Commit-ID: 279b5165ffadf57e2596e0ad438cb9b69b76f320
Gitweb: http://git.kernel.org/tip/279b5165ffadf57e2596e0ad438cb9b69b76f320
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Thu, 16 Feb 2017 10:28:37 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 24 Feb 2017 08:56:32 +0100
perf/core: Remove confusing comment and move put_ctx()
Since commit:
321027c1fe77 ("perf/core: Fix concurrent sys_perf_event_open() vs. 'move_group' race")
... the code looks like (assuming move_group==1):
gctx = __perf_event_ctx_lock_double(group_leader, ctx);
perf_remove_from_context(group_leader, 0);
list_for_each_entry(sibling, &group_leader->sibling_list, group_entry) {
perf_remove_from_context(sibling, 0);
put_ctx(gctx);
}
/* ... */
/* misleading comment about how this is the last reference */
put_ctx(gctx);
perf_event_ctx_unlock(group_leader, gctx);
What that 'last' put_ctx() does is drop @group_leader's reference on
gctx after having dropped all its potential sibling references.
But the thing is that __perf_event_ctx_lock_double() returns with a
reference _and_ a held lock, and perf_event_ctx_unlock() unlocks that
lock and drops that reference. Therefore that put_ctx() cannot be the
'last' of anything, nor is there an unbalance in puts.
To reduce confusion, remove the comment and place the put_ctx() next
to the remove_from_context() call.
Reported-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Stephane Eranian <eranian@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Vince Weaver <vincent.weaver@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
kernel/events/core.c | 8 +-------
1 file changed, 1 insertion(+), 7 deletions(-)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 77a932b..94d7b9a 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -9955,6 +9955,7 @@ SYSCALL_DEFINE5(perf_event_open,
* of swizzling perf_event::ctx.
*/
perf_remove_from_context(group_leader, 0);
+ put_ctx(gctx);
list_for_each_entry(sibling, &group_leader->sibling_list,
group_entry) {
@@ -9993,13 +9994,6 @@ SYSCALL_DEFINE5(perf_event_open,
perf_event__state_init(group_leader);
perf_install_in_context(ctx, group_leader, group_leader->cpu);
get_ctx(ctx);
-
- /*
- * Now that all events are installed in @ctx, nothing
- * references @gctx anymore, so drop the last reference we have
- * on it.
- */
- put_ctx(gctx);
}
/*
--
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