From 279b5165ffadf57e2596e0ad438cb9b69b76f320 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 16 Feb 2017 10:28:37 +0100 Subject: [PATCH] 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 Signed-off-by: Peter Zijlstra (Intel) Cc: Alexander Shishkin Cc: Arnaldo Carvalho de Melo Cc: Jiri Olsa Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Stephane Eranian Cc: Thomas Gleixner Cc: Vince Weaver Signed-off-by: Ingo Molnar --- 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 77a932b54a64..94d7b9aae925 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); } /*