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

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Message-ID: <20120718115700.GB6233@somewhere.redhat.com>
Date:	Wed, 18 Jul 2012 13:57:03 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.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 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?

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