[PATCH 1/2] perf: Allow a pmu to pin context

[Frederic Weisbecker <frederic@kernel.org>]

Some PMUs rely on the fact that struct perf_event::hw::target always
point to the actual event target during the whole event lifecycle, ie:
from pmu::init() to event::destroy().

Now perf event ctx swapping on task sched switch breaks that guarantee.

Solve that with providing a way for a PMU to pin the context of an event.

Reported-by: syzbot+370a6b0f11867bf13515@syzkaller.appspotmail.com
Cc: Borislav Petkov <bp@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 include/linux/perf_event.h | 2 ++
 kernel/events/core.c       | 6 ++++++
 2 files changed, 8 insertions(+)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 2ddae518dce6..ea700a193865 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -273,6 +273,8 @@ struct pmu {
 	/* number of address filters this PMU can do */
 	unsigned int			nr_addr_filters;
 
+	int				pin_ctx;
+
 	/*
 	 * Fully disable/enable this PMU, can be used to protect from the PMI
 	 * as well as for lazy/batch writing of the MSRs.
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 23efe6792abc..6b4bc6fc47aa 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1689,6 +1689,9 @@ list_add_event(struct perf_event *event, struct perf_event_context *ctx)
 		ctx->nr_stat++;
 
 	ctx->generation++;
+
+	if (event->pmu->pin_ctx)
+		ctx->pin_count++;
 }
 
 /*
@@ -1885,6 +1888,9 @@ list_del_event(struct perf_event *event, struct perf_event_context *ctx)
 		perf_event_set_state(event, PERF_EVENT_STATE_OFF);
 
 	ctx->generation++;
+
+	if (event->pmu->pin_ctx)
+		ctx->pin_count--;
 }
 
 static void perf_group_detach(struct perf_event *event)
-- 
2.21.0