mm/slab: use prandom if !allow_spin

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): [<ffffffff8298b77a>] exc_nmi+0x11a/0x240
  hardirqs last disabled at (76): [<ffffffff8298b991>] sysvec_irq_work+0x11/0x110
  softirqs last  enabled at (0): [<ffffffff813b2dda>] 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);
    <Interrupt>
      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
Fixes: af92793 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Cc: stable@vger.kernel.org
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20260210081900.329447-3-harry.yoo@oracle.com
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Author: hygoni

mm/slub.c

@@ -43,6 +43,7 @@
 #include <linux/prefetch.h>
 #include <linux/memcontrol.h>
 #include <linux/random.h>
+#include <linux/prandom.h>
 #include <kunit/test.h>
 #include <kunit/test-bug.h>
 #include <linux/sort.h>
@@ -3311,8 +3312,11 @@ 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;
@@ -3323,7 +3327,19 @@ 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;
+
+		/*
+		 * An interrupt or NMI handler might interrupt and change
+		 * the state in the middle, but that's safe.
+		 */
+		state = &get_cpu_var(slab_rnd_state);
+		pos = prandom_u32_state(state) % freelist_count;
+		put_cpu_var(slab_rnd_state);
+	}
 
 	page_limit = slab->objects * s->size;
 	start = fixup_red_left(s, slab_address(slab));
@@ -3350,7 +3366,8 @@ static inline int init_cache_random_seq(struct kmem_cache *s)
 	return 0;
 }
 static inline void init_freelist_randomization(void) { }
-static inline bool shuffle_freelist(struct kmem_cache *s, struct slab *slab)
+static inline bool shuffle_freelist(struct kmem_cache *s, struct slab *slab,
+				    bool allow_spin)
 {
 	return false;
 }
@@ -3441,7 +3458,7 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
 	alloc_slab_obj_exts_early(s, slab);
 	account_slab(slab, oo_order(oo), s, flags);
 
-	shuffle = shuffle_freelist(s, slab);
+	shuffle = shuffle_freelist(s, slab, allow_spin);
 
 	if (!shuffle) {
 		start = fixup_red_left(s, start);
@@ -8341,6 +8358,9 @@ void __init kmem_cache_init_late(void)
 {
 	flushwq = alloc_workqueue("slub_flushwq", WQ_MEM_RECLAIM, 0);
 	WARN_ON(!flushwq);
+#ifdef CONFIG_SLAB_FREELIST_RANDOM
+	prandom_init_once(&slab_rnd_state);
+#endif
 }
 
 int do_kmem_cache_create(struct kmem_cache *s, const char *name,