On Tue, May 23, 2023 at 6:33 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> 在 2023/05/24 2:05, Song Liu 写道:
> > On Mon, May 22, 2023 at 6:30 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
> >>
> >> From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >>
> >> Commit 5792a2856a63 ("[PATCH] md: avoid a deadlock when removing a device
> >> from an md array via sysfs") delays the deletion of rdev, however, this
> >> introduces a window that rdev can be added again while the deletion is
> >> not done yet, and sysfs will complain about duplicate filename.
> >>
> >> Follow up patches try to fix this problem by flushing workqueue, however,
> >> flush_rdev_wq() is just dead code, the progress in
> >> md_kick_rdev_from_array():
> >>
> >> 1) list_del_rcu(&rdev->same_set);
> >> 2) synchronize_rcu();
> >> 3) queue_work(md_rdev_misc_wq, &rdev->del_work);
> >>
> >> So in flush_rdev_wq(), if rdev is found in the list, work_pending() can
> >> never pass, in the meantime, if work is queued, then rdev can never be
> >> found in the list.
> >>
> >> flush_rdev_wq() can be replaced by flush_workqueue() directly, however,
> >> this approach is not good:
> >> - the workqueue is global, this synchronization for all raid disks is
> >> not necessary.
> >> - flush_workqueue can't be called under 'reconfig_mutex', there is still
> >> a small window between flush_workqueue() and mddev_lock() that other
> >> contexts can queue new work, hence the problem is not solved completely.
> >>
> >> sysfs already has apis to support delete itself through writer, and
> >> these apis, specifically sysfs_break/unbreak_active_protection(), is used
> >> to support deleting rdev synchronously. Therefore, the above commit can be
> >> reverted, and sysfs duplicate filename can be avoided.
> >>
> >> A new mdadm regression test is proposed as well([1]).
> >>
> >> [1] https://lore.kernel.org/linux-raid/20230428062845.1975462-1-yukuai1@xxxxxxxxxxxxxxx/
> >> Fixes: 5792a2856a63 ("[PATCH] md: avoid a deadlock when removing a device from an md array via sysfs")
> >> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > Thanks for the fix! I made the following changes and applied it
> > to md-next:
> >
> > 1. remove md_rdev->del_work, which is not used any more;
> > 2. change list_empty_safe to list_empty protected by the mutex, as
> > list_empty_safe doesn't seem safe here.
Hmm.. it appears that I missed a circular locking dependency with mdadm
test 21raid5cache (delete_mutex and open_mutex).
Please take a look at this.
Thanks,
Song
[ 239.802277] ======================================================
[ 239.803650] WARNING: possible circular locking dependency detected
[ 239.804929] 6.4.0-rc2+ #772 Not tainted
[ 239.805569] ------------------------------------------------------
[ 239.806568] kworker/20:1/222 is trying to acquire lock:
[ 239.807406] ffff88815335b3f0 (&mddev->delete_mutex){+.+.}-{3:3},
at: mddev_unlock+0xe0/0x2d0
[ 239.808653]
but task is already holding lock:
[ 239.809481] ffffc9000246fe00
((work_completion)(&log->disable_writeback_work)){+.+.}-{0:0}, at:
process_one_work+0x462/0xa50
[ 239.811049]
which lock already depends on the new lock.
[ 239.812187]
the existing dependency chain (in reverse order) is:
[ 239.813230]
-> #3
((work_completion)(&log->disable_writeback_work)){+.+.}-{0:0}:
[ 239.814468] __flush_work+0xdb/0x690
[ 239.815068] r5l_exit_log+0x59/0xc0
[ 239.815649] free_conf+0x34/0x320
[ 239.816243] raid5_free+0x11/0x40
[ 239.816788] __md_stop+0x9f/0x140
[ 239.817336] do_md_stop+0x2af/0xaf0
[ 239.817901] md_ioctl+0xb34/0x1e30
[ 239.818469] blkdev_ioctl+0x2bf/0x3d0
[ 239.819079] __x64_sys_ioctl+0xbe/0x100
[ 239.819701] do_syscall_64+0x3a/0x90
[ 239.820294] entry_SYSCALL_64_after_hwframe+0x72/0xdc
[ 239.821076]
-> #2 (&mddev->open_mutex){+.+.}-{3:3}:
[ 239.821971] __mutex_lock+0x11d/0x13f0
[ 239.822592] md_open+0xad/0x180
[ 239.822937] kobject: 'md0' (ffff88811c8e5498): kobject_uevent_env
[ 239.823113] blkdev_get_whole+0x50/0x120
[ 239.824216] kobject: 'md0' (ffff88811c8e5498): fill_kobj_path: path
= '/devices/virtual/block/md0'
[ 239.824725] blkdev_get_by_dev+0x309/0x4f0
[ 239.826747] blkdev_open+0x8a/0x110
[ 239.827319] do_dentry_open+0x2a5/0x7b0
[ 239.827939] path_openat+0xcee/0x1070
[ 239.828545] do_filp_open+0x148/0x1d0
[ 239.829137] do_sys_openat2+0x2ec/0x470
[ 239.829791] do_sys_open+0x8a/0xd0
[ 239.830352] do_syscall_64+0x3a/0x90
[ 239.830935] entry_SYSCALL_64_after_hwframe+0x72/0xdc
[ 239.831728]
-> #1 (&disk->open_mutex){+.+.}-{3:3}:
[ 239.832615] __mutex_lock+0x11d/0x13f0
[ 239.833231] blkdev_put+0x65/0x350
[ 239.833785] export_rdev.isra.63+0x72/0xe0
[ 239.834456] mddev_unlock+0x1b1/0x2d0
[ 239.835043] md_ioctl+0x96c/0x1e30
[ 239.835616] blkdev_ioctl+0x2bf/0x3d0
[ 239.836213] __x64_sys_ioctl+0xbe/0x100
[ 239.836828] do_syscall_64+0x3a/0x90
[ 239.837414] entry_SYSCALL_64_after_hwframe+0x72/0xdc
[ 239.838225]
-> #0 (&mddev->delete_mutex){+.+.}-{3:3}:
[ 239.839145] __lock_acquire+0x1e42/0x34b0
[ 239.839797] lock_acquire+0x161/0x3d0
[ 239.840395] __mutex_lock+0x11d/0x13f0
[ 239.840999] mddev_unlock+0xe0/0x2d0
[ 239.841582] r5c_disable_writeback_async+0x261/0x270
[ 239.842354] process_one_work+0x55f/0xa50
[ 239.842996] worker_thread+0x69/0x660
[ 239.843595] kthread+0x1a1/0x1e0
[ 239.844131] ret_from_fork+0x2c/0x50
[ 239.844712]
other info that might help us debug this:
[ 239.845813] Chain exists of:
&mddev->delete_mutex --> &mddev->open_mutex -->
(work_completion)(&log->disable_writeback_work)
[ 239.847776] Possible unsafe locking scenario:
[ 239.848623] CPU0 CPU1
[ 239.849262] ---- ----
[ 239.849927] lock((work_completion)(&log->disable_writeback_work));
[ 239.850831] lock(&mddev->open_mutex);
[ 239.851719]
lock((work_completion)(&log->disable_writeback_work));
[ 239.852945] lock(&mddev->delete_mutex);
[ 239.853517]
*** DEADLOCK ***
[ 239.854345] 2 locks held by kworker/20:1/222:
[ 239.854960] #0: ffff8881000b2748
((wq_completion)events){+.+.}-{0:0}, at: process_one_work+0x462/0xa50
[ 239.856287] #1: ffffc9000246fe00
((work_completion)(&log->disable_writeback_work)){+.+.}-{0:0}, at:
process_one_work+0x462/0xa50
[ 239.857939]
stack backtrace:
[ 239.858567] CPU: 20 PID: 222 Comm: kworker/20:1 Not tainted 6.4.0-rc2+ #772
[ 239.859533] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS rel-1.15.0-0-g2dd4b9b3f840-prebuilt.qemu.org 04/01/2014
[ 239.861135] Workqueue: events r5c_disable_writeback_async
[ 239.861903] Call Trace:
[ 239.862281] <TASK>
[ 239.862606] dump_stack_lvl+0x46/0x80
[ 239.863144] check_noncircular+0x1ff/0x240
[ 239.863737] ? __pfx_check_noncircular+0x10/0x10
[ 239.864401] ? mark_lock.part.45+0x11a/0x1350
[ 239.865019] ? add_chain_block+0x23b/0x310
[ 239.865624] __lock_acquire+0x1e42/0x34b0
[ 239.866202] ? select_task_rq_fair+0x2b0/0x1e50
[ 239.866845] ? __pfx___lock_acquire+0x10/0x10
[ 239.867473] ? ttwu_queue_wakelist+0x1cc/0x1f0
[ 239.868107] ? __smp_call_single_queue+0x137/0x2a0
[ 239.868816] ? __default_send_IPI_dest_field+0x2b/0xa0
[ 239.869548] ? __lock_acquire+0xa5d/0x34b0
[ 239.870131] lock_acquire+0x161/0x3d0
[ 239.870660] ? mddev_unlock+0xe0/0x2d0
[ 239.871216] ? __pfx_lock_acquire+0x10/0x10
[ 239.871808] ? lock_is_held_type+0xd8/0x130
[ 239.872405] __mutex_lock+0x11d/0x13f0
[ 239.872936] ? mddev_unlock+0xe0/0x2d0
[ 239.873474] ? mddev_unlock+0xe0/0x2d0
[ 239.874006] ? __mutex_unlock_slowpath+0x12c/0x410
[ 239.874695] ? __pfx___mutex_lock+0x10/0x10
[ 239.875297] ? __pfx_rcu_read_lock_held+0x10/0x10
[ 239.875986] ? mddev_unlock+0xe0/0x2d0
[ 239.876530] mddev_unlock+0xe0/0x2d0
[ 239.877044] r5c_disable_writeback_async+0x261/0x270
[ 239.877753] ? __pfx_r5c_disable_writeback_async+0x10/0x10
[ 239.878531] ? __switch_to+0x2d8/0x770
[ 239.879066] ? __pfx_autoremove_wake_function+0x10/0x10
[ 239.879813] process_one_work+0x55f/0xa50
[ 239.880398] ? __pfx_process_one_work+0x10/0x10
[ 239.881073] ? _raw_spin_lock_irq+0x5c/0x90
[ 239.881675] worker_thread+0x69/0x660
[ 239.882206] ? __kthread_parkme+0xe4/0x100
[ 239.882786] ? __pfx_worker_thread+0x10/0x10
[ 239.883395] kthread+0x1a1/0x1e0
[ 239.883863] ? __pfx_kthread+0x10/0x10
[ 239.884406] ret_from_fork+0x2c/0x50
[ 239.884925] </TASK>
