[tip:locking/core] locking/lockdep: Fix the rollback and overwrite detection logic in crossrelease

[tip-bot for Byungchul Park]

Commit-ID:  907dc16d7e23ec81a126c9585435494fa1b3a4b7
Gitweb:     http://git.kernel.org/tip/907dc16d7e23ec81a126c9585435494fa1b3a4b7
Author:     Byungchul Park <byungchul.park@xxxxxxx>
AuthorDate: Mon, 14 Aug 2017 16:00:52 +0900
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 14 Aug 2017 12:52:17 +0200

locking/lockdep: Fix the rollback and overwrite detection logic in crossrelease

As Boqun Feng pointed out, current->hist_id should be aligned with the
latest valid xhlock->hist_id so that hist_id_save[] storing current->hist_id
can be comparable with xhlock->hist_id. Fix it.

Additionally, the condition for overwrite-detection should be the
opposite. Fix the code and the comments as well.

           <- direction to visit
hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh (h: history)
                 ^^        ^
                 ||        start from here
                 |previous entry
                 current entry

Reported-by: Boqun Feng <boqun.feng@xxxxxxxxx>
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: 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/1502694052-16085-3-git-send-email-byungchul.park@xxxxxxx
[ Improve the comments some more. ]
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/locking/lockdep.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 257931e..66011c9 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4853,7 +4853,7 @@ static void add_xhlock(struct held_lock *hlock)
 
 	/* Initialize hist_lock's members */
 	xhlock->hlock = *hlock;
-	xhlock->hist_id = current->hist_id++;
+	xhlock->hist_id = ++current->hist_id;
 
 	xhlock->trace.nr_entries = 0;
 	xhlock->trace.max_entries = MAX_XHLOCK_TRACE_ENTRIES;
@@ -5029,12 +5029,12 @@ static void commit_xhlocks(struct cross_lock *xlock)
 				break;
 
 			/*
-			 * Filter out the cases that the ring buffer was
-			 * overwritten and the previous entry has a bigger
-			 * hist_id than the following one, which is impossible
-			 * otherwise.
+			 * Filter out the cases where the ring buffer was
+			 * overwritten and the current entry has a bigger
+			 * hist_id than the previous one, which is impossible
+			 * otherwise:
 			 */
-			if (unlikely(before(xhlock->hist_id, prev_hist_id)))
+			if (unlikely(before(prev_hist_id, xhlock->hist_id)))
 				break;
 
 			prev_hist_id = xhlock->hist_id;