* [PATCH] nbd: reclassify u->iolock of AF_UNIX sockets
@ 2026-07-13 18:14 Christian Borntraeger
2026-07-14 16:19 ` Christian Borntraeger
0 siblings, 1 reply; 2+ messages in thread
From: Christian Borntraeger @ 2026-07-13 18:14 UTC (permalink / raw)
To: Josef Bacik
Cc: Jens Axboe, Christian Brauner, linux-block, nbd, linux-kernel,
Eric Dumazet, Christian Borntraeger, Christian Borntraeger
From: Christian Borntraeger <borntraeger@kernel.org>
in our CI we do get the following when unloading the nbd module.
[ 120.008698] block nbd0: NBD_DISCONNECT
[ 120.009078] block nbd0: Disconnected due to user request.
[ 120.009413] block nbd0: shutting down sockets
[ 120.269755] ======================================================
[ 120.269757] WARNING: possible circular locking dependency detected
[ 120.269759] 7.2.0-rc2-00005-g3e3aa6da87d3 #117 Not tainted
[ 120.269761] ------------------------------------------------------
[ 120.269762] modprobe/8168 is trying to acquire lock:
[ 120.269764] 000001e67da57138 (set->srcu){.+.?}-{0:0}, at: __synchronize_srcu+0x36/0x210
[ 120.269775]
but task is already holding lock:
[ 120.269776] 000001bf63462d90 (&q->elevator_lock){+.+.}-{3:3}, at: elevator_change+0xac/0x1d0
[ 120.269784] which lock already depends on the new lock.
[ 120.269786] the existing dependency chain (in reverse order) is:
[ 120.269788]
-> #5 (&q->elevator_lock){+.+.}-{3:3}:
[ 120.269792] lock_acquire+0x150/0x3f0
[ 120.269795] __mutex_lock+0xba/0xdc0
[ 120.269799] mutex_lock_nested+0x32/0x40
[ 120.269802] elevator_change+0xac/0x1d0
[ 120.269804] elv_iosched_store+0x194/0x1e0
[ 120.269807] kernfs_fop_write_iter+0x18c/0x270
[ 120.269810] vfs_write+0x23c/0x380
[ 120.269813] ksys_write+0x88/0x120
[ 120.269816] __do_syscall+0x170/0x750
[ 120.269818] system_call+0x72/0x90
[ 120.269821]
-> #4 (&q->q_usage_counter(io)#18){++++}-{0:0}:
[ 120.269871] lock_acquire+0x150/0x3f0
[ 120.269873] blk_alloc_queue+0x2fe/0x340
[ 120.269876] blk_mq_alloc_queue+0x6c/0xe0
[ 120.269879] __blk_mq_alloc_disk+0x2a/0x80
[ 120.269881] dasd_gendisk_alloc+0xd4/0x200
[ 120.269885] dasd_state_known_to_basic+0x34/0x1f0
[ 120.269890] dasd_change_state+0x28a/0x570
[ 120.269892] dasd_set_target_state+0x76/0xf0
[ 120.269895] dasd_generic_set_online+0xc8/0x2c0
[ 120.269897] ccw_device_set_online+0xc6/0x550
[ 120.269901] online_store+0xd2/0x270
[ 120.269904] kernfs_fop_write_iter+0x18c/0x270
[ 120.269906] vfs_write+0x23c/0x380
[ 120.269909] ksys_write+0x88/0x120
[ 120.269912] __do_syscall+0x170/0x750
[ 120.269914] system_call+0x72/0x90
[ 120.269916]
-> #3 (fs_reclaim){+.+.}-{0:0}:
[ 120.269920] lock_acquire+0x150/0x3f0
[ 120.269922] __fs_reclaim_acquire+0x44/0x50
[ 120.269925] fs_reclaim_acquire+0xbe/0x100
[ 120.269927] kmem_cache_alloc_noprof+0x6c/0x6d0
[ 120.269931] alloc_empty_file+0x58/0x150
[ 120.269933] dentry_open+0x3c/0xa0
[ 120.269935] pidfs_alloc_file+0xba/0x130
[ 120.269940] pidfd_prepare+0x8c/0xf0
[ 120.269945] scm_recv_unix+0xb0/0x130
[ 120.269949] __unix_dgram_recvmsg+0x2aa/0x430
[ 120.269954] sock_recvmsg+0x70/0xd0
[ 120.269956] ____sys_recvmsg+0x8a/0x190
[ 120.269958] ___sys_recvmsg+0x88/0xd0
[ 120.269960] __sys_recvmsg+0x76/0xd0
[ 120.269962] __do_sys_socketcall+0x40a/0x460
[ 120.269964] __do_syscall+0x170/0x750
[ 120.269967] system_call+0x72/0x90
[ 120.269969]
-> #2 (&u->iolock){+.+.}-{3:3}:
[ 120.269973] lock_acquire+0x150/0x3f0
[ 120.269975] __mutex_lock+0xba/0xdc0
[ 120.269978] mutex_lock_nested+0x32/0x40
[ 120.269981] unix_stream_read_generic+0x11e/0xa60
[ 120.269985] unix_stream_recvmsg+0xae/0xc0
[ 120.269988] sock_recvmsg+0x70/0xd0
[ 120.269990] __sock_xmit+0xf0/0x1b0 [nbd]
[ 120.269997] nbd_handle_reply+0x284/0x770 [nbd]
[ 120.270001] recv_work+0x10c/0x290 [nbd]
[ 120.270004] process_one_work+0x2ba/0x800
[ 120.270009] worker_thread+0x21a/0x400
[ 120.270012] kthread+0x164/0x190
[ 120.270016] __ret_from_fork+0x4c/0x340
[ 120.270020] ret_from_fork+0xa/0x30
[ 120.270022]
-> #1 (&cmd->lock){+.+.}-{3:3}:
[ 120.270026] lock_acquire+0x150/0x3f0
[ 120.270028] __mutex_lock+0xba/0xdc0
[ 120.270031] mutex_lock_nested+0x32/0x40
[ 120.270034] nbd_queue_rq+0x38/0x70 [nbd]
[ 120.270037] blk_mq_dispatch_rq_list+0x19c/0x540
[ 120.270040] __blk_mq_do_dispatch_sched+0x3ce/0x3e0
[ 120.270043] __blk_mq_sched_dispatch_requests+0x198/0x1e0
[ 120.270046] blk_mq_sched_dispatch_requests+0x3c/0x90
[ 120.270049] blk_mq_run_work_fn+0x9e/0x180
[ 120.270052] process_one_work+0x2ba/0x800
[ 120.270055] worker_thread+0x21a/0x400
[ 120.270058] kthread+0x164/0x190
[ 120.270060] __ret_from_fork+0x4c/0x340
[ 120.270063] ret_from_fork+0xa/0x30
[ 120.270065]
-> #0 (set->srcu){.+.?}-{0:0}:
[ 120.270069] check_prev_add+0x160/0xf40
[ 120.270073] __lock_acquire+0x12aa/0x15a0
[ 120.270075] lock_sync+0xf4/0x190
[ 120.270077] __synchronize_srcu+0x5c/0x210
[ 120.270080] elevator_switch+0x190/0x340
[ 120.270082] elevator_change+0x152/0x1d0
[ 120.270084] elevator_set_none+0x44/0x90
[ 120.270087] blk_unregister_queue+0xd4/0x130
[ 120.270090] __del_gendisk+0x172/0x3c0
[ 120.270093] del_gendisk+0x86/0xb0
[ 120.270096] nbd_dev_remove+0x30/0x90 [nbd]
[ 120.270100] nbd_cleanup+0x11a/0x3a8 [nbd]
[ 120.270104] __do_sys_delete_module+0x18e/0x340
[ 120.270107] __do_syscall+0x170/0x750
[ 120.270110] system_call+0x72/0x90
[ 120.270112]
other info that might help us debug this:
[ 120.270114] Chain exists of:
set->srcu --> &q->q_usage_counter(io)#18 --> &q->elevator_lock
[ 120.270120] Possible unsafe locking scenario:
[ 120.270121] CPU0 CPU1
[ 120.270122] ---- ----
[ 120.270124] lock(&q->elevator_lock);
[ 120.270126] lock(&q->q_usage_counter(io)#18);
[ 120.270129] lock(&q->elevator_lock);
[ 120.270132] sync(set->srcu);
[ 120.270134]
*** DEADLOCK ***
[ 120.270136] 2 locks held by modprobe/8168:
[ 120.270138] #0: 000001bf44ce11e0 (&set->update_nr_hwq_lock){++++}-{3:3}, at: del_gendisk+0x76/0xb0
[ 120.270145] #1: 000001bf63462d90 (&q->elevator_lock){+.+.}-{3:3}, at: elevator_change+0xac/0x1d0
[ 120.270152]
stack backtrace:
[ 120.270155] CPU: 48 UID: 0 PID: 8168 Comm: modprobe Not tainted 7.2.0-rc2-00005-g3e3aa6da87d3 #117 PREEMPT
[ 120.270157] Hardware name: IBM 9175 ME1 701 (LPAR)
[ 120.270158] Call Trace:
[ 120.270158] [<000002c7d1164dde>] dump_stack_lvl+0xae/0x108
[ 120.270160] [<000002c7d126e5b4>] print_circular_bug+0x1a4/0x230
[ 120.270163] [<000002c7d126e7cc>] check_noncircular+0x18c/0x1b0
[ 120.270165] [<000002c7d126fb80>] check_prev_add+0x160/0xf40
[ 120.270167] [<000002c7d12736fa>] __lock_acquire+0x12aa/0x15a0
[ 120.270168] [<000002c7d1275ce4>] lock_sync+0xf4/0x190
[ 120.270169] [<000002c7d129c8fc>] __synchronize_srcu+0x5c/0x210
[ 120.270171] [<000002c7d1bb4790>] elevator_switch+0x190/0x340
[ 120.270173] [<000002c7d1bb4a92>] elevator_change+0x152/0x1d0
[ 120.270174] [<000002c7d1bb53c4>] elevator_set_none+0x44/0x90
[ 120.270176] [<000002c7d1bbee84>] blk_unregister_queue+0xd4/0x130
[ 120.270177] [<000002c7d1bdff42>] __del_gendisk+0x172/0x3c0
[ 120.270179] [<000002c7d1be0216>] del_gendisk+0x86/0xb0
[ 120.270182] [<000002c751b2adc0>] nbd_dev_remove+0x30/0x90 [nbd]
[ 120.270185] [<000002c751b30d72>] nbd_cleanup+0x11a/0x3a8 [nbd]
[ 120.270189] [<000002c7d12c72ce>] __do_sys_delete_module+0x18e/0x340
[ 120.270191] [<000002c7d22dc170>] __do_syscall+0x170/0x750
[ 120.270192] [<000002c7d22f0db2>] system_call+0x72/0x90
[ 120.270194] INFO: lockdep is turned off.
this seems to be a false positive.
Commit d532cddb6c60 ("nbd: Reclassify sockets to avoid lockdep
circular dependency") already reclassifies sk_lock and slock of
sockets handed to nbd, including AF_UNIX ones, precisely to keep
lockdep from reporting.
That is not sufficient for AF_UNIX: the unix stream/dgram recvmsg and
sendmsg paths do not serialize on sk_lock but on unix_sk(sk)->iolock,
and that mutex keeps the generic "&u->iolock" class shared by every
AF_UNIX socket in the system.
Teach lockdep about the lock instances to avoid this report.
This gets rid of the lockdep warning.
Assisted-by: Claude:claude-fable-5
Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
---
drivers/block/nbd.c | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 8f10762e90ef..a811e431b47a 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -32,6 +32,7 @@
#include <linux/err.h>
#include <linux/kernel.h>
#include <linux/slab.h>
+#include <net/af_unix.h>
#include <net/sock.h>
#include <linux/net.h>
#include <linux/kthread.h>
@@ -1241,6 +1242,7 @@ static struct socket *nbd_get_socket(struct nbd_device *nbd, unsigned long fd,
#ifdef CONFIG_DEBUG_LOCK_ALLOC
static struct lock_class_key nbd_key[3];
static struct lock_class_key nbd_slock_key[3];
+static struct lock_class_key nbd_unix_iolock_key;
static void nbd_reclassify_socket(struct socket *sock)
{
@@ -1267,6 +1269,17 @@ static void nbd_reclassify_socket(struct socket *sock)
&nbd_slock_key[2],
"sk_lock-AF_UNIX-NBD",
&nbd_key[2]);
+ /*
+ * The AF_UNIX stream recvmsg/sendmsg paths serialize on
+ * u->iolock, not sk_lock, so it must be reclassified as
+ * well. A held mutex cannot be reclassified; skip it in
+ * that case, as sock_allow_reclassification() does for
+ * sk_lock.
+ */
+ if (!mutex_is_locked(&unix_sk(sk)->iolock))
+ lockdep_set_class_and_name(&unix_sk(sk)->iolock,
+ &nbd_unix_iolock_key,
+ "&u->iolock-NBD");
break;
}
}
--
2.53.0
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] nbd: reclassify u->iolock of AF_UNIX sockets
2026-07-13 18:14 [PATCH] nbd: reclassify u->iolock of AF_UNIX sockets Christian Borntraeger
@ 2026-07-14 16:19 ` Christian Borntraeger
0 siblings, 0 replies; 2+ messages in thread
From: Christian Borntraeger @ 2026-07-14 16:19 UTC (permalink / raw)
To: Josef Bacik, Eric Dumazet
Cc: Jens Axboe, Christian Brauner, linux-block, nbd, linux-kernel
Am 13.07.26 um 20:14 schrieb Christian Borntraeger:
I just realized I had my kernel.org address in from.
question is, is this the right fix and shall I do a proper patch wit correct from and signoff.
[..]
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 8f10762e90ef..a811e431b47a 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -32,6 +32,7 @@
> #include <linux/err.h>
> #include <linux/kernel.h>
> #include <linux/slab.h>
> +#include <net/af_unix.h>
> #include <net/sock.h>
> #include <linux/net.h>
> #include <linux/kthread.h>
> @@ -1241,6 +1242,7 @@ static struct socket *nbd_get_socket(struct nbd_device *nbd, unsigned long fd,
> #ifdef CONFIG_DEBUG_LOCK_ALLOC
> static struct lock_class_key nbd_key[3];
> static struct lock_class_key nbd_slock_key[3];
> +static struct lock_class_key nbd_unix_iolock_key;
>
> static void nbd_reclassify_socket(struct socket *sock)
> {
> @@ -1267,6 +1269,17 @@ static void nbd_reclassify_socket(struct socket *sock)
> &nbd_slock_key[2],
> "sk_lock-AF_UNIX-NBD",
> &nbd_key[2]);
> + /*
> + * The AF_UNIX stream recvmsg/sendmsg paths serialize on
> + * u->iolock, not sk_lock, so it must be reclassified as
> + * well. A held mutex cannot be reclassified; skip it in
> + * that case, as sock_allow_reclassification() does for
> + * sk_lock.
> + */
> + if (!mutex_is_locked(&unix_sk(sk)->iolock))
> + lockdep_set_class_and_name(&unix_sk(sk)->iolock,
> + &nbd_unix_iolock_key,
> + "&u->iolock-NBD");
> break;
> }
> }
^ permalink raw reply [flat|nested] 2+ messages in thread
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