linux-kernel - dm-snap: there are issues for the rt kernel

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Message-ID: <20251128072524.1462256-1-jiping.ma2@windriver.com>
Date: Fri, 28 Nov 2025 07:25:24 +0000
From: Jiping Ma <jiping.ma2@...driver.com>
To: agk@...hat.com, snitzer@...nel.org, mpatocka@...hat.com,
        dm-devel@...ts.linux.dev
Cc: jiping.ma2@...driver.com, linux-kernel@...r.kernel.org
Subject: dm-snap: there are issues for the rt kernel

We observed instability for the rt kernel under the upgreade and rollback test in 6.6, 6.12 and mainline.

The issue is related with dm_exception_table_lock(&lock), in which function preempt_disable() is called twice.
The code block is between dm_exception_table_lock(&lock) and dm_exception_table_unlock(&lock),if the code involves rt_spin_lock that will trigger such as "BUG: scheduling while atomic: kworker/u72:11/349/0x00000003" because the preempt number is 3 in this time.

There are several places that involve the same issue in dm-snap.c, such as dm_add_exception(), pending_complete() and snapshot_map().

Do we need reimplement dm_exception_table_lock?
Any suggestions or assistance would be appreciated.

[  862.410151] Kernel panic - not syncing: scheduling while atomic: panic_on_warn set ...
[  862.580196] CPU: 2 UID: 0 PID: 349 Comm: kworker/u72:11 Kdump: loaded Tainted: G           O       6.12.0-1-rt-amd64 #1  Debian 6.12.40-1.stx.130
[  862.593223] Tainted: [O]=OOT_MODULE
[  862.596714] Hardware name: Dell Inc. PowerEdge R740xd/00WGD1, BIOS 2.24.0 03/27/2025
[  862.604453] Workqueue: writeback wb_workfn (flush-253:21)
[  862.609852] Call Trace:
[  862.612306]  <TASK>
[  862.614411]  panic+0x34a/0x370
[  862.617470]  check_panic_on_warn+0x50/0x50
[  862.621569]  __schedule_bug+0x4d/0x60
[  862.625236]  __schedule+0xa0c/0xbb0
[  862.628729]  schedule_rtlock+0x1a/0x30
[  862.632481]  rtlock_slowlock_locked+0x20b/0xcc0
[  862.637014]  rt_spin_lock+0x40/0x60
[  862.640506]  __insert_pending_exception+0x4e/0xe0 [dm_snapshot]
[  862.646424]  __origin_write+0x2fb/0x360 [dm_snapshot]
[  862.651477]  do_origin+0xd5/0xe0 [dm_snapshot]
[  862.655923]  __map_bio+0x17c/0x1b0 [dm_mod]
[  862.660117]  dm_submit_bio+0x1ad/0x5a0 [dm_mod]
[  862.664649]  __submit_bio+0x144/0x240
[  862.668315]  ? __submit_bio+0xc1/0x240
[  862.672067]  submit_bio_noacct_nocheck+0x19a/0x3c0
[  862.676860]  iomap_submit_ioend+0x42/0x80
[  862.680873]  iomap_writepages+0x5f8/0x8d0
[  862.684886]  xfs_vm_writepages+0x62/0x90 [xfs]
[  862.689473]  do_writepages+0xcc/0x240
[  862.693136]  __writeback_single_inode+0x41/0x330
[  862.697756]  writeback_sb_inodes+0x21c/0x4d0
[  862.702028]  wb_writeback+0x7c/0x2f0
[  862.705607]  wb_workfn+0xc1/0x450
[  862.708926]  process_one_work+0x179/0x390
[  862.712940]  worker_thread+0x237/0x340
[  862.716691]  ? __pfx_worker_thread+0x10/0x10
[  862.720964]  kthread+0xc6/0x100
[  862.724111]  ? __pfx_kthread+0x10/0x10
[  862.727863]  ret_from_fork+0x2d/0x50
[  862.731441]  ? __pfx_kthread+0x10/0x10
[  862.735193]  ret_from_fork_asm+0x1a/0x30
[  862.739122]  </TASK>

and

[   36.563812] BUG: scheduling while atomic: lvm/1380/0x00000003
......
[   36.563841] CPU: 32 PID: 1380 Comm: lvm Tainted: G           O       6.6.0-1-rt-amd64 #1  Debian 6.6.71-1.stx.104
[   36.563844] Hardware name: ZTSYSTEMS Galene EI/Galene, BIOS 1.01 12/07/2023
[   36.563845] Call Trace:
[   36.563848]  <TASK>
[   36.563849]  dump_stack_lvl+0x37/0x50
[   36.563855]  __schedule_bug+0x52/0x60
[   36.563859]  __schedule+0x87d/0xb10
[   36.563861]  ? update_load_avg+0x7e/0x750
[   36.563865]  schedule_rtlock+0x1f/0x40
[   36.563866]  rtlock_slowlock_locked+0x232/0xd40
[   36.563870]  ? __set_cpus_allowed_ptr+0x55/0xa0
[   36.563873]  ? dm_add_exception+0xb4/0xf0 [dm_snapshot]
[   36.563879]  rt_spin_lock+0x45/0x60
[   36.563881]  kmem_cache_free+0x182/0x480
[   36.563884]  dm_add_exception+0xb4/0xf0 [dm_snapshot]
[   36.563889]  persistent_read_metadata+0x29d/0x550 [dm_snapshot]
[   36.563895]  ? __pfx_dm_add_exception+0x10/0x10 [dm_snapshot]
[   36.563900]  snapshot_ctr+0x60b/0x8f0 [dm_snapshot]
[   36.563905]  dm_table_add_target+0x246/0x3b0 [dm_mod]
[   36.563919]  table_load+0x136/0x4b0 [dm_mod]
[   36.563930]  ? __pfx_table_load+0x10/0x10 [dm_mod]
[   36.563940]  ctl_ioctl+0x1b3/0x500 [dm_mod]
[   36.563950]  dm_ctl_ioctl+0xe/0x20 [dm_mod]
[   36.563960]  __x64_sys_ioctl+0x8f/0xd0
[   36.563964]  do_syscall_64+0x58/0xb0
[   36.563967]  ? dm_ctl_ioctl+0xe/0x20 [dm_mod]
[   36.563976]  ? __ct_user_enter+0x2f/0xd0
[   36.563978]  ? syscall_exit_to_user_mode+0x32/0x40
[   36.563980]  ? do_syscall_64+0x65/0xb0
[   36.563983]  ? exit_to_user_mode_prepare+0xa9/0x190
[   36.563985]  ? __ct_user_enter+0x2f/0xd0
[   36.563987]  ? syscall_exit_to_user_mode+0x32/0x40

Thanks,
Jiping