linux-kernel - Re: Q: perf_install_in_context/perf_event

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Date:	Wed, 26 Jan 2011 20:33:03 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Alan Stern <stern@...land.harvard.edu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Q: perf_install_in_context/perf_event_enable are racy?

On Wed, 2011-01-26 at 20:05 +0100, Peter Zijlstra wrote:
> On Wed, 2011-01-26 at 19:49 +0100, Oleg Nesterov wrote:
> > On 01/26, Oleg Nesterov wrote:
> > >
> > > Please see the untested patch below. It doesn't change perf_event_enable(),
> > > only perf_install_in_context().
> > 
> > Forgot to mention... Also, it doesn't try to fix the race with do_exit(),
> > this needs another change.
> > 
> > And, damn, can't resist. This is mostly cosmetic issue, but I feel
> > discomfort every time I look at task_oncpu_function_call(). It _looks_
> > obviously wrong, even if the problem doesn't exist in practice. I'll
> > send the pedantic fix to keep the maintainers busy ;)
> 
> I've been trying to sit down and work my way through it today, your last
> suggestion very nearly seemed to make sense, but I kept getting
> distracted.
> 
> FWIW I think perf_event_enable() has the very same issue...

Wouldn't something like the below cure things too?


---
 kernel/sched.c |   23 ++++++++++++++++++++++-
 1 files changed, 22 insertions(+), 1 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 18d38e4..7eadbcf 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2265,6 +2265,22 @@ void kick_process(struct task_struct *p)
 EXPORT_SYMBOL_GPL(kick_process);
 #endif /* CONFIG_SMP */
 
+struct task_function_call {
+	struct task_struct *p;
+	void (*func)(void *info);
+	void *info;
+};
+
+void task_function_trampoline(void *data)
+{
+	struct task_function_call *tfc = data;
+
+	if (this_rq()->curr != tfc->p)
+		return;
+
+	tfc->func(tfc->data);
+}
+
 /**
  * task_oncpu_function_call - call a function on the cpu on which a task runs
  * @p:		the task to evaluate
@@ -2278,11 +2294,16 @@ void task_oncpu_function_call(struct task_struct *p,
 			      void (*func) (void *info), void *info)
 {
 	int cpu;
+	struct task_function_call data = {
+		.p = p,
+		.func = func,
+		.info = info,
+	};
 
 	preempt_disable();
 	cpu = task_cpu(p);
 	if (task_curr(p))
-		smp_call_function_single(cpu, func, info, 1);
+		smp_call_function_single(cpu, task_function_trampoline, &data, 1);
 	preempt_enable();
 }
 

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