linux-kernel - [PATCH] perf/core: Fix the same task check in perf_event_set

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Date:   Mon, 11 Jul 2022 11:07:06 -0700
From:   kan.liang@...ux.intel.com
To:     peterz@...radead.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org
Cc:     alexander.shishkin@...ux.intel.com, ak@...ux.intel.com,
        adrian.hunter@...el.com, nadav.amit@...il.com,
        Kan Liang <kan.liang@...ux.intel.com>,
        Zhengjun Xing <zhengjun.xing@...ux.intel.com>
Subject: [PATCH] perf/core: Fix the same task check in perf_event_set_output

From: Kan Liang <kan.liang@...ux.intel.com>

With the --per-thread option, perf record errors out when sampling with
a hardware event and a software event as below.

 $ perf record -e cycles,dummy --per-thread ls
 failed to mmap with 22 (Invalid argument)

The same task is sampled with the two events. The IOC_OUTPUT is invoked
to share the mmap memory of the task between the events. In the
perf_event_set_output(), the event->ctx is used to check whether the
two events are attached to the same task. However, a hardware event and
a software event are from different task context. The check always
fails.

The task struct is stored in the event->hw.target for each per-thread
event. It can be used to determine whether two events are attached to
the same task.

The patch can also fix another issue reported months ago.
https://lore.kernel.org/all/92645262-D319-4068-9C44-2409EF44888E@gmail.com/
The event->ctx is not ready when the perf_event_set_output() is invoked
in the perf_event_open(), while the event->hw.target has been assigned
at the moment.

The problem should be a long time issue since commit c3f00c70276d
("perf: Separate find_get_context() from event initialization"). The
event->hw.target doesn't exist at that time. Here, the patch which
introduces the event->hw.target is used by the Fixes tag.

The problem should still exists between the broken patch and the
event->hw.target patch. This patch does not intend to fix that case.

Fixes: 50f16a8bf9d7 ("perf: Remove type specific target pointers")
Reviewed-by: Zhengjun Xing <zhengjun.xing@...ux.intel.com>
Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
---
 kernel/events/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index b4d62210c3e5..22df79d3f19d 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -12080,7 +12080,7 @@ perf_event_set_output(struct perf_event *event, struct perf_event *output_event)
 	/*
 	 * If its not a per-cpu rb, it must be the same task.
 	 */
-	if (output_event->cpu == -1 && output_event->ctx != event->ctx)
+	if (output_event->cpu == -1 && output_event->hw.target != event->hw.target)
 		goto out;

 	/*
-- 
2.35.1