mm/slab: allow kfree_rcu_sheaf() on PREEMPT_RT

As suggested by Vlastimil Babka [1], kfree_rcu_sheaf() can be used
on PREEMPT_RT if we always assume spinning is not allowed on PREEMPT_RT.
This is because local_trylock and spinlock_t are safe to use with
trylock and unlock as long as the kernel does not spin and the context
is not NMI and not hardirq.

Now that __kfree_rcu_sheaf() knows how to handle SLAB_FREE_NOLOCK,
relax the limitation and try the sheaves path on PREEMPT_RT as well.

Keep the lockdep map on non RT kernels. However, do not use the lockdep
map on PREEMPT_RT to avoid suppressing valid lockdep warnings.

As pointed by Vlastimil Babka [2], on PREEMPT_RT it is unnecessary to
defer call_rcu() under IRQ-disabled section or raw spinlock. However,
let us avoid adding more complexity as the scenario is not supposed
to be common on PREEMPT_RT, with a hope that call_rcu_nolock() will be
soon supported in RCU.

Link: https://lore.kernel.org/linux-mm/6811cc17-8ee4-48c8-8cbf-6bf4d9f98162@kernel.org [1]
Link: https://lore.kernel.org/linux-mm/40591888-3a87-433e-b3d2-cda1cab543be@kernel.org [2]
Suggested-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
Link: https://patch.msgid.link/20260729-kfree_rcu_nolock-v5-5-a28cdcda9673@kernel.org
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

2 files changed, 20 insertions, 9 deletions

diff --git a/mm/slab_common.c b/mm/slab_common.c
index ed49b9abfab2..e776fc4516c8 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1612,6 +1612,14 @@ static bool kfree_rcu_sheaf(void *obj)
 {
 	struct kmem_cache *s;
 	struct slab *slab;
+	unsigned int free_flags = SLAB_FREE_DEFAULT;
+
+	/*
+	 * It is not safe to spin on PREEMPT_RT because the kernel might be
+	 * holding a raw spinlock and slab acquires sleeping locks.
+	 */
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		free_flags = SLAB_FREE_NOLOCK;
 
 	if (is_vmalloc_addr(obj))
 		return false;
@@ -1622,7 +1630,7 @@ static bool kfree_rcu_sheaf(void *obj)
 
 	s = slab->slab_cache;
 	if (likely(!IS_ENABLED(CONFIG_NUMA) || slab_nid(slab) == numa_mem_id()))
-		return __kfree_rcu_sheaf(s, obj, SLAB_FREE_DEFAULT);
+		return __kfree_rcu_sheaf(s, obj, free_flags);
 
 	return false;
 }
@@ -1971,7 +1979,7 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
 	if (!head)
 		might_sleep();
 
-	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && kfree_rcu_sheaf(ptr))
+	if (kfree_rcu_sheaf(ptr))
 		return;
 
 	// Queue the object but don't yet schedule the batch.
diff --git a/mm/slub.c b/mm/slub.c
index c0cc6d3126c7..4a07ba92b64f 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -6060,12 +6060,13 @@ empty:
  * kvfree_call_rcu() can be called while holding a raw_spinlock_t. Since
  * __kfree_rcu_sheaf() may acquire a spinlock_t (sleeping lock on PREEMPT_RT),
  * this would violate lock nesting rules. Therefore, kvfree_call_rcu() avoids
- * this problem by bypassing the sheaves layer entirely on PREEMPT_RT.
+ * this problem by passing SLAB_FREE_NOLOCK on PREEMPT_RT.
  *
  * However, lockdep still complains that it is invalid to acquire spinlock_t
  * while holding raw_spinlock_t, even on !PREEMPT_RT where spinlock_t is a
  * spinning lock. Tell lockdep that acquiring spinlock_t is valid here
- * by temporarily raising the wait-type to LD_WAIT_CONFIG.
+ * by temporarily raising the wait-type to LD_WAIT_CONFIG. Skip the lockdep map
+ * on PREEMPT_RT to avoid suppressing valid lockdep warnings.
  */
 static DEFINE_WAIT_OVERRIDE_MAP(kfree_rcu_sheaf_map, LD_WAIT_CONFIG);
 
@@ -6075,10 +6076,10 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj, unsigned int free_flags)
 	struct slab_sheaf *rcu_sheaf;
 	bool allow_spin = free_flags_allow_spinning(free_flags);
 
-	if (WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT)))
-		return false;
+	VM_WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT) && allow_spin);
 
-	lock_map_acquire_try(&kfree_rcu_sheaf_map);
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		lock_map_acquire_try(&kfree_rcu_sheaf_map);
 
 	if (!local_trylock(&s->cpu_sheaves->lock))
 		goto fail;
@@ -6177,12 +6178,14 @@ do_free:
 	local_unlock(&s->cpu_sheaves->lock);
 
 	stat(s, FREE_RCU_SHEAF);
-	lock_map_release(&kfree_rcu_sheaf_map);
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		lock_map_release(&kfree_rcu_sheaf_map);
 	return true;
 
 fail:
 	stat(s, FREE_RCU_SHEAF_FAIL);
-	lock_map_release(&kfree_rcu_sheaf_map);
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		lock_map_release(&kfree_rcu_sheaf_map);
 	return false;
 }