bcache: fix NULL pointer in cache_set_flush()

bcache: fix NULL pointer in cache_set_flush()

1. LINE#1794 - LINE#1887 is some codes about function of
   bch_cache_set_alloc().
2. LINE#2078 - LINE#2142 is some codes about function of
   register_cache_set().
3. register_cache_set() will call bch_cache_set_alloc() in LINE#2098.

 1794 struct cache_set *bch_cache_set_alloc(struct cache_sb *sb)
 1795 {
 ...
 1860         if (!(c->devices = kcalloc(c->nr_uuids, sizeof(void *), GFP_KERNEL)) ||
 1861             mempool_init_slab_pool(&c->search, 32, bch_search_cache) ||
 1862             mempool_init_kmalloc_pool(&c->bio_meta, 2,
 1863                                 sizeof(struct bbio) + sizeof(struct bio_vec) *
 1864                                 bucket_pages(c)) ||
 1865             mempool_init_kmalloc_pool(&c->fill_iter, 1, iter_size) ||
 1866             bioset_init(&c->bio_split, 4, offsetof(struct bbio, bio),
 1867                         BIOSET_NEED_BVECS|BIOSET_NEED_RESCUER) ||
 1868             !(c->uuids = alloc_bucket_pages(GFP_KERNEL, c)) ||
 1869             !(c->moving_gc_wq = alloc_workqueue("bcache_gc",
 1870                                                 WQ_MEM_RECLAIM, 0)) ||
 1871             bch_journal_alloc(c) ||
 1872             bch_btree_cache_alloc(c) ||
 1873             bch_open_buckets_alloc(c) ||
 1874             bch_bset_sort_state_init(&c->sort, ilog2(c->btree_pages)))
 1875                 goto err;
                      ^^^^^^^^
 1876
 ...
 1883         return c;
 1884 err:
 1885         bch_cache_set_unregister(c);
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 1886         return NULL;
 1887 }
 ...
 2078 static const char *register_cache_set(struct cache *ca)
 2079 {
 ...
 2098         c = bch_cache_set_alloc(&ca->sb);
 2099         if (!c)
 2100                 return err;
                      ^^^^^^^^^^
 ...
 2128         ca->set = c;
 2129         ca->set->cache[ca->sb.nr_this_dev] = ca;
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 ...
 2138         return NULL;
 2139 err:
 2140         bch_cache_set_unregister(c);
 2141         return err;
 2142 }

(1) If LINE#1860 - LINE#1874 is true, then do 'goto err'(LINE#1875) and
    call bch_cache_set_unregister()(LINE#1885).
(2) As (1) return NULL(LINE#1886), LINE#2098 - LINE#2100 would return.
(3) As (2) has returned, LINE#2128 - LINE#2129 would do *not* give the
    value to c->cache[], it means that c->cache[] is NULL.

LINE#1624 - LINE#1665 is some codes about function of cache_set_flush().
As (1), in LINE#1885 call
bch_cache_set_unregister()
---> bch_cache_set_stop()
     ---> closure_queue()
          -.-> cache_set_flush() (as below LINE#1624)

 1624 static void cache_set_flush(struct closure *cl)
 1625 {
 ...
 1654         for_each_cache(ca, c, i)
 1655                 if (ca->alloc_thread)
                          ^^
 1656                         kthread_stop(ca->alloc_thread);
 ...
 1665 }

(4) In LINE#1655 ca is NULL(see (3)) in cache_set_flush() then the
    kernel crash occurred as below:
[  846.712887] bcache: register_cache() error drbd6: cannot allocate memory
[  846.713242] bcache: register_bcache() error : failed to register device
[  846.713336] bcache: cache_set_free() Cache set 2f84bdc1-498a-4f2f-98a7-01946bf54287 unregistered
[  846.713768] BUG: unable to handle kernel NULL pointer dereference at 00000000000009f8
[  846.714790] PGD 0 P4D 0
[  846.715129] Oops: 0000 [#1] SMP PTI
[  846.715472] CPU: 19 PID: 5057 Comm: kworker/19:16 Kdump: loaded Tainted: G           OE    --------- -  - 4.18.0-147.5.1.el8_1.5es.3.x86_64 #1
[  846.716082] Hardware name: ESPAN GI-25212/X11DPL-i, BIOS 2.1 06/15/2018
[  846.716451] Workqueue: events cache_set_flush [bcache]
[  846.716808] RIP: 0010:cache_set_flush+0xc9/0x1b0 [bcache]
[  846.717155] Code: 00 4c 89 a5 b0 03 00 00 48 8b 85 68 f6 ff ff a8 08 0f 84 88 00 00 00 31 db 66 83 bd 3c f7 ff ff 00 48 8b 85 48 ff ff ff 74 28 <48> 8b b8 f8 09 00 00 48 85 ff 74 05 e8 b6 58 a2 e1 0f b7 95 3c f7
[  846.718026] RSP: 0018:ffffb56dcf85fe70 EFLAGS: 00010202
[  846.718372] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[  846.718725] RDX: 0000000000000001 RSI: 0000000040000001 RDI: 0000000000000000
[  846.719076] RBP: ffffa0ccc0f20df8 R08: ffffa0ce1fedb118 R09: 000073746e657665
[  846.719428] R10: 8080808080808080 R11: 0000000000000000 R12: ffffa0ce1fee8700
[  846.719779] R13: ffffa0ccc0f211a8 R14: ffffa0cd1b902840 R15: ffffa0ccc0f20e00
[  846.720132] FS:  0000000000000000(0000) GS:ffffa0ce1fec0000(0000) knlGS:0000000000000000
[  846.720726] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  846.721073] CR2: 00000000000009f8 CR3: 00000008ba00a005 CR4: 00000000007606e0
[  846.721426] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  846.721778] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  846.722131] PKRU: 55555554
[  846.722467] Call Trace:
[  846.722814]  process_one_work+0x1a7/0x3b0
[  846.723157]  worker_thread+0x30/0x390
[  846.723501]  ? create_worker+0x1a0/0x1a0
[  846.723844]  kthread+0x112/0x130
[  846.724184]  ? kthread_flush_work_fn+0x10/0x10
[  846.724535]  ret_from_fork+0x35/0x40

Now, check whether that ca is NULL in LINE#1655 to fix the issue.

Signed-off-by: Linggang Zeng <linggang.zeng@easystack.cn>
Signed-off-by: Mingzhe Zou <mingzhe.zou@easystack.cn>
Signed-off-by: Coly Li <colyli@kernel.org>
Link: https://lore.kernel.org/r/20250527051601.74407-2-colyli@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>