linux-kernel - Re: [PATCH v2 RESEND] Hardware breakpoints: Invoke __perf_event_disable() if interrupts are already disabled

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Date:	Thu, 19 Jul 2012 16:46:28 +0530
From:	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	paulus@...ba.org, a.p.zijlstra@...llo.nl,
	linux-kernel@...r.kernel.org, mingo@...hat.com,
	emachado@...ux.vnet.ibm.com, acme@...stprotocols.net,
	prasad.krishnan@...il.com
Subject: Re: [PATCH v2 RESEND] Hardware breakpoints: Invoke __perf_event_disable()
 if interrupts are already disabled

On 07/18/2012 05:27 PM, Frederic Weisbecker wrote:
> On Wed, Jul 18, 2012 at 04:00:46PM +0530, Naveen N. Rao wrote:
>> Please find v2 of the patch from Prasad, based on Peter Zijlstra's
>> feedback. This applies on top of v3.5-rc7. This has been tested and
>> found to work fine by Edjunior.
>>
>> Regards,
>> Naveen
>> ______
>>
>> From: K.Prasad <Prasad.Krishnan@...il.com>
>>
>> While debugging a warning message on PowerPC while using hardware
>> breakpoints, it was discovered that when perf_event_disable is invoked
>> through hw_breakpoint_handler function with interrupts disabled, a
>> subsequent IPI in the code path would trigger a WARN_ON_ONCE message in
>> smp_call_function_single function.
>>
>> This patch calls __perf_event_disable() when interrupts are already
>> disabled, instead of perf_event_disable().
>>
>> Reported-by: Edjunior Barbosa Machado <emachado@...ux.vnet.ibm.com>
>> Signed-off-by: K.Prasad <Prasad.Krishnan@...il.com>
>> Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
>> ---
>>   include/linux/perf_event.h    |    2 ++
>>   kernel/events/core.c          |    2 +-
>>   kernel/events/hw_breakpoint.c |   10 +++++++++-
>>   3 files changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index 45db49f..c289ba0 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -1292,6 +1292,7 @@ extern int perf_swevent_get_recursion_context(void);
>>   extern void perf_swevent_put_recursion_context(int rctx);
>>   extern void perf_event_enable(struct perf_event *event);
>>   extern void perf_event_disable(struct perf_event *event);
>> +extern int __perf_event_disable(void *info);
>>   extern void perf_event_task_tick(void);
>>   #else
>>   static inline void
>> @@ -1330,6 +1331,7 @@ static inline int  perf_swevent_get_recursion_context(void)		{ return -1; }
>>   static inline void perf_swevent_put_recursion_context(int rctx)		{ }
>>   static inline void perf_event_enable(struct perf_event *event)		{ }
>>   static inline void perf_event_disable(struct perf_event *event)		{ }
>> +static inline int __perf_event_disable(void *info)			{ }
>>   static inline void perf_event_task_tick(void)				{ }
>>   #endif
>>
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index d7d71d6..0ad0fc9 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -1253,7 +1253,7 @@ retry:
>>   /*
>>    * Cross CPU call to disable a performance event
>>    */
>> -static int __perf_event_disable(void *info)
>> +int __perf_event_disable(void *info)
>>   {
>>   	struct perf_event *event = info;
>>   	struct perf_event_context *ctx = event->ctx;
>> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
>> index bb38c4d..483f14a 100644
>> --- a/kernel/events/hw_breakpoint.c
>> +++ b/kernel/events/hw_breakpoint.c
>> @@ -453,7 +453,15 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
>>   	int old_type = bp->attr.bp_type;
>>   	int err = 0;
>>
>> -	perf_event_disable(bp);
>> +	/*
>> +	 * modify_user_hw_breakpoint can be invoked with IRQs disabled and hence it
>> +	 * will not be possible to raise IPIs that invoke __perf_event_disable.
>> +	 * So call the function directly.
>> +	 */
>> +	if (irqs_disabled())
>> +		__perf_event_disable(bp);
>> +	else
>> +		perf_event_disable(bp);
>
> This only works if we are sure the bp is on the current CPU. Do we
> have that guarantee?

Yes. This is being hit during bp exception processing and is specific to 
ppc where we disable interrupts: 
hw_breakpoint_handler->perf_bp_event->ptrace_triggered->modify_user_hw_breakpoint()


Regards,
Naveen

>
>>
>>   	bp->attr.bp_addr = attr->bp_addr;
>>   	bp->attr.bp_type = attr->bp_type;
>>
>

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