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perf: Robustify task_function_call()

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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: Peter 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: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2016-02-24 18:45:51 +01:00 committed by Ingo Molnar
parent a096309bc4
commit 0da4cf3e0a
1 changed files with 20 additions and 20 deletions
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40
kernel/events/core.c
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@ -64,8 +64,17 @@ static void remote_function(void *data)
struct task_struct *p = tfc->p;
if (p) {
tfc->ret = -EAGAIN;
if (task_cpu(p) != smp_processor_id() || !task_curr(p))
/* -EAGAIN */
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;
}
@ -92,13 +101,17 @@ task_function_call(struct task_struct *p, remote_function_f func, void *info)
.p = p,
.func = func,
.info = info,
.ret = -ESRCH, /* No such (running) process */
.ret = -EAGAIN,
};
int ret;
if (task_curr(p))
smp_call_function_single(task_cpu(p), remote_function, &data, 1);
do {
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().
* 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.
*/
@ -212,7 +212,7 @@ static int event_function(void *info)
*/
if (ctx->task) {
if (ctx->task != current) {
ret = -EAGAIN;
ret = -ESRCH;
goto unlock;
}