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Date: Wed, 21 Jun 2023 10:25:06 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Song Liu <song@...nel.org>, Yu Kuai <yukuai1@...weicloud.com>
Cc: pmenzel@...gen.mpg.de, akpm@...ux-foundation.org, neilb@...e.de,
linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
yi.zhang@...wei.com, yangerkun@...wei.com,
"yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v3] md: fix duplicate filename for rdev
Hi,
在 2023/06/21 5:38, Song Liu 写道:
> On Tue, May 23, 2023 at 6:33 PM Yu Kuai <yukuai1@...weicloud.com> wrote:
>>
>> Hi,
>>
>> 在 2023/05/24 2:05, Song Liu 写道:
>>> On Mon, May 22, 2023 at 6:30 PM Yu Kuai <yukuai1@...weicloud.com> wrote:
>>>>
>>>> From: Yu Kuai <yukuai3@...wei.com>
>>>>
>>>> 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@huaweicloud.com/
>>>> Fixes: 5792a2856a63 ("[PATCH] md: avoid a deadlock when removing a device from an md array via sysfs")
>>>> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
>>>
>>> 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, I think following patch can fix this, but run this test keep
failing in my VM. Song, can you pass this test?
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 1be64ab76f5c..62b7d2d0e873 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -766,17 +766,29 @@ static void md_free_rdev(struct mddev *mddev)
{
struct md_rdev *rdev;
struct md_rdev *tmp;
+ LIST_HEAD(delete);
- mutex_lock(&mddev->delete_mutex);
- if (list_empty(&mddev->deleting))
- goto out;
+retry:
+ if (list_empty_careful(&mddev->deleting))
+ return;
+
+ if (!mutex_trylock(&mddev->delete_mutex))
+ goto retry;
- list_for_each_entry_safe(rdev, tmp, &mddev->deleting, same_set) {
+ /*
+ * Clear 'mddev->deleting' first, because export_rdev can trigger
+ * mddev_unlock(), and grab 'delete_mutex' again will cause
deadlock.
+ * make sure md_free_rdev() will exit directly untill all rdev is
+ * deleted.
+ */
+ list_splice(&delete, &mddev->deleting);
+
+ list_for_each_entry_safe(rdev, tmp, &delete, same_set) {
list_del_init(&rdev->same_set);
kobject_del(&rdev->kobj);
export_rdev(rdev, mddev);
}
-out:
+
mutex_unlock(&mddev->delete_mutex);
}
> 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>
> .
>
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