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Re: [PATCH] synchronize_irq needs a barrier

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From: Linus Torvalds <torvalds@...>
To: Benjamin Herrenschmidt <benh@...>
Cc: Herbert Xu <herbert@...>, <akpm@...>, <linux-kernel@...>, <linuxppc-dev@...>
Subject: Re: [PATCH] synchronize_irq needs a barrier
Date: Thursday, October 18, 2007 - 6:52 pm

On Fri, 19 Oct 2007, Benjamin Herrenschmidt wrote:

No, it doesn't really guarantee that.

The thing is, there is no such thing as "globally visible".

There is a "ordering of visibility wrt CPU's", but it's not global, it's 
quite potentially per-CPU. So a barrier on one CPU doesn't guarantee 
anything at all without a barrier on the *other* CPU.

That said, the interrupt handling itself contains various barriers on the 
CPU's that receive interrupts, thanks to the spinlocking. But I do agree 
with Herbert that adding a "smb_mb()" is certainly in no way "obviously 
correct", because it doesn't talk about what the other side does wrt 
barriers and that word in memory.

		Linus
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Messages in current thread:
[PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Wed Oct 17, 9:25 pm)
Re: [PATCH] synchronize_irq needs a barrier, Maxim Levitsky, (Fri Oct 19, 10:02 pm)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Fri Oct 19, 11:56 pm)
Re: [PATCH] synchronize_irq needs a barrier, Maxim Levitsky, (Sat Oct 20, 12:24 am)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Sat Oct 20, 1:04 am)
Re: [PATCH] synchronize_irq needs a barrier, Maxim Levitsky, (Sat Oct 20, 1:36 am)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Sat Oct 20, 1:46 am)
Re: [PATCH] synchronize_irq needs a barrier, Maxim Levitsky, (Sat Oct 20, 2:06 am)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Sat Oct 20, 2:13 am)
Re: [PATCH] synchronize_irq needs a barrier, Herbert Xu, (Fri Oct 19, 11:37 pm)
Re: [PATCH] synchronize_irq needs a barrier, Linus Torvalds, (Fri Oct 19, 10:25 pm)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Sat Oct 20, 12:04 am)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Sat Oct 20, 12:09 am)
Re: [PATCH] synchronize_irq needs a barrier, Maxim Levitsky, (Fri Oct 19, 11:10 pm)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Sat Oct 20, 12:06 am)
Re: [PATCH] synchronize_irq needs a barrier, Linus Torvalds, (Wed Oct 17, 10:12 pm)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Wed Oct 17, 10:40 pm)
Re: [PATCH] synchronize_irq needs a barrier, Herbert Xu, (Thu Oct 18, 10:35 am)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Thu Oct 18, 5:35 pm)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Wed Oct 17, 10:57 pm)
Re: [PATCH] synchronize_irq needs a barrier, Herbert Xu, (Thu Oct 18, 10:56 am)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Thu Oct 18, 6:05 pm)
Re: [PATCH] synchronize_irq needs a barrier, Linus Torvalds, (Thu Oct 18, 6:52 pm)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Thu Oct 18, 7:17 pm)
Re: [PATCH] synchronize_irq needs a barrier, Linus Torvalds, (Thu Oct 18, 7:39 pm)
Re: [PATCH] synchronize_irq needs a barrier, Herbert Xu, (Thu Oct 18, 10:32 pm)
Re: [PATCH] synchronize_irq needs a barrier, Linus Torvalds, (Thu Oct 18, 10:55 pm)
Re: [PATCH] synchronize_irq needs a barrier, Linus Torvalds, (Thu Oct 18, 11:26 pm)
Re: [PATCH] synchronize_irq needs a barrier, Herbert Xu, (Fri Oct 19, 12:20 am)
[NET]: Fix possible dev_deactivate race condition, Herbert Xu, (Fri Oct 19, 1:36 am)
Re: [NET]: Fix possible dev_deactivate race condition, Peter Zijlstra, (Fri Oct 19, 3:35 am)
Re: [NET]: Fix possible dev_deactivate race condition, Herbert Xu, (Fri Oct 19, 5:29 am)
Re: [NET]: Fix possible dev_deactivate race condition, David Miller, (Fri Oct 19, 1:38 am)
Re: [PATCH] synchronize_irq needs a barrier, Herbert Xu, (Fri Oct 19, 12:48 am)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Fri Oct 19, 12:58 am)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Sun Oct 21, 5:10 pm)
[IRQ]: Fix synchronize_irq races with IRQ handler, Herbert Xu, (Mon Oct 22, 11:26 pm)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Fri Oct 19, 12:35 am)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Fri Oct 19, 12:29 am)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Fri Oct 19, 12:11 am)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Fri Oct 19, 12:26 am)
Re: [PATCH] synchronize_irq needs a barrier, Herbert Xu, (Fri Oct 19, 1:53 am)
Re: [PATCH] synchronize_irq needs a barrier, Nick Piggin, (Thu Oct 18, 10:52 pm)
Re: [PATCH] synchronize_irq needs a barrier, Herbert Xu, (Thu Oct 18, 11:28 pm)
Re: [PATCH] synchronize_irq needs a barrier, Nick Piggin, (Fri Oct 19, 12:49 am)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Thu Oct 18, 7:52 pm)
Re: [PATCH] synchronize_irq needs a barrier, Andrew Morton, (Wed Oct 17, 9:45 pm)
Re: [PATCH] synchronize_irq needs a barrier, Benjamin Herrenschmidt, (Wed Oct 17, 9:55 pm)