Re: [PATCH] btrfs: don't take dev_replace rwsem on task already holding it
From: Filipe Manana
Date: Mon Sep 09 2024 - 08:29:40 EST
On Mon, Sep 9, 2024 at 11:35 AM Johannes Thumshirn
<Johannes.Thumshirn@xxxxxxx> wrote:
>
> On 26.08.24 13:11, Filipe Manana wrote:
> > On Thu, Aug 15, 2024 at 1:57 PM Johannes Thumshirn <jth@xxxxxxxxxx> wrote:
> >>
> >> From: Johannes Thumshirn <johannes.thumshirn@xxxxxxx>
> >>
> >> Running fstests btrfs/011 with MKFS_OPTIONS="-O rst" to force the usage of
> >> the RAID stripe-tree, we get the following splat from lockdep:
> >>
> >> BTRFS info (device sdd): dev_replace from /dev/sdd (devid 1) to /dev/sdb started
> >>
> >> ============================================
> >> WARNING: possible recursive locking detected
> >> 6.11.0-rc3-btrfs-for-next #599 Not tainted
> >> --------------------------------------------
> >> btrfs/2326 is trying to acquire lock:
> >> ffff88810f215c98 (&fs_info->dev_replace.rwsem){++++}-{3:3}, at: btrfs_map_block+0x39f/0x2250
> >>
> >> but task is already holding lock:
> >> ffff88810f215c98 (&fs_info->dev_replace.rwsem){++++}-{3:3}, at: btrfs_map_block+0x39f/0x2250
> >>
> >> other info that might help us debug this:
> >> Possible unsafe locking scenario:
> >>
> >> CPU0
> >> ----
> >> lock(&fs_info->dev_replace.rwsem);
> >> lock(&fs_info->dev_replace.rwsem);
> >>
> >> *** DEADLOCK ***
> >>
> >> May be due to missing lock nesting notation
> >>
> >> 1 lock held by btrfs/2326:
> >> #0: ffff88810f215c98 (&fs_info->dev_replace.rwsem){++++}-{3:3}, at: btrfs_map_block+0x39f/0x2250
> >>
> >> stack backtrace:
> >> CPU: 1 UID: 0 PID: 2326 Comm: btrfs Not tainted 6.11.0-rc3-btrfs-for-next #599
> >> Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> >> Call Trace:
> >> <TASK>
> >> dump_stack_lvl+0x5b/0x80
> >> __lock_acquire+0x2798/0x69d0
> >> ? __pfx___lock_acquire+0x10/0x10
> >> ? __pfx___lock_acquire+0x10/0x10
> >> lock_acquire+0x19d/0x4a0
> >> ? btrfs_map_block+0x39f/0x2250
> >> ? __pfx_lock_acquire+0x10/0x10
> >> ? find_held_lock+0x2d/0x110
> >> ? lock_is_held_type+0x8f/0x100
> >> down_read+0x8e/0x440
> >> ? btrfs_map_block+0x39f/0x2250
> >> ? __pfx_down_read+0x10/0x10
> >> ? do_raw_read_unlock+0x44/0x70
> >> ? _raw_read_unlock+0x23/0x40
> >> btrfs_map_block+0x39f/0x2250
> >> ? btrfs_dev_replace_by_ioctl+0xd69/0x1d00
> >> ? btrfs_bio_counter_inc_blocked+0xd9/0x2e0
> >> ? __kasan_slab_alloc+0x6e/0x70
> >> ? __pfx_btrfs_map_block+0x10/0x10
> >> ? __pfx_btrfs_bio_counter_inc_blocked+0x10/0x10
> >> ? kmem_cache_alloc_noprof+0x1f2/0x300
> >> ? mempool_alloc_noprof+0xed/0x2b0
> >> btrfs_submit_chunk+0x28d/0x17e0
> >> ? __pfx_btrfs_submit_chunk+0x10/0x10
> >> ? bvec_alloc+0xd7/0x1b0
> >> ? bio_add_folio+0x171/0x270
> >> ? __pfx_bio_add_folio+0x10/0x10
> >> ? __kasan_check_read+0x20/0x20
> >> btrfs_submit_bio+0x37/0x80
> >> read_extent_buffer_pages+0x3df/0x6c0
> >> btrfs_read_extent_buffer+0x13e/0x5f0
> >> read_tree_block+0x81/0xe0
> >> read_block_for_search+0x4bd/0x7a0
> >> ? __pfx_read_block_for_search+0x10/0x10
> >> btrfs_search_slot+0x78d/0x2720
> >> ? __pfx_btrfs_search_slot+0x10/0x10
> >> ? lock_is_held_type+0x8f/0x100
> >> ? kasan_save_track+0x14/0x30
> >> ? __kasan_slab_alloc+0x6e/0x70
> >> ? kmem_cache_alloc_noprof+0x1f2/0x300
> >> btrfs_get_raid_extent_offset+0x181/0x820
> >> ? __pfx_lock_acquire+0x10/0x10
> >> ? __pfx_btrfs_get_raid_extent_offset+0x10/0x10
> >> ? down_read+0x194/0x440
> >> ? __pfx_down_read+0x10/0x10
> >> ? do_raw_read_unlock+0x44/0x70
> >> ? _raw_read_unlock+0x23/0x40
> >> btrfs_map_block+0x5b5/0x2250
> >> ? __pfx_btrfs_map_block+0x10/0x10
> >> scrub_submit_initial_read+0x8fe/0x11b0
> >> ? __pfx_scrub_submit_initial_read+0x10/0x10
> >> submit_initial_group_read+0x161/0x3a0
> >> ? lock_release+0x20e/0x710
> >> ? __pfx_submit_initial_group_read+0x10/0x10
> >> ? __pfx_lock_release+0x10/0x10
> >> scrub_simple_mirror.isra.0+0x3eb/0x580
> >> scrub_stripe+0xe4d/0x1440
> >> ? lock_release+0x20e/0x710
> >> ? __pfx_scrub_stripe+0x10/0x10
> >> ? __pfx_lock_release+0x10/0x10
> >> ? do_raw_read_unlock+0x44/0x70
> >> ? _raw_read_unlock+0x23/0x40
> >> scrub_chunk+0x257/0x4a0
> >> scrub_enumerate_chunks+0x64c/0xf70
> >> ? __mutex_unlock_slowpath+0x147/0x5f0
> >> ? __pfx_scrub_enumerate_chunks+0x10/0x10
> >> ? bit_wait_timeout+0xb0/0x170
> >> ? __up_read+0x189/0x700
> >> ? scrub_workers_get+0x231/0x300
> >> ? up_write+0x490/0x4f0
> >> btrfs_scrub_dev+0x52e/0xcd0
> >> ? create_pending_snapshots+0x230/0x250
> >> ? __pfx_btrfs_scrub_dev+0x10/0x10
> >> btrfs_dev_replace_by_ioctl+0xd69/0x1d00
> >> ? lock_acquire+0x19d/0x4a0
> >> ? __pfx_btrfs_dev_replace_by_ioctl+0x10/0x10
> >> ? lock_release+0x20e/0x710
> >> ? btrfs_ioctl+0xa09/0x74f0
> >> ? __pfx_lock_release+0x10/0x10
> >> ? do_raw_spin_lock+0x11e/0x240
> >> ? __pfx_do_raw_spin_lock+0x10/0x10
> >> btrfs_ioctl+0xa14/0x74f0
> >> ? lock_acquire+0x19d/0x4a0
> >> ? find_held_lock+0x2d/0x110
> >> ? __pfx_btrfs_ioctl+0x10/0x10
> >> ? lock_release+0x20e/0x710
> >> ? do_sigaction+0x3f0/0x860
> >> ? __pfx_do_vfs_ioctl+0x10/0x10
> >> ? do_raw_spin_lock+0x11e/0x240
> >> ? lockdep_hardirqs_on_prepare+0x270/0x3e0
> >> ? _raw_spin_unlock_irq+0x28/0x50
> >> ? do_sigaction+0x3f0/0x860
> >> ? __pfx_do_sigaction+0x10/0x10
> >> ? __x64_sys_rt_sigaction+0x18e/0x1e0
> >> ? __pfx___x64_sys_rt_sigaction+0x10/0x10
> >> ? __x64_sys_close+0x7c/0xd0
> >> __x64_sys_ioctl+0x137/0x190
> >> do_syscall_64+0x71/0x140
> >> entry_SYSCALL_64_after_hwframe+0x76/0x7e
> >> RIP: 0033:0x7f0bd1114f9b
> >> Code: Unable to access opcode bytes at 0x7f0bd1114f71.
> >> RSP: 002b:00007ffc8a8c3130 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> >> RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f0bd1114f9b
> >> RDX: 00007ffc8a8c35e0 RSI: 00000000ca289435 RDI: 0000000000000003
> >> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000007
> >> R10: 0000000000000008 R11: 0000000000000246 R12: 00007ffc8a8c6c85
> >> R13: 00000000398e72a0 R14: 0000000000004361 R15: 0000000000000004
> >> </TASK>
> >>
> >> This happens because on RAID stripe-tree filesystems we recurse back into
> >> btrfs_map_block() on scrub to perform the logical to device physical
> >> mapping.
> >>
> >> But as the device replace task is already holding the dev_replace::rwsem
> >> we deadlock.
> >>
> >> So don't take the dev_replace::rwsem in case our task is the task performing
> >> the device replace.
> >>
> >> Suggested-by: Filipe Manana <fdmanana@xxxxxxxx>
> >> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@xxxxxxx>
> >> ---
> >> fs/btrfs/dev-replace.c | 2 ++
> >> fs/btrfs/fs.h | 2 ++
> >> fs/btrfs/volumes.c | 4 +++-
> >> 3 files changed, 7 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
> >> index 83d5cdd77f29..604399e59a3d 100644
> >> --- a/fs/btrfs/dev-replace.c
> >> +++ b/fs/btrfs/dev-replace.c
> >> @@ -641,6 +641,7 @@ static int btrfs_dev_replace_start(struct btrfs_fs_info *fs_info,
> >> return ret;
> >>
> >> down_write(&dev_replace->rwsem);
> >> + dev_replace->replace_task = current;
> >> switch (dev_replace->replace_state) {
> >> case BTRFS_IOCTL_DEV_REPLACE_STATE_NEVER_STARTED:
> >> case BTRFS_IOCTL_DEV_REPLACE_STATE_FINISHED:
> >> @@ -994,6 +995,7 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
> >> list_add(&tgt_device->dev_alloc_list, &fs_devices->alloc_list);
> >> fs_devices->rw_devices++;
> >>
> >> + dev_replace->replace_task = NULL;
> >> up_write(&dev_replace->rwsem);
> >> btrfs_rm_dev_replace_blocked(fs_info);
> >>
> >> diff --git a/fs/btrfs/fs.h b/fs/btrfs/fs.h
> >> index 3d6d4b503220..53824da92cc3 100644
> >> --- a/fs/btrfs/fs.h
> >> +++ b/fs/btrfs/fs.h
> >> @@ -317,6 +317,8 @@ struct btrfs_dev_replace {
> >>
> >> struct percpu_counter bio_counter;
> >> wait_queue_head_t replace_wait;
> >> +
> >> + struct task_struct *replace_task;
> >> };
> >>
> >> /*
> >> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> >> index 4b9b647a7e29..d2e80a1f258d 100644
> >> --- a/fs/btrfs/volumes.c
> >> +++ b/fs/btrfs/volumes.c
> >> @@ -6481,7 +6481,9 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
> >> *length = min_t(u64, map->chunk_len - map_offset, max_len);
> >>
> >> down_read(&dev_replace->rwsem);
> >> - dev_replace_is_ongoing = btrfs_dev_replace_is_ongoing(dev_replace);
> >> + if (btrfs_dev_replace_is_ongoing(dev_replace) && dev_replace->replace_task != current)
> >> + dev_replace_is_ongoing = 1;
> >
> > So the idea I suggested was not like this.
> > The idea was just to not ever down_read() the semaphore if the current
> > task is the replace task.
>
> So your idea is sth like the following (not yet tested):
Yes, exactly.
>
> diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
> index 83d5cdd77f29..604399e59a3d 100644
> --- a/fs/btrfs/dev-replace.c
> +++ b/fs/btrfs/dev-replace.c
> @@ -641,6 +641,7 @@ static int btrfs_dev_replace_start(struct
> btrfs_fs_info *fs_info,
> return ret;
>
> down_write(&dev_replace->rwsem);
> + dev_replace->replace_task = current;
> switch (dev_replace->replace_state) {
> case BTRFS_IOCTL_DEV_REPLACE_STATE_NEVER_STARTED:
> case BTRFS_IOCTL_DEV_REPLACE_STATE_FINISHED:
> @@ -994,6 +995,7 @@ static int btrfs_dev_replace_finishing(struct
> btrfs_fs_info *fs_info,
> list_add(&tgt_device->dev_alloc_list, &fs_devices->alloc_list);
> fs_devices->rw_devices++;
>
> + dev_replace->replace_task = NULL;
> up_write(&dev_replace->rwsem);
> btrfs_rm_dev_replace_blocked(fs_info);
>
> diff --git a/fs/btrfs/fs.h b/fs/btrfs/fs.h
> index 79f64e383edd..cbfb225858a5 100644
> --- a/fs/btrfs/fs.h
> +++ b/fs/btrfs/fs.h
> @@ -317,6 +317,8 @@ struct btrfs_dev_replace {
>
> struct percpu_counter bio_counter;
> wait_queue_head_t replace_wait;
> +
> + struct task_struct *replace_task;
> };
>
> /*
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 8f340ad1d938..995b0647f538 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -6480,13 +6480,15 @@ int btrfs_map_block(struct btrfs_fs_info
> *fs_info, enum btrfs_map_op op,
> max_len = btrfs_max_io_len(map, map_offset, &io_geom);
> *length = min_t(u64, map->chunk_len - map_offset, max_len);
>
> - down_read(&dev_replace->rwsem);
> + if (dev_replace->replace_task != current)
> + down_read(&dev_replace->rwsem);
> +
> dev_replace_is_ongoing = btrfs_dev_replace_is_ongoing(dev_replace);
> /*
> * Hold the semaphore for read during the whole operation, write is
> * requested at commit time but must wait.
> */
> - if (!dev_replace_is_ongoing)
> + if (!dev_replace_is_ongoing && dev_replace->replace_task != current)
> up_read(&dev_replace->rwsem);
>
> switch (map->type & BTRFS_BLOCK_GROUP_PROFILE_MASK) {
> @@ -6626,7 +6628,7 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info,
> enum btrfs_map_op op,
> bioc->mirror_num = io_geom.mirror_num;
>
> out:
> - if (dev_replace_is_ongoing) {
> + if (dev_replace_is_ongoing && dev_replace->replace_task !=
> current) {
> lockdep_assert_held(&dev_replace->rwsem);
> /* Unlock and let waiting writers proceed */
> up_read(&dev_replace->rwsem);
>
>
> >
> > What this patch is doing is fishy, because it's setting
> > dev_replace_is_ongoing to false when there's an actual dev replace in
> > progress.
> > By setting it to false we are preventing the redirection of a write to
> > the new device.
> >
> > Was that intentional? If so, why (the changelog doesn't mention it,
> > and that's a very important detail)?
>
> Nope that was unintentional. Thanks for spotting.
>
> > For my suggestion, since we will skip the locking of the semaphore,
> > we'll have to compare "current" with "dev_replace->replace_task"
> > without any locking,
> > but that's ok and we can use data_race() to silence KCSAN.
>
> I'll check if KCSAN actually complains about this.
>
> As for David's remark, do you prefere a pid or a task_struct to be
> stored in struct dev_replace?
I'm fine with either one.
Thanks.
>
> Thanks,
> Johannes