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Commit 0da4cf3e authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

perf: Robustify task_function_call() 



Since there is no serialization between task_function_call() doing
task_curr() and the other CPU doing context switches, we could end
up not sending an IPI even if we had to.

And I'm not sure I still buy my own argument we're OK.

Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dvyukov@google.com
Cc: eranian@google.com
Cc: oleg@redhat.com
Cc: panand@redhat.com
Cc: sasha.levin@oracle.com
Cc: vince@deater.net
Link: http://lkml.kernel.org/r/20160224174948.340031200@infradead.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent a096309b

kernel/events/core.c

+20 −20
Original line number Original line Diff line number Diff line
@@ -64,8 +64,17 @@ static void remote_function(void *data) 
	struct task_struct *p = tfc->p;

	struct task_struct *p = tfc->p;





	if (p) {

	if (p) {

		tfc->ret = -EAGAIN;

		/* -EAGAIN */

		if (task_cpu(p) != smp_processor_id() || !task_curr(p))

		if (task_cpu(p) != smp_processor_id())

			return;



		/*

		 * Now that we're on right CPU with IRQs disabled, we can test

		 * if we hit the right task without races.

		 */



		tfc->ret = -ESRCH; /* No such (running) process */

		if (p != current)

			return;

			return;

	}

	}
@@ -92,13 +101,17 @@ task_function_call(struct task_struct *p, remote_function_f func, void *info) 
		.p	= p,

		.p	= p,

		.func	= func,

		.func	= func,

		.info	= info,

		.info	= info,

		.ret	= -ESRCH, /* No such (running) process */

		.ret	= -EAGAIN,

	};

	};

	int ret;





	if (task_curr(p))

	do {

		smp_call_function_single(task_cpu(p), remote_function, &data, 1);

		ret = smp_call_function_single(task_cpu(p), remote_function, &data, 1);

		if (!ret)

			ret = data.ret;

	} while (ret == -EAGAIN);





	return data.ret;

	return ret;

}

}





/**

/**
@@ -169,19 +182,6 @@ static bool is_kernel_event(struct perf_event *event) 
 *    rely on ctx->is_active and therefore cannot use event_function_call().

 *    rely on ctx->is_active and therefore cannot use event_function_call().

 *    See perf_install_in_context().

 *    See perf_install_in_context().

 *

 *

 * This is because we need a ctx->lock serialized variable (ctx->is_active)

 * to reliably determine if a particular task/context is scheduled in. The

 * task_curr() use in task_function_call() is racy in that a remote context

 * switch is not a single atomic operation.

 *

 * As is, the situation is 'safe' because we set rq->curr before we do the

 * actual context switch. This means that task_curr() will fail early, but

 * we'll continue spinning on ctx->is_active until we've passed

 * perf_event_task_sched_out().

 *

 * Without this ctx->lock serialized variable we could have race where we find

 * the task (and hence the context) would not be active while in fact they are.

 *

 * If ctx->nr_events, then ctx->is_active and cpuctx->task_ctx are set.

 * If ctx->nr_events, then ctx->is_active and cpuctx->task_ctx are set.

 */

 */
@@ -212,7 +212,7 @@ static int event_function(void *info) 
	 */

	 */

	if (ctx->task) {

	if (ctx->task) {

		if (ctx->task != current) {

		if (ctx->task != current) {

			ret = -EAGAIN;

			ret = -ESRCH;

			goto unlock;

			goto unlock;

		}

		}