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mm, slub: move slub_debug static key enabling outside slab_mutex

Paul E. McKenney reported [1] that commit 1f0723a4c0df ("mm, slub: enable slub_debug static key when creating cache with explicit debug flags") results in the lockdep complaint: ====================================================== WARNING: possible circular locking dependency detected 5.12.0+ #15 Not tainted ------------------------------------------------------ rcu_torture_sta/109 is trying to acquire lock: ffffffff96063cd0 (cpu_hotplug_lock){++++}-{0:0}, at: static_key_enable+0x9/0x20 but task is already holding lock: ffffffff96173c28 (slab_mutex){+.+.}-{3:3}, at: kmem_cache_create_usercopy+0x2d/0x250 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #1 (slab_mutex){+.+.}-{3:3}: lock_acquire+0xb9/0x3a0 __mutex_lock+0x8d/0x920 slub_cpu_dead+0x15/0xf0 cpuhp_invoke_callback+0x17a/0x7c0 cpuhp_invoke_callback_range+0x3b/0x80 _cpu_down+0xdf/0x2a0 cpu_down+0x2c/0x50 device_offline+0x82/0xb0 remove_cpu+0x1a/0x30 torture_offline+0x80/0x140 torture_onoff+0x147/0x260 kthread+0x10a/0x140 ret_from_fork+0x22/0x30 -> #0 (cpu_hotplug_lock){++++}-{0:0}: check_prev_add+0x8f/0xbf0 __lock_acquire+0x13f0/0x1d80 lock_acquire+0xb9/0x3a0 cpus_read_lock+0x21/0xa0 static_key_enable+0x9/0x20 __kmem_cache_create+0x38d/0x430 kmem_cache_create_usercopy+0x146/0x250 kmem_cache_create+0xd/0x10 rcu_torture_stats+0x79/0x280 kthread+0x10a/0x140 ret_from_fork+0x22/0x30 other info that might help us debug this: Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(slab_mutex); lock(cpu_hotplug_lock); lock(slab_mutex); lock(cpu_hotplug_lock); *** DEADLOCK *** 1 lock held by rcu_torture_sta/109: #0: ffffffff96173c28 (slab_mutex){+.+.}-{3:3}, at: kmem_cache_create_usercopy+0x2d/0x250 stack backtrace: CPU: 3 PID: 109 Comm: rcu_torture_sta Not tainted 5.12.0+ #15 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-1ubuntu1.1 04/01/2014 Call Trace: dump_stack+0x6d/0x89 check_noncircular+0xfe/0x110 ? lock_is_held_type+0x98/0x110 check_prev_add+0x8f/0xbf0 __lock_acquire+0x13f0/0x1d80 lock_acquire+0xb9/0x3a0 ? static_key_enable+0x9/0x20 ? mark_held_locks+0x49/0x70 cpus_read_lock+0x21/0xa0 ? static_key_enable+0x9/0x20 static_key_enable+0x9/0x20 __kmem_cache_create+0x38d/0x430 kmem_cache_create_usercopy+0x146/0x250 ? rcu_torture_stats_print+0xd0/0xd0 kmem_cache_create+0xd/0x10 rcu_torture_stats+0x79/0x280 ? rcu_torture_stats_print+0xd0/0xd0 kthread+0x10a/0x140 ? kthread_park+0x80/0x80 ret_from_fork+0x22/0x30 This is because there's one order of locking from the hotplug callbacks: lock(cpu_hotplug_lock); // from hotplug machinery itself lock(slab_mutex); // in e.g. slab_mem_going_offline_callback() And commit 1f0723a4c0df made the reverse sequence possible: lock(slab_mutex); // in kmem_cache_create_usercopy() lock(cpu_hotplug_lock); // kmem_cache_open() -> static_key_enable() The simplest fix is to move static_key_enable() to a place before slab_mutex is taken. That means kmem_cache_create_usercopy() in mm/slab_common.c which is not ideal for SLUB-specific code, but the #ifdef CONFIG_SLUB_DEBUG makes it at least self-contained and obvious. [1] https://lore.kernel.org/lkml/20210502171827.GA3670492@paulmck-ThinkPad-P17-Gen-1/

Vlastimil Babka committed 5 years ago

Commit afe0c261 parent 84894e12 files · +10 −9Browse files

Unified · Split

2 files
  1. mm/slab_common.c +10 −0
  2. mm/slub.c +0 −9
mm/slab_common.c+10−0
@@ -318,6 +318,16 @@ kmem_cache_create_usercopy(const char *name, · 317 unmodified lines
318318 const char *cache_name;
319319 int err;
320320
321+#ifdef CONFIG_SLUB_DEBUG
322+ /*
323+ * If no slub_debug was enabled globally, the static key is not yet
324+ * enabled by setup_slub_debug(). Enable it if the cache is being
325+ * created with any of the debugging flags passed explicitly.
326+ */
327+ if (flags & SLAB_DEBUG_FLAGS)
328+ static_branch_enable(&slub_debug_enabled);
329+#endif
330+
321331 mutex_lock(&slab_mutex);
322332
323333 err = kmem_cache_sanity_check(name, size);
mm/slub.c+0−9
@@ -3828,15 +3828,6 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order) · 3827 unmodified lines
38283828
38293829 static int kmem_cache_open(struct kmem_cache *s, slab_flags_t flags)
38303830 {
3831−#ifdef CONFIG_SLUB_DEBUG
3832− /*
3833− * If no slub_debug was enabled globally, the static key is not yet
3834− * enabled by setup_slub_debug(). Enable it if the cache is being
3835− * created with any of the debugging flags passed explicitly.
3836− */
3837− if (flags & SLAB_DEBUG_FLAGS)
3838− static_branch_enable(&slub_debug_enabled);
3839−#endif
38403831 s->flags = kmem_cache_flags(s->size, flags, s->name);
38413832 #ifdef CONFIG_SLAB_FREELIST_HARDENED
38423833 s->random = get_random_long();