[PATCH 16/23] kernel/locking: Ensure identical trylocks are nested too.

[Maarten Lankhorst]

i915 will soon gain an eviction path that trylock a whole lot of locks
for eviction, getting dmesg failures like below:

BUG: MAX_LOCK_DEPTH too low!
turning off the locking correctness validator.
depth: 48  max: 48!
48 locks held by i915_selftest/5776:
 #0: ffff888101a79240 (&dev->mutex){....}-{3:3}, at: __driver_attach+0x88/0x160
 #1: ffffc900009778c0 (reservation_ww_class_acquire){+.+.}-{0:0}, at: i915_vma_pin.constprop.63+0x39/0x1b0 [i915]
 #2: ffff88800cf74de8 (reservation_ww_class_mutex){+.+.}-{3:3}, at: i915_vma_pin.constprop.63+0x5f/0x1b0 [i915]
 #3: ffff88810c7f9e38 (&vm->mutex/1){+.+.}-{3:3}, at: i915_vma_pin_ww+0x1c4/0x9d0 [i915]
 #4: ffff88810bad5768 (reservation_ww_class_mutex){+.+.}-{3:3}, at: i915_gem_evict_something+0x110/0x860 [i915]
 #5: ffff88810bad60e8 (reservation_ww_class_mutex){+.+.}-{3:3}, at: i915_gem_evict_something+0x110/0x860 [i915]
...
 #46: ffff88811964d768 (reservation_ww_class_mutex){+.+.}-{3:3}, at: i915_gem_evict_something+0x110/0x860 [i915]
 #47: ffff88811964e0e8 (reservation_ww_class_mutex){+.+.}-{3:3}, at: i915_gem_evict_something+0x110/0x860 [i915]
INFO: lockdep is turned off.

Fixing eviction to nest into ww_class_acquire is a high priority,
but it requires a rework of the entire driver, which can only be
done one step at a time.

As an intermediate solution, fold all identical trylocks, making the
above lockdep splat disappear.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Ingo Molnar <mingo at redhat.com>
Cc: Will Deacon <will at kernel.org>
Cc: Waiman Long <longman at redhat.com>
Cc: Boqun Feng <boqun.feng at gmail.com>
Cc: linux-kernel at vger.kernel.org
---
 kernel/locking/lockdep.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index b274b3491850..1858a1c01015 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4922,7 +4922,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 
 	if (depth) { /* we're holding locks */
 		hlock = curr->held_locks + depth - 1;
-		if (hlock->class_idx == class_idx && nest_lock) {
+		if (hlock->class_idx == class_idx && (nest_lock || trylock)) {
 			if (!references)
 				references++;
 
-- 
2.33.0