The patch below does not apply to the 6.6-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.
To reproduce the conflict and resubmit, you may use the following commands:
git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y git checkout FETCH_HEAD git cherry-pick -x 54db6d1bdd71fa90172a2a6aca3308bbf7fa7eb5 # <resolve conflicts, build, test, etc.> git commit -s git send-email --to 'stable@vger.kernel.org' --in-reply-to '2025051954-gratified-manager-da96@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..
Possible dependencies:
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 54db6d1bdd71fa90172a2a6aca3308bbf7fa7eb5 Mon Sep 17 00:00:00 2001 From: Filipe Manana fdmanana@suse.com Date: Mon, 5 May 2025 16:03:16 +0100 Subject: [PATCH] btrfs: fix discard worker infinite loop after disabling discard
If the discard worker is running and there's currently only one block group, that block group is a data block group, it's in the unused block groups discard list and is being used (it got an extent allocated from it after becoming unused), the worker can end up in an infinite loop if a transaction abort happens or the async discard is disabled (during remount or unmount for example).
This happens like this:
1) Task A, the discard worker, is at peek_discard_list() and find_next_block_group() returns block group X;
2) Block group X is in the unused block groups discard list (its discard index is BTRFS_DISCARD_INDEX_UNUSED) since at some point in the past it become an unused block group and was added to that list, but then later it got an extent allocated from it, so its ->used counter is not zero anymore;
3) The current transaction is aborted by task B and we end up at __btrfs_handle_fs_error() in the transaction abort path, where we call btrfs_discard_stop(), which clears BTRFS_FS_DISCARD_RUNNING from fs_info, and then at __btrfs_handle_fs_error() we set the fs to RO mode (setting SB_RDONLY in the super block's s_flags field);
4) Task A calls __add_to_discard_list() with the goal of moving the block group from the unused block groups discard list into another discard list, but at __add_to_discard_list() we end up doing nothing because btrfs_run_discard_work() returns false, since the super block has SB_RDONLY set in its flags and BTRFS_FS_DISCARD_RUNNING is not set anymore in fs_info->flags. So block group X remains in the unused block groups discard list;
5) Task A then does a goto into the 'again' label, calls find_next_block_group() again we gets block group X again. Then it repeats the previous steps over and over since there are not other block groups in the discard lists and block group X is never moved out of the unused block groups discard list since btrfs_run_discard_work() keeps returning false and therefore __add_to_discard_list() doesn't move block group X out of that discard list.
When this happens we can get a soft lockup report like this:
[71.957] watchdog: BUG: soft lockup - CPU#0 stuck for 27s! [kworker/u4:3:97] [71.957] Modules linked in: xfs af_packet rfkill (...) [71.957] CPU: 0 UID: 0 PID: 97 Comm: kworker/u4:3 Tainted: G W 6.14.2-1-default #1 openSUSE Tumbleweed 968795ef2b1407352128b466fe887416c33af6fa [71.957] Tainted: [W]=WARN [71.957] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.2-3-gd478f380-rebuilt.opensuse.org 04/01/2014 [71.957] Workqueue: btrfs_discard btrfs_discard_workfn [btrfs] [71.957] RIP: 0010:btrfs_discard_workfn+0xc4/0x400 [btrfs] [71.957] Code: c1 01 48 83 (...) [71.957] RSP: 0018:ffffafaec03efe08 EFLAGS: 00000246 [71.957] RAX: ffff897045500000 RBX: ffff8970413ed8d0 RCX: 0000000000000000 [71.957] RDX: 0000000000000001 RSI: ffff8970413ed8d0 RDI: 0000000a8f1272ad [71.957] RBP: 0000000a9d61c60e R08: ffff897045500140 R09: 8080808080808080 [71.957] R10: ffff897040276800 R11: fefefefefefefeff R12: ffff8970413ed860 [71.957] R13: ffff897045500000 R14: ffff8970413ed868 R15: 0000000000000000 [71.957] FS: 0000000000000000(0000) GS:ffff89707bc00000(0000) knlGS:0000000000000000 [71.957] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [71.957] CR2: 00005605bcc8d2f0 CR3: 000000010376a001 CR4: 0000000000770ef0 [71.957] PKRU: 55555554 [71.957] Call Trace: [71.957] <TASK> [71.957] process_one_work+0x17e/0x330 [71.957] worker_thread+0x2ce/0x3f0 [71.957] ? __pfx_worker_thread+0x10/0x10 [71.957] kthread+0xef/0x220 [71.957] ? __pfx_kthread+0x10/0x10 [71.957] ret_from_fork+0x34/0x50 [71.957] ? __pfx_kthread+0x10/0x10 [71.957] ret_from_fork_asm+0x1a/0x30 [71.957] </TASK> [71.957] Kernel panic - not syncing: softlockup: hung tasks [71.987] CPU: 0 UID: 0 PID: 97 Comm: kworker/u4:3 Tainted: G W L 6.14.2-1-default #1 openSUSE Tumbleweed 968795ef2b1407352128b466fe887416c33af6fa [71.989] Tainted: [W]=WARN, [L]=SOFTLOCKUP [71.989] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.2-3-gd478f380-rebuilt.opensuse.org 04/01/2014 [71.991] Workqueue: btrfs_discard btrfs_discard_workfn [btrfs] [71.992] Call Trace: [71.993] <IRQ> [71.994] dump_stack_lvl+0x5a/0x80 [71.994] panic+0x10b/0x2da [71.995] watchdog_timer_fn.cold+0x9a/0xa1 [71.996] ? __pfx_watchdog_timer_fn+0x10/0x10 [71.997] __hrtimer_run_queues+0x132/0x2a0 [71.997] hrtimer_interrupt+0xff/0x230 [71.998] __sysvec_apic_timer_interrupt+0x55/0x100 [71.999] sysvec_apic_timer_interrupt+0x6c/0x90 [72.000] </IRQ> [72.000] <TASK> [72.001] asm_sysvec_apic_timer_interrupt+0x1a/0x20 [72.002] RIP: 0010:btrfs_discard_workfn+0xc4/0x400 [btrfs] [72.002] Code: c1 01 48 83 (...) [72.005] RSP: 0018:ffffafaec03efe08 EFLAGS: 00000246 [72.006] RAX: ffff897045500000 RBX: ffff8970413ed8d0 RCX: 0000000000000000 [72.006] RDX: 0000000000000001 RSI: ffff8970413ed8d0 RDI: 0000000a8f1272ad [72.007] RBP: 0000000a9d61c60e R08: ffff897045500140 R09: 8080808080808080 [72.008] R10: ffff897040276800 R11: fefefefefefefeff R12: ffff8970413ed860 [72.009] R13: ffff897045500000 R14: ffff8970413ed868 R15: 0000000000000000 [72.010] ? btrfs_discard_workfn+0x51/0x400 [btrfs 23b01089228eb964071fb7ca156eee8cd3bf996f] [72.011] process_one_work+0x17e/0x330 [72.012] worker_thread+0x2ce/0x3f0 [72.013] ? __pfx_worker_thread+0x10/0x10 [72.014] kthread+0xef/0x220 [72.014] ? __pfx_kthread+0x10/0x10 [72.015] ret_from_fork+0x34/0x50 [72.015] ? __pfx_kthread+0x10/0x10 [72.016] ret_from_fork_asm+0x1a/0x30 [72.017] </TASK> [72.017] Kernel Offset: 0x15000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff) [72.019] Rebooting in 90 seconds..
So fix this by making sure we move a block group out of the unused block groups discard list when calling __add_to_discard_list().
Fixes: 2bee7eb8bb81 ("btrfs: discard one region at a time in async discard") Link: https://bugzilla.suse.com/show_bug.cgi?id=1242012 CC: stable@vger.kernel.org # 5.10+ Reviewed-by: Boris Burkov boris@bur.io Reviewed-by: Daniel Vacek neelx@suse.com Signed-off-by: Filipe Manana fdmanana@suse.com Signed-off-by: David Sterba dsterba@suse.com
diff --git a/fs/btrfs/discard.c b/fs/btrfs/discard.c index d6eef4bd9e9d..de23c4b3515e 100644 --- a/fs/btrfs/discard.c +++ b/fs/btrfs/discard.c @@ -94,8 +94,6 @@ static void __add_to_discard_list(struct btrfs_discard_ctl *discard_ctl, struct btrfs_block_group *block_group) { lockdep_assert_held(&discard_ctl->lock); - if (!btrfs_run_discard_work(discard_ctl)) - return;
if (list_empty(&block_group->discard_list) || block_group->discard_index == BTRFS_DISCARD_INDEX_UNUSED) { @@ -118,6 +116,9 @@ static void add_to_discard_list(struct btrfs_discard_ctl *discard_ctl, if (!btrfs_is_block_group_data_only(block_group)) return;
+ if (!btrfs_run_discard_work(discard_ctl)) + return; + spin_lock(&discard_ctl->lock); __add_to_discard_list(discard_ctl, block_group); spin_unlock(&discard_ctl->lock); @@ -244,6 +245,18 @@ static struct btrfs_block_group *peek_discard_list( block_group->used != 0) { if (btrfs_is_block_group_data_only(block_group)) { __add_to_discard_list(discard_ctl, block_group); + /* + * The block group must have been moved to other + * discard list even if discard was disabled in + * the meantime or a transaction abort happened, + * otherwise we can end up in an infinite loop, + * always jumping into the 'again' label and + * keep getting this block group over and over + * in case there are no other block groups in + * the discard lists. + */ + ASSERT(block_group->discard_index != + BTRFS_DISCARD_INDEX_UNUSED); } else { list_del_init(&block_group->discard_list); btrfs_put_block_group(block_group);