[tip:locking/core] locking/lockdep: Add a comment about crossrelease_hist_end() in lockdep_sys_exit()

[tip-bot for Byungchul Park]

Commit-ID:  a10b5c564741cd3b6708f085a1fa892b63c2063d
Gitweb:     http://git.kernel.org/tip/a10b5c564741cd3b6708f085a1fa892b63c2063d
Author:     Byungchul Park <byungchul.park@xxxxxxx>
AuthorDate: Mon, 14 Aug 2017 16:00:51 +0900
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 14 Aug 2017 12:52:17 +0200

locking/lockdep: Add a comment about crossrelease_hist_end() in lockdep_sys_exit()

In lockdep_sys_exit(), crossrelease_hist_end() is called unconditionally
even when getting here without having started e.g. just after forking.

But it's no problem since it would roll back to an invalid entry anyway.
Add a comment to explain this.

Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: akpm@xxxxxxxxxxxxxxxxxxxx
Cc: boqun.feng@xxxxxxxxx
Cc: kernel-team@xxxxxxx
Cc: kirill@xxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: npiggin@xxxxxxxxx
Cc: walken@xxxxxxxxxx
Cc: willy@xxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/1502694052-16085-2-git-send-email-byungchul.park@xxxxxxx
[ Improved the description and the comments. ]
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/locking/lockdep.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 1114dc4..257931e 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4623,6 +4623,10 @@ asmlinkage __visible void lockdep_sys_exit(void)
 	/*
 	 * The lock history for each syscall should be independent. So wipe the
 	 * slate clean on return to userspace.
+	 *
+	 * crossrelease_hist_end() works well here even when getting here
+	 * without starting (i.e. just after forking), because it rolls back
+	 * the index to point to the last entry, which is already invalid.
 	 */
 	crossrelease_hist_end(XHLOCK_PROC);
 	crossrelease_hist_start(XHLOCK_PROC);