From: Anand Jain <anand.jain@oracle.com>
To: dsterba@suse.cz, linux-btrfs@vger.kernel.org, dsterba@suse.com,
l@damenly.su
Subject: Re: [PATCH RFC V5 2/2] btrfs: consolidate device_list_mutex in prepare_sprout to its parent
Date: Sat, 18 Sep 2021 08:10:18 +0800 [thread overview]
Message-ID: <6e62c59b-91f1-c090-931f-96434fd08ac4@oracle.com> (raw)
In-Reply-To: <20210917153720.GW9286@twin.jikos.cz>
On 17/09/2021 23:37, David Sterba wrote:
> On Tue, Aug 31, 2021 at 09:21:29AM +0800, Anand Jain wrote:
>> btrfs_prepare_sprout() moves seed devices into its own struct fs_devices,
>> so that its parent function btrfs_init_new_device() can add the new sprout
>> device to fs_info->fs_devices.
>>
>> Both btrfs_prepare_sprout() and btrfs_init_new_device() needs
>> device_list_mutex. But they are holding it sequentially, thus creates a
>> small window to an opportunity to race. Close this opportunity and hold
>> device_list_mutex common to both btrfs_init_new_device() and
>> btrfs_prepare_sprout().
>
> I don't se what exactly would go wrong with the separate device list
> locking, but I see at least one potential problem with the new code.
>
>>
>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>> ---
>> RFC because IMO the cleanup of device_list_mutex makes sense even though
>> there isn't another thread that could race potentially race as of now.
>>
>> Depends on
>> [PATCH v2] btrfs: fix lockdep warning while mounting sprout fs
>> which removed the device_list_mutex from clone_fs_devices() otherwise
>> this patch will cause a double mutex error.
>>
>> v2: fix the missing mutex_unlock in the error return
>> v3: -
>> v4: -
>> v5: - (Except for the change in below SO comments)
>>
>> fs/btrfs/volumes.c | 7 ++++---
>> 1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>> index fa9fe47b5b68..53ead67b625c 100644
>> --- a/fs/btrfs/volumes.c
>> +++ b/fs/btrfs/volumes.c
>> @@ -2369,6 +2369,8 @@ static int btrfs_prepare_sprout(struct btrfs_fs_info *fs_info)
>> u64 super_flags;
>>
>> lockdep_assert_held(&uuid_mutex);
>> + lockdep_assert_held(&fs_devices->device_list_mutex);
>> +
>> if (!fs_devices->seeding)
>> return -EINVAL;
>>
>> @@ -2400,7 +2402,6 @@ static int btrfs_prepare_sprout(struct btrfs_fs_info *fs_info)
>> INIT_LIST_HEAD(&seed_devices->alloc_list);
>> mutex_init(&seed_devices->device_list_mutex);
BTW mutex_init here will go, as the sprout's private
fs_devices::device_list_mutex is unused. It is a pending cleanup.
> A few lines before this one there's alloc_fs_devices and
> clone_fs_devices, both allocating memory. This would happen under a big
> lock as device_list_mutex also protects superblock write. This is a
> pattern to avoid.
Oh. That's right. Thx. One way is to flag NOFS alloc.
> A rough idea would be to split btrfs_prepare_sprout into parts where the
> allocations are not done under the lock and the locked part. It could be
> partially inlined to btrfs_init_new_device.
I think you mean something like this...
btrfs_init_new_device()
<snip>
if seeding_dev
alloc_prepare_sprout
mutex_lock(&fs_devices->device_list_mutex);
if seeding_dev
finish_prepare_sprout
<snip>
mutex_unlock(&fs_devices->device_list_mutex);
I am trying.
Thanks, Anand
>>
>> - mutex_lock(&fs_devices->device_list_mutex);
>> list_splice_init_rcu(&fs_devices->devices, &seed_devices->devices,
>> synchronize_rcu);
>> list_for_each_entry(device, &seed_devices->devices, dev_list)
>> @@ -2416,7 +2417,6 @@ static int btrfs_prepare_sprout(struct btrfs_fs_info *fs_info)
>> generate_random_uuid(fs_devices->fsid);
>> memcpy(fs_devices->metadata_uuid, fs_devices->fsid, BTRFS_FSID_SIZE);
>> memcpy(disk_super->fsid, fs_devices->fsid, BTRFS_FSID_SIZE);
>> - mutex_unlock(&fs_devices->device_list_mutex);
>>
>> super_flags = btrfs_super_flags(disk_super) &
>> ~BTRFS_SUPER_FLAG_SEEDING;
>> @@ -2591,10 +2591,12 @@ int btrfs_init_new_device(struct btrfs_fs_info *fs_info, const char *device_path
>> device->dev_stats_valid = 1;
>> set_blocksize(device->bdev, BTRFS_BDEV_BLOCKSIZE);
>>
>> + mutex_lock(&fs_devices->device_list_mutex);
>> if (seeding_dev) {
>> btrfs_clear_sb_rdonly(sb);
>> ret = btrfs_prepare_sprout(fs_info);
>> if (ret) {
>> + mutex_unlock(&fs_devices->device_list_mutex);
>> btrfs_abort_transaction(trans, ret);
>> goto error_trans;
>> }
>> @@ -2604,7 +2606,6 @@ int btrfs_init_new_device(struct btrfs_fs_info *fs_info, const char *device_path
>>
>> device->fs_devices = fs_devices;
>>
>> - mutex_lock(&fs_devices->device_list_mutex);
>> mutex_lock(&fs_info->chunk_mutex);
>> list_add_rcu(&device->dev_list, &fs_devices->devices);
>> list_add(&device->dev_alloc_list, &fs_devices->alloc_list);
>> --
>> 2.31.1
prev parent reply other threads:[~2021-09-18 0:10 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-31 1:21 [PATCH V5 0/2] btrfs: device_list_mutex fix lockdep warn and cleanup Anand Jain
2021-08-31 1:21 ` [PATCH V5 1/2] btrfs: fix lockdep warning while mounting sprout fs Anand Jain
2021-08-31 8:18 ` Nikolay Borisov
2021-09-02 23:51 ` Anand Jain
2021-08-31 12:37 ` Nikolay Borisov
2021-09-01 0:49 ` Su Yue
2021-09-02 15:28 ` David Sterba
2021-08-31 1:21 ` [PATCH RFC V5 2/2] btrfs: consolidate device_list_mutex in prepare_sprout to its parent Anand Jain
2021-08-31 13:03 ` Nikolay Borisov
2021-09-03 3:08 ` Anand Jain
2021-09-17 15:37 ` David Sterba
2021-09-18 0:10 ` Anand Jain [this message]
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