Message-ID: <CAL3q7H6NP_Rr33cdeV3+=GvseOafowaED-maWhtUw65P4Bti+w@mail.gmail.com>
Date: Mon, 26 Aug 2024 12:11:04 +0100
From: Filipe Manana <fdmanana@...nel.org>
To: Johannes Thumshirn <jth@...nel.org>
Cc: Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>, David Sterba <dsterba@...e.com>, 
	"open list:BTRFS FILE SYSTEM" <linux-btrfs@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>, 
	Johannes Thumshirn <johannes.thumshirn@....com>
Subject: Re: [PATCH] btrfs: don't take dev_replace rwsem on task already
 holding it

On Thu, Aug 15, 2024 at 1:57 PM Johannes Thumshirn <jth@...nel.org> wrote:
>
> From: Johannes Thumshirn <johannes.thumshirn@....com>
>
> 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@...e.com>
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@....com>
> ---
>  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.

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)?

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.

Thanks.

> +
>         /*
>          * Hold the semaphore for read during the whole operation, write is
>          * requested at commit time but must wait.
> --
> 2.43.0
>
>

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