[tip: locking/urgent] locking/ww_mutex: Treat ww_mutex_lock() like a trylock

tip-bot2 for Waiman Long
2021-03-17T12:39:15Z

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID: b058f2e4d0a70c060e21ed122b264e9649cad57f
Gitweb: https://git.kernel.org/tip/b058f2e4d0a70c060e21ed122b264e9649cad57f
Author: Waiman Long <longman@redhat.com>
AuthorDate: Tue, 16 Mar 2021 11:31:18 -04:00
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Wed, 17 Mar 2021 09:56:46 +01:00

locking/ww_mutex: Treat ww_mutex_lock() like a trylock

It was found that running the ww_mutex_lock-torture test produced the
following lockdep splat almost immediately:

[ 103.892638] ======================================================
[ 103.892639] WARNING: possible circular locking dependency detected
[ 103.892641] 5.12.0-rc3-debug+ #2 Tainted: G S W
[ 103.892643] ------------------------------------------------------
[ 103.892643] lock_torture_wr/3234 is trying to acquire lock:
[ 103.892646] ffffffffc0b35b10 (torture_ww_mutex_2.base){+.+.}-{3:3}, at: torture_ww_mutex_lock+0x316/0x720 [locktorture]
[ 103.892660]
[ 103.892660] but task is already holding lock:
[ 103.892661] ffffffffc0b35cd0 (torture_ww_mutex_0.base){+.+.}-{3:3}, at: torture_ww_mutex_lock+0x3e2/0x720 [locktorture]
[ 103.892669]
[ 103.892669] which lock already depends on the new lock.
[ 103.892669]
[ 103.892670]
[ 103.892670] the existing dependency chain (in reverse order) is:
[ 103.892671]
[ 103.892671] -> #2 (torture_ww_mutex_0.base){+.+.}-{3:3}:
[ 103.892675] lock_acquire+0x1c5/0x830
[ 103.892682] __ww_mutex_lock.constprop.15+0x1d1/0x2e50
[ 103.892687] ww_mutex_lock+0x4b/0x180
[ 103.892690] torture_ww_mutex_lock+0x316/0x720 [locktorture]
[ 103.892694] lock_torture_writer+0x142/0x3a0 [locktorture]
[ 103.892698] kthread+0x35f/0x430
[ 103.892701] ret_from_fork+0x1f/0x30
[ 103.892706]
[ 103.892706] -> #1 (torture_ww_mutex_1.base){+.+.}-{3:3}:
[ 103.892709] lock_acquire+0x1c5/0x830
[ 103.892712] __ww_mutex_lock.constprop.15+0x1d1/0x2e50
[ 103.892715] ww_mutex_lock+0x4b/0x180
[ 103.892717] torture_ww_mutex_lock+0x316/0x720 [locktorture]
[ 103.892721] lock_torture_writer+0x142/0x3a0 [locktorture]
[ 103.892725] kthread+0x35f/0x430
[ 103.892727] ret_from_fork+0x1f/0x30
[ 103.892730]
[ 103.892730] -> #0 (torture_ww_mutex_2.base){+.+.}-{3:3}:
[ 103.892733] check_prevs_add+0x3fd/0x2470
[ 103.892736] __lock_acquire+0x2602/0x3100
[ 103.892738] lock_acquire+0x1c5/0x830
[ 103.892740] __ww_mutex_lock.constprop.15+0x1d1/0x2e50
[ 103.892743] ww_mutex_lock+0x4b/0x180
[ 103.892746] torture_ww_mutex_lock+0x316/0x720 [locktorture]
[ 103.892749] lock_torture_writer+0x142/0x3a0 [locktorture]
[ 103.892753] kthread+0x35f/0x430
[ 103.892755] ret_from_fork+0x1f/0x30
[ 103.892757]
[ 103.892757] other info that might help us debug this:
[ 103.892757]
[ 103.892758] Chain exists of:
[ 103.892758] torture_ww_mutex_2.base --> torture_ww_mutex_1.base --> torture_ww_mutex_0.base
[ 103.892758]
[ 103.892763] Possible unsafe locking scenario:
[ 103.892763]
[ 103.892764] CPU0 CPU1
[ 103.892765] ---- ----
[ 103.892765] lock(torture_ww_mutex_0.base);
[ 103.892767] lock(torture_ww_mutex_1.base);
[ 103.892770] lock(torture_ww_mutex_0.base);
[ 103.892772] lock(torture_ww_mutex_2.base);
[ 103.892774]
[ 103.892774] *** DEADLOCK ***

Since ww_mutex is supposed to be deadlock-proof if used properly, such
deadlock scenario should not happen. To avoid this false positive splat,
treat ww_mutex_lock() like a trylock().

After applying this patch, the locktorture test can run for a long time
without triggering the circular locking dependency splat.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by Davidlohr Bueso <dbueso@suse.de>
Link: https://lore.kernel.org/r/20210316153119.13802-4-longman@redhat.com
---
kernel/locking/mutex.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 622ebdf..bb89393 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -946,7 +946,10 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
}

preempt_disable();
- mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
+ /*
+ * Treat as trylock for ww_mutex.
+ */
+ mutex_acquire_nest(&lock->dep_map, subclass, !!ww_ctx, nest_lock, ip);

if (__mutex_trylock(lock) ||
mutex_optimistic_spin(lock, ww_ctx, NULL)) {