linux-kernel - Re: [PATCH] prevent sparc64 from invoking irq handlers on offline CPUs

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Message-Id: <20080902.171630.193505044.davem@davemloft.net>
Date:	Tue, 02 Sep 2008 17:16:30 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, wli@...omorphy.com,
	sparclinux@...r.kernel.org, manfred@...orfullife.com,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] prevent sparc64 from invoking irq handlers on offline
 CPUs

From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Date: Sun, 31 Aug 2008 10:33:49 -0700

> Make sparc64 refrain from clearing a given to-be-offlined CPU's bit in the
> cpu_online_mask until it has processed pending irqs.  This change
> prevents other CPUs from being blindsided by an apparently offline CPU
> nevertheless changing globally visible state.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

I wonder what the 'call_lock' thing protects :-)

That lock is a cobweb from the sparc64 code before I switched it over
to use the generic smp_call_function() code in kernel/smp.c

So this lock doesn't protect anything any more.

kernel/smp.c has a call_function_lock, which isn't marked static
but isn't declared in any header file.

My instinct is that the intention is that I could use this lock
for the synchronization previously provided by sparc64's local
"call_lock", and it even seems the author of kernel/smp.c intended
this kind of usage.

Anyways, if this code is still using the worthless call_lock, it
isn't protecting against anything.

So I'd like to hold off on this patch until this locking issue is
resolved.
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