Re: [PATCH] btrfs: don't take dev_replace rwsem on task already holding it

From: Johannes Thumshirn
Date: Mon Sep 09 2024 - 06:35:50 EST

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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):

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?

Thanks,
Johannes

Next message: Vlastimil Babka: "Re: linux-next: duplicate patches in the slab tree"
Previous message: Konrad Dybcio: "Re: [PATCH] drm/msm: allow returning NULL from crete_address_space"
Next in thread: Filipe Manana: "Re: [PATCH] btrfs: don't take dev_replace rwsem on task already holding it"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]