On 3/29/25 7:29 AM, Ming Lei wrote: > On Fri, Mar 28, 2025 at 07:37:25AM -0700, syzbot wrote: >> Hello, >> >> syzbot found the following issue on: >> >> HEAD commit: 1a9239bb4253 Merge tag 'net-next-6.15' of git://git.kernel.. >> git tree: upstream >> console output: https://syzkaller.appspot.com/x/log.txt?x=1384b43f980000 >> kernel config: https://syzkaller.appspot.com/x/.config?x=c7163a109ac459a8 >> dashboard link: https://syzkaller.appspot.com/bug?extid=4c7e0f9b94ad65811efb >> compiler: gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40 >> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=178cfa4c580000 >> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=11a8ca4c580000 >> >> Downloadable assets: >> disk image: https://storage.googleapis.com/syzbot-assets/fc7dc9f0d9a7/disk-1a9239bb.raw.xz >> vmlinux: https://storage.googleapis.com/syzbot-assets/f555a3ae03d3/vmlinux-1a9239bb.xz >> kernel image: https://storage.googleapis.com/syzbot-assets/55f6ea74eaf2/bzImage-1a9239bb.xz >> >> IMPORTANT: if you fix the issue, please add the following tag to the commit: >> Reported-by: syzbot+4c7e0f9b94ad65811efb@xxxxxxxxxxxxxxxxxxxxxxxxx >> > > ... > >> >> If you want syzbot to run the reproducer, reply with: >> #syz test: git://repo/address.git branch-or-commit-hash >> If you attach or paste a git patch, syzbot will apply it before testing. > > > diff --git a/block/blk-mq.c b/block/blk-mq.c > index ae8494d88897..d7a103dc258b 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -4465,14 +4465,12 @@ static struct blk_mq_hw_ctx *blk_mq_alloc_and_init_hctx( > return NULL; > } > > -static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set, > - struct request_queue *q) > +static void __blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set, > + struct request_queue *q) > { > struct blk_mq_hw_ctx *hctx; > unsigned long i, j; > > - /* protect against switching io scheduler */ > - mutex_lock(&q->elevator_lock); > for (i = 0; i < set->nr_hw_queues; i++) { > int old_node; > int node = blk_mq_get_hctx_node(set, i); > @@ -4505,7 +4503,19 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set, > > xa_for_each_start(&q->hctx_table, j, hctx, j) > blk_mq_exit_hctx(q, set, hctx, j); > - mutex_unlock(&q->elevator_lock); > +} > + > +static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set, > + struct request_queue *q, bool lock) > +{ > + if (lock) { > + /* protect against switching io scheduler */ > + mutex_lock(&q->elevator_lock); > + __blk_mq_realloc_hw_ctxs(set, q); > + mutex_unlock(&q->elevator_lock); > + } else { > + __blk_mq_realloc_hw_ctxs(set, q); > + } > > /* unregister cpuhp callbacks for exited hctxs */ > blk_mq_remove_hw_queues_cpuhp(q); > @@ -4537,7 +4547,7 @@ int blk_mq_init_allocated_queue(struct blk_mq_tag_set *set, > > xa_init(&q->hctx_table); > > - blk_mq_realloc_hw_ctxs(set, q); > + blk_mq_realloc_hw_ctxs(set, q, false); > if (!q->nr_hw_queues) > goto err_hctxs; > > @@ -5033,7 +5043,7 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, > fallback: > blk_mq_update_queue_map(set); > list_for_each_entry(q, &set->tag_list, tag_set_list) { > - blk_mq_realloc_hw_ctxs(set, q); > + blk_mq_realloc_hw_ctxs(set, q, true); > > if (q->nr_hw_queues != set->nr_hw_queues) { > int i = prev_nr_hw_queues; > This patch looks good to me, however after we fix this one, I found another splat. I see that these new splats are side effect of commit ffa1e7ada456 ("block: Make request_queue lockdep splats show up earlier"). IMO in the block layer code (unless it's in an IO submission path or a path where we have already frozen queue) we may still want to allow memory allocation with GFP_KERNEL. So in that sense, for example, we may acquire ->elevator_lock followed by fs_reclaim. Or in another words, shouldn't it be legitimate to acquire blk layer specific lock and then allocate memory using GFP_KERNEL assuming we haven't freezed queue or we're not in IO submission path. But this commit ffa1e7ada456 ("block: Make request_queue lockdep splats show up earlier") now showing up some false-positive splat as well, please see below: ====================================================== WARNING: possible circular locking dependency detected 6.14.0+ #147 Not tainted ------------------------------------------------------ bash/5903 is trying to acquire lock: c0000000ba0c6ad8 (&q->elevator_lock){+.+.}-{4:4}, at: elv_iosched_store+0x11c/0x5d4 but task is already holding lock: c0000000ba0c65b8 (&q->q_usage_counter(io)#20){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x28/0x40 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #2 (&q->q_usage_counter(io)#20){++++}-{0:0}: blk_alloc_queue+0x3a8/0x3e4 blk_mq_alloc_queue+0x88/0x11c __blk_mq_alloc_disk+0x34/0xd8 null_add_dev+0x3c8/0x914 [null_blk] null_init+0x1e0/0x4bc [null_blk] do_one_initcall+0x8c/0x4b8 do_init_module+0x7c/0x2c4 init_module_from_file+0xb4/0x108 idempotent_init_module+0x26c/0x368 sys_finit_module+0x98/0x150 system_call_exception+0x134/0x360 system_call_vectored_common+0x15c/0x2ec -> #1 (fs_reclaim){+.+.}-{0:0}: fs_reclaim_acquire+0xe4/0x120 kmem_cache_alloc_noprof+0x74/0x570 __kernfs_new_node+0x98/0x378 kernfs_new_node+0x80/0xc4 kernfs_create_dir_ns+0x44/0xec sysfs_create_dir_ns+0x94/0x160 kobject_add_internal+0xf4/0x3c8 kobject_add+0x70/0x10c elv_register_queue+0x70/0x14c blk_register_queue+0x1d8/0x2bc add_disk_fwnode+0x3b4/0x5d0 sd_probe+0x3bc/0x5b4 [sd_mod] really_probe+0x104/0x4c4 __driver_probe_device+0xb8/0x200 driver_probe_device+0x54/0x128 __driver_attach_async_helper+0x7c/0x150 async_run_entry_fn+0x60/0x1bc process_one_work+0x2ac/0x7e4 worker_thread+0x238/0x460 kthread+0x158/0x188 start_kernel_thread+0x14/0x18 -> #0 (&q->elevator_lock){+.+.}-{4:4}: __lock_acquire+0x1b6c/0x2ae0 lock_acquire+0x140/0x430 __mutex_lock+0xf0/0xb00 elv_iosched_store+0x11c/0x5d4 queue_attr_store+0x12c/0x164 sysfs_kf_write+0x6c/0xb0 kernfs_fop_write_iter+0x1ac/0x2a8 vfs_write+0x410/0x584 ksys_write+0x84/0x140 system_call_exception+0x134/0x360 system_call_vectored_common+0x15c/0x2ec other info that might help us debug this: Chain exists of: &q->elevator_lock --> fs_reclaim --> &q->q_usage_counter(io)#20 Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&q->q_usage_counter(io)#20); lock(fs_reclaim); lock(&q->q_usage_counter(io)#20); lock(&q->elevator_lock); *** DEADLOCK *** 5 locks held by bash/5903: #0: c00000005cb7f400 (sb_writers#3){.+.+}-{0:0}, at: ksys_write+0x84/0x140 #1: c000000008711288 (&of->mutex#2){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x168/0x2a8 #2: c00000000a1e2c08 (kn->active#57){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x174/0x2a8 #3: c0000000ba0c65b8 (&q->q_usage_counter(io)#20){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x28/0x40 #4: c0000000ba0c65f0 (&q->q_usage_counter(queue)#21){+.+.}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x28/0x40 stack backtrace: CPU: 17 UID: 0 PID: 5903 Comm: bash Kdump: loaded Not tainted 6.14.0+ #147 VOLUNTARY Hardware name: IBM,9043-MRX POWER10 (architected) 0x800200 0xf000006 of:IBM,FW1060.00 (NM1060_028) hv:phyp pSeries Call Trace: [c0000000955df580] [c0000000011a7ef8] dump_stack_lvl+0x108/0x18c (unreliable) [c0000000955df5b0] [c000000000225b0c] print_circular_bug+0x448/0x604 [c0000000955df660] [c000000000225f14] check_noncircular+0x24c/0x26c [c0000000955df730] [c00000000022c3e8] __lock_acquire+0x1b6c/0x2ae0 [c0000000955df860] [c000000000229700] lock_acquire+0x140/0x430 [c0000000955df960] [c0000000011e84e8] __mutex_lock+0xf0/0xb00 [c0000000955dfa90] [c0000000008fb6f8] elv_iosched_store+0x11c/0x5d4 [c0000000955dfb50] [c000000000903ec0] queue_attr_store+0x12c/0x164 [c0000000955dfc60] [c0000000007ca58c] sysfs_kf_write+0x6c/0xb0 [c0000000955dfca0] [c0000000007c8df0] kernfs_fop_write_iter+0x1ac/0x2a8 [c0000000955dfcf0] [c0000000006a8c9c] vfs_write+0x410/0x584 [c0000000955dfdc0] [c0000000006a9148] ksys_write+0x84/0x140 [c0000000955dfe10] [c000000000031814] system_call_exception+0x134/0x360 [c0000000955dfe50] [c00000000000cedc] system_call_vectored_common+0x15c/0x2ec What do you think? Thanks, --Nilay
