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

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Message-ID: <20120731134124.GD17078@somewhere.redhat.com>
Date:	Tue, 31 Jul 2012 15:41:28 +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 25, 2012 at 05:02:56PM +0530, Naveen N. Rao wrote:
> >>>@@ -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.

Yeah, please make sure that the targeted task is "current".
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