Re: [PATCH v2 3/6] locking/local_lock: Introduce local_lock_lockdep_start/end()

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Jul 15, 2025 at 08:56:21AM +0200, Vlastimil Babka wrote:
> > the addresses of the locks are different and they're different
> > kmalloc buckets, but lockdep cannot understand this without
> > explicit local_lock_lockdep_start().
> > The same thing I'm trying to explain in the commit log.
> 
> Thanks for the explanation and sorry for being so dense.
> Maybe lockdep's lock classes can be used here somehow instead of having to
> teach lockdep completely new tricks, but I don't know enough about those to
> know for sure.

I tried that with a separate lock_key for each local_trylock_t
and it's sort-of kinda works for 16 cpus, but doesn't scale
when number of cpus is large.
There is no good way to pick LOCKDEP_BITS.

It can be made optional on PREEMP_RT only
and used for kmalloc buckets only that kmalloc_nolock() is using,
but still feels less clean than
local_lock_lockdep_start/end()
since it makes lockdep work harder.

Better ideas?

From da2b3bac08950929da105836fbff7e2ea4ecbc0e Mon Sep 17 00:00:00 2001
From: Alexei Starovoitov <ast@kernel.org>
Date: Tue, 15 Jul 2025 10:16:42 -0700
Subject: [PATCH] lockdep

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
 lib/Kconfig.debug |  2 +-
 mm/slub.c         | 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index ebe33181b6e6..94c07b84ecd0 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1526,7 +1526,7 @@ config LOCKDEP_BITS
 	int "Size for MAX_LOCKDEP_ENTRIES (as Nth power of 2)"
 	depends on LOCKDEP && !LOCKDEP_SMALL
 	range 10 24
-	default 15
+	default 16
 	help
 	  Try increasing this value if you hit "BUG: MAX_LOCKDEP_ENTRIES too low!" message.
 
diff --git a/mm/slub.c b/mm/slub.c
index 2f30b85fbf68..2ae6bf3ebcd0 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -395,6 +395,7 @@ struct kmem_cache_cpu {
 	struct slab *partial;	/* Partially allocated slabs */
 #endif
 	local_trylock_t lock;	/* Protects the fields above */
+	struct lock_class_key lock_key;
 #ifdef CONFIG_SLUB_STATS
 	unsigned int stat[NR_SLUB_STAT_ITEMS];
 #endif
@@ -3083,6 +3084,8 @@ static void init_kmem_cache_cpus(struct kmem_cache *s)
 	for_each_possible_cpu(cpu) {
 		c = per_cpu_ptr(s->cpu_slab, cpu);
 		local_trylock_init(&c->lock);
+		lockdep_register_key(&c->lock_key);
+		lockdep_set_class(&c->lock, &c->lock_key);
 		c->tid = init_tid(cpu);
 	}
 }
@@ -5953,6 +5956,16 @@ void __kmem_cache_release(struct kmem_cache *s)
 {
 	cache_random_seq_destroy(s);
 #ifndef CONFIG_SLUB_TINY
+	{
+		int cpu;
+
+		for_each_possible_cpu(cpu) {
+			struct kmem_cache_cpu *c = per_cpu_ptr(s->cpu_slab,
+							       cpu);
+
+			lockdep_unregister_key(&c->lock_key);
+		}
+	}
 	free_percpu(s->cpu_slab);
 #endif
 	free_kmem_cache_nodes(s);
-- 
2.47.1