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owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Feb 06, 2026 at 11:22:27AM -0800, Alexei Starovoitov wrote: > On Fri, Feb 6, 2026 at 10:27 AM Vlastimil Babka wrote: > > > > On 2/6/26 18:13, Harry Yoo wrote: > > > When CONFIG_SLAB_FREELIST_RANDOM is enabled and get_random_u32() > > > is called in an NMI context, lockdep complains because it acquires > > > a local_lock: > > > > > > ================================ > > > WARNING: inconsistent lock state > > > 6.19.0-rc5-slab-for-next+ #325 Tainted: G N > > > -------------------------------- > > > inconsistent {INITIAL USE} -> {IN-NMI} usage. > > > kunit_try_catch/8312 [HC2[2]:SC0[0]:HE0:SE1] takes: > > > ffff88a02ec49cc0 (batched_entropy_u32.lock){-.-.}-{3:3}, at: get_random_u32+0x7f/0x2e0 > > > {INITIAL USE} state was registered at: > > > lock_acquire+0xd9/0x2f0 > > > get_random_u32+0x93/0x2e0 > > > __get_random_u32_below+0x17/0x70 > > > cache_random_seq_create+0x121/0x1c0 > > > init_cache_random_seq+0x5d/0x110 > > > do_kmem_cache_create+0x1e0/0xa30 > > > __kmem_cache_create_args+0x4ec/0x830 > > > create_kmalloc_caches+0xe6/0x130 > > > kmem_cache_init+0x1b1/0x660 > > > mm_core_init+0x1d8/0x4b0 > > > start_kernel+0x620/0xcd0 > > > x86_64_start_reservations+0x18/0x30 > > > x86_64_start_kernel+0xf3/0x140 > > > common_startup_64+0x13e/0x148 > > > irq event stamp: 76 > > > hardirqs last enabled at (75): [] exc_nmi+0x11a/0x240 > > > hardirqs last disabled at (76): [] sysvec_irq_work+0x11/0x110 > > > softirqs last enabled at (0): [] copy_process+0xc7a/0x2350 > > > softirqs last disabled at (0): [<0000000000000000>] 0x0 > > > > > > other info that might help us debug this: > > > Possible unsafe locking scenario: > > > > > > CPU0 > > > ---- > > > lock(batched_entropy_u32.lock); > > > > > > lock(batched_entropy_u32.lock); > > > > > > *** DEADLOCK *** > > > > > > Fix this by using pseudo-random number generator if !allow_spin. > > > This means kmalloc_nolock() users won't get truly random numbers, > > > but there is not much we can do about it. > > > > > > Note that an NMI handler might interrupt prandom_u32_state() and > > > change the random state, but that's safe. > > > > > > Link: https://lore.kernel.org/all/0c33bdee-6de8-4d9f-92ca-4f72c1b6fb9f@suse.cz > > > Signed-off-by: Harry Yoo > > > --- > > > mm/slub.c | 28 ++++++++++++++++++++++++---- > > > 1 file changed, 24 insertions(+), 4 deletions(-) > > > > > > diff --git a/mm/slub.c b/mm/slub.c > > > index d46464654c15..4d76af84f018 100644 > > > --- a/mm/slub.c > > > +++ b/mm/slub.c > > > @@ -43,6 +43,7 @@ > > > #include > > > #include > > > #include > > > +#include > > > #include > > > #include > > > #include > > > @@ -3308,8 +3309,11 @@ static void *next_freelist_entry(struct kmem_cache *s, > > > return (char *)start + idx; > > > } > > > > > > +static DEFINE_PER_CPU(struct rnd_state, slab_rnd_state); > > > + > > > /* Shuffle the single linked freelist based on a random pre-computed sequence */ > > > -static bool shuffle_freelist(struct kmem_cache *s, struct slab *slab) > > > +static bool shuffle_freelist(struct kmem_cache *s, struct slab *slab, > > > + bool allow_spin) > > > { > > > void *start; > > > void *cur; > > > @@ -3320,7 +3324,19 @@ static bool shuffle_freelist(struct kmem_cache *s, struct slab *slab) > > > return false; > > > > > > freelist_count = oo_objects(s->oo); > > > - pos = get_random_u32_below(freelist_count); > > > + if (allow_spin) { > > > + pos = get_random_u32_below(freelist_count); > > > + } else { > > > + struct rnd_state *state; > > > + > > > + /* > > > + * kmalloc_nolock() called in an NMI context might interrupt > > > + * and change the state in the middle. > > > + */ > > > + state = &get_cpu_var(slab_rnd_state); > > > + pos = prandom_u32_state(state) % freelist_count; > > > + put_cpu_var(slab_rnd_state); > > > > I don't think this prevents the changing in the middle? We just stored the > > pointer in a local variable state, but the prandom call will still access > > the percpu variable through that? > > > > So we might need to disable irq here, and have another percpu state that's > > used when in_nmi()? Oh, my intention was not preventing state changes in the middle. I was thinking "Hmm if we can't disable NMI, do we even need to disable IRQ? just add some comment saying it might be interrupted in the middle". I was even thinking of using raw_cpu_ptr() instead without disabling preemption through get/put_cpu_var()... > imo this is all overkill. > Just prandom_u32_state() without any protection is fine. > Even if it reenters there is no harm. Just more randomness. Yeah. -- Cheers, Harry / Hyeonggon