linux-kernel - [tip:locking/core] locking/lockdep: Fix merging of hlocks with non-zero references

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Date:   Mon, 3 Jun 2019 06:32:39 -0700
From:   tip-bot for Imre Deak <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     tglx@...utronix.de, torvalds@...ux-foundation.org,
        will.deacon@....com, ville.syrjala@...ux.intel.com,
        mingo@...nel.org, hpa@...or.com, imre.deak@...el.com,
        linux-kernel@...r.kernel.org, peterz@...radead.org
Subject: [tip:locking/core] locking/lockdep: Fix merging of hlocks with
 non-zero references

Commit-ID:  d9349850e188b8b59e5322fda17ff389a1c0cd7d
Gitweb:     https://git.kernel.org/tip/d9349850e188b8b59e5322fda17ff389a1c0cd7d
Author:     Imre Deak <imre.deak@...el.com>
AuthorDate: Fri, 24 May 2019 23:15:09 +0300
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 3 Jun 2019 12:32:56 +0200

locking/lockdep: Fix merging of hlocks with non-zero references

The sequence

	static DEFINE_WW_CLASS(test_ww_class);

	struct ww_acquire_ctx ww_ctx;
	struct ww_mutex ww_lock_a;
	struct ww_mutex ww_lock_b;
	struct ww_mutex ww_lock_c;
	struct mutex lock_c;

	ww_acquire_init(&ww_ctx, &test_ww_class);

	ww_mutex_init(&ww_lock_a, &test_ww_class);
	ww_mutex_init(&ww_lock_b, &test_ww_class);
	ww_mutex_init(&ww_lock_c, &test_ww_class);

	mutex_init(&lock_c);

	ww_mutex_lock(&ww_lock_a, &ww_ctx);

	mutex_lock(&lock_c);

	ww_mutex_lock(&ww_lock_b, &ww_ctx);
	ww_mutex_lock(&ww_lock_c, &ww_ctx);

	mutex_unlock(&lock_c);	(*)

	ww_mutex_unlock(&ww_lock_c);
	ww_mutex_unlock(&ww_lock_b);
	ww_mutex_unlock(&ww_lock_a);

	ww_acquire_fini(&ww_ctx); (**)

will trigger the following error in __lock_release() when calling
mutex_release() at **:

	DEBUG_LOCKS_WARN_ON(depth <= 0)

The problem is that the hlock merging happening at * updates the
references for test_ww_class incorrectly to 3 whereas it should've
updated it to 4 (representing all the instances for ww_ctx and
ww_lock_[abc]).

Fix this by updating the references during merging correctly taking into
account that we can have non-zero references (both for the hlock that we
merge into another hlock or for the hlock we are merging into).

Signed-off-by: Imre Deak <imre.deak@...el.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: =?UTF-8?q?Ville=20Syrj=C3=A4l=C3=A4?= <ville.syrjala@...ux.intel.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Will Deacon <will.deacon@....com>
Link: https://lkml.kernel.org/r/20190524201509.9199-2-imre.deak@intel.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/locking/lockdep.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 6c97f67ec321..48a840adb281 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3796,17 +3796,17 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 	if (depth) {
 		hlock = curr->held_locks + depth - 1;
 		if (hlock->class_idx == class_idx && nest_lock) {
-			if (hlock->references) {
-				/*
-				 * Check: unsigned int references:12, overflow.
-				 */
-				if (DEBUG_LOCKS_WARN_ON(hlock->references == (1 << 12)-1))
-					return 0;
+			if (!references)
+				references++;
 
+			if (!hlock->references)
 				hlock->references++;
-			} else {
-				hlock->references = 2;
-			}
+
+			hlock->references += references;
+
+			/* Overflow */
+			if (DEBUG_LOCKS_WARN_ON(hlock->references < references))
+				return 0;
 
 			return 2;
 		}