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Commit 3a80b4a3 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

perf_counter: Fix a race on perf_counter_ctx 



While extending perfcounters with BTS hw-tracing, Markus
Metzger managed to trigger this warning:

   [  995.557128] WARNING: at kernel/perf_counter.c:1191 __perf_counter_task_sched_out+0x48/0x6b()

triggers because commit
9f498cc5 (perf_counter: Full
task tracing) removed clearing of tsk->perf_counter_ctxp out
from under ctx->lock which introduced a race (against
perf_lock_task_context).

Move it back and deal with the exit notification by explicitly
passing along the former task context.

Reported-by: default avatarMarkus T Metzger <markus.t.metzger@intel.com>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1249667341.17467.5.camel@twins>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 3a43ce68

kernel/perf_counter.c

+15 −15
Original line number Diff line number Diff line
@@ -2851,6 +2851,7 @@ perf_counter_read_event(struct perf_counter *counter, 


struct perf_task_event {

	struct task_struct		*task;

	struct perf_counter_context	*task_ctx;



	struct {

		struct perf_event_header	header;
@@ -2910,24 +2911,23 @@ static void perf_counter_task_ctx(struct perf_counter_context *ctx, 
static void perf_counter_task_event(struct perf_task_event *task_event)

{

	struct perf_cpu_context *cpuctx;

	struct perf_counter_context *ctx;

	struct perf_counter_context *ctx = task_event->task_ctx;



	cpuctx = &get_cpu_var(perf_cpu_context);

	perf_counter_task_ctx(&cpuctx->ctx, task_event);

	put_cpu_var(perf_cpu_context);



	rcu_read_lock();

	/*

	 * doesn't really matter which of the child contexts the

	 * events ends up in.

	 */

	ctx = rcu_dereference(current->perf_counter_ctxp);

	if (!ctx)

		ctx = rcu_dereference(task_event->task->perf_counter_ctxp);

	if (ctx)

		perf_counter_task_ctx(ctx, task_event);

	rcu_read_unlock();

}



static void perf_counter_task(struct task_struct *task, int new)

static void perf_counter_task(struct task_struct *task,

			      struct perf_counter_context *task_ctx,

			      int new)

{

	struct perf_task_event task_event;
@@ -2938,6 +2938,7 @@ static void perf_counter_task(struct task_struct *task, int new) 


	task_event = (struct perf_task_event){

		.task	  = task,

		.task_ctx = task_ctx,

		.event    = {

			.header = {

				.type = new ? PERF_EVENT_FORK : PERF_EVENT_EXIT,
@@ -2956,7 +2957,7 @@ static void perf_counter_task(struct task_struct *task, int new) 


void perf_counter_fork(struct task_struct *task)

{

	perf_counter_task(task, 1);

	perf_counter_task(task, NULL, 1);

}



/*
@@ -4310,7 +4311,7 @@ void perf_counter_exit_task(struct task_struct *child) 
	unsigned long flags;



	if (likely(!child->perf_counter_ctxp)) {

		perf_counter_task(child, 0);

		perf_counter_task(child, NULL, 0);

		return;

	}
@@ -4330,6 +4331,7 @@ void perf_counter_exit_task(struct task_struct *child) 
	 * incremented the context's refcount before we do put_ctx below.

	 */

	spin_lock(&child_ctx->lock);

	child->perf_counter_ctxp = NULL;

	/*

	 * If this context is a clone; unclone it so it can't get

	 * swapped to another process while we're removing all
@@ -4343,9 +4345,7 @@ void perf_counter_exit_task(struct task_struct *child) 
	 * won't get any samples after PERF_EVENT_EXIT. We can however still

	 * get a few PERF_EVENT_READ events.

	 */

	perf_counter_task(child, 0);



	child->perf_counter_ctxp = NULL;

	perf_counter_task(child, child_ctx, 0);



	/*

	 * We can recurse on the same lock type through: