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

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Date:	Wed, 25 Jul 2012 17:02:56 +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 07/19/2012 04:46 PM, Naveen N. Rao wrote:
> 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()

Frederick,
Is this acceptable, or do you have other scenarios where this won't 
work? I can add a check to ensure we call __perf_event_disable only if 
the task is on the current CPU, but the above scenario is the only one 
where we're seeing this issue.

Thanks,
Naveen

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