debugobjects,locking: Annotate debug_object_fill_pool() wait type violation

There is an explicit wait-type violation in debug_object_fill_pool()
for PREEMPT_RT=n kernels which allows them to more easily fill the
object pool and reduce the chance of allocation failures.

Lockdep's wait-type checks are designed to check the PREEMPT_RT
locking rules even for PREEMPT_RT=n kernels and object to this, so
create a lockdep annotation to allow this to stand.

Specifically, create a 'lock' type that overrides the inner wait-type
while it is held -- allowing one to temporarily raise it, such that
the violation is hidden.

Reported-by: Vlastimil Babka <[email protected]>
Reported-by: Qi Zheng <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Tested-by: Qi Zheng <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

1 files changed, 13 insertions, 2 deletions

diff --git a/lib/debugobjects.c b/lib/debugobjects.c
index 003edc5ebd67..826c617b10a7 100644
--- a/lib/debugobjects.c
+++ b/lib/debugobjects.c
@@ -591,10 +591,21 @@ static void debug_objects_fill_pool(void)
 {
 	/*
 	 * On RT enabled kernels the pool refill must happen in preemptible
-	 * context:
+	 * context -- for !RT kernels we rely on the fact that spinlock_t and
+	 * raw_spinlock_t are basically the same type and this lock-type
+	 * inversion works just fine.
 	 */
-	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || preemptible())
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || preemptible()) {
+		/*
+		 * Annotate away the spinlock_t inside raw_spinlock_t warning
+		 * by temporarily raising the wait-type to WAIT_SLEEP, matching
+		 * the preemptible() condition above.
+		 */
+		static DEFINE_WAIT_OVERRIDE_MAP(fill_pool_map, LD_WAIT_SLEEP);
+		lock_map_acquire_try(&fill_pool_map);
 		fill_pool();
+		lock_map_release(&fill_pool_map);
+	}
 }
 
 static void