Re: [PATCH] smp_call_function WARN_ON

Re: [PATCH] smp_call_function WARN_ON

[Manfred Spraul]

Zwane wrote:

>The WARN_ON() is really only true if you're waiting for the other
>processors.
>
>  
>
Wrong: At least on i386 the code always waits for delivery of the IPI, 
it just doesn't wait for completion of the callback.
Do you need IPIs from irq or bh context? It would be tricky to change 
the current code: an IPI just delivers an interrupt without any payload. 
The global 'call_data' variable contains the description of the IPI and 
accesses to it must be synchronized.

IPIs from BH context do no work either: they can deadlock against the 
tasklist_lock: the first cpu calls read_lock(&tasklist_lock) from 
process context, another cpu calls write_lock_irq(&tasklist_lock), and 
then the first cpu tries smp_call_function() from bh context.

--
    Manfred

Re: [PATCH] smp_call_function WARN_ON

[Zwane Mwaikambo]

On Sun, 15 Aug 2004, Manfred Spraul wrote:

> Wrong: At least on i386 the code always waits for delivery of the IPI,
> it just doesn't wait for completion of the callback.
> Do you need IPIs from irq or bh context? It would be tricky to change
> the current code: an IPI just delivers an interrupt without any payload.
> The global 'call_data' variable contains the description of the IPI and
> accesses to it must be synchronized.

Oops yes you're right, i'd have to setup a seperate vector to be able to
do it without the wait entirely. Sorry about the noise.

Thanks,
	Zwane

[PATCH] smp_call_function WARN_ON

[Zwane Mwaikambo]

The WARN_ON() is really only true if you're waiting for the other
processors.

Signed-off-by: Zwane Mwaikambo <zwane@fsmlabs.com>

Index: linux-2.6.8-rc4-mm1/arch/i386/kernel/smp.c
===================================================================
RCS file: /home/cvsroot/linux-2.6.8-rc4-mm1/arch/i386/kernel/smp.c,v
retrieving revision 1.1.1.1
diff -u -p -B -r1.1.1.1 smp.c
--- linux-2.6.8-rc4-mm1/arch/i386/kernel/smp.c	10 Aug 2004 14:49:26 -0000	1.1.1.1
+++ linux-2.6.8-rc4-mm1/arch/i386/kernel/smp.c	13 Aug 2004 18:46:11 -0000
@@ -538,7 +538,7 @@ int smp_call_function (void (*func) (voi
 	}

 	/* Can deadlock when called with interrupts disabled */
-	WARN_ON(irqs_disabled());
+	WARN_ON(wait && irqs_disabled());

 	data.func = func;
 	data.info = info;
Index: linux-2.6.8-rc4-mm1/arch/x86_64/kernel/smp.c
===================================================================
RCS file: /home/cvsroot/linux-2.6.8-rc4-mm1/arch/x86_64/kernel/smp.c,v
retrieving revision 1.1.1.1
diff -u -p -B -r1.1.1.1 smp.c
--- linux-2.6.8-rc4-mm1/arch/x86_64/kernel/smp.c	10 Aug 2004 14:49:30 -0000	1.1.1.1
+++ linux-2.6.8-rc4-mm1/arch/x86_64/kernel/smp.c	13 Aug 2004 18:45:54 -0000
@@ -417,7 +417,7 @@ int smp_call_function (void (*func) (voi
 		return 0;

 	/* Can deadlock when called with interrupts disabled */
-	WARN_ON(irqs_disabled());
+	WARN_ON(wait && irqs_disabled());

 	data.func = func;
 	data.info = info;