all is good, or after, in which case the store to foo will be
> visible to the IRQ handler as it will be ordered with the unlock in the
> code above.

Note that napi_synchronize needs a slightly different treatement.

Here, the situation boils down to:

one CPU does:

	foo = 1;
	while(test_bit(bar))
		barrier();

and the other:

	if (!test_and_set_bit(bar)) {
		read & use foo
		smp_mb__before_clear_bit();
		clear_bit(bar);
	}

The good thing here is that read & use foo is part of the critical
section (I hate hand-made locks ...) defined by bar which makes
things somewhat easier than the synchronize_irq() case.

I think a simple smp_mb(); here after foo = 1; is enough, which means
basically just having an smp_mp(); inside napi_synchronize(), before
the test_bit(). Or do I miss something ?

Cheers,
Ben.


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