Subject: Re: [RFC PATCH 08/15] smp: Don't warn if irq are disabled but we
 don't wait for the ipi
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>,
        Lai Jiangshan <laijs@cn.fujitsu.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Anton Blanchard <anton@au1.ibm.com>,
        Tim Pepper <lnxninja@linux.vnet.ibm.com>
In-Reply-To: <1292858662-5650-9-git-send-email-fweisbec@gmail.com>
References: <1292858662-5650-1-git-send-email-fweisbec@gmail.com>
	 <1292858662-5650-9-git-send-email-fweisbec@gmail.com>
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Date: Mon, 20 Dec 2010 17:03:59 +0100
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On Mon, 2010-12-20 at 16:24 +0100, Frederic Weisbecker wrote:
> The comment in smp_call_function_single() says it wants the irqs
> to be enabled otherwise it may deadlock.
> 
> I can't find the reason for that though, except if we had to wait
> for a self triggered IPI but we execute the local IPI by just
> calling the function in place.
> 
> In doubt, only suppress the warning if we are not waiting for the
> IPI to complete as it should really not raise any deadlock.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Anton Blanchard <anton@au1.ibm.com>
> Cc: Tim Pepper <lnxninja@linux.vnet.ibm.com>
> ---
>  kernel/smp.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index 12ed8b0..886a406 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -289,7 +289,7 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
>  	 * send smp call function interrupt to this cpu and as such deadlocks
>  	 * can't happen.
>  	 */
> -	WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()
> +	WARN_ON_ONCE(cpu_online(this_cpu) && wait && irqs_disabled()
>  		     && !oops_in_progress);
>  
>  	if (cpu == this_cpu) {

You just deadlocked the machine.. note how you can still wait on the
previous csd in csd_lock().
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