Re: [PATCH 0/15] Review uuid_mutex usage

[Anand Jain <anand.jain@oracle.com>]

On 05/16/2018 12:33 AM, David Sterba wrote:
> On Thu, Apr 12, 2018 at 10:29:23AM +0800, Anand Jain wrote:
>> uuid_mutex lock is not a per-fs lock but a global lock. The main aim of
>> this patch-set is to critically review the usage of this lock, and delete
>> the unnecessary once. By doing this we improve the concurrency of
>> device operations across multiple btrfs filesystems is in the system.
>>
>> patch 1: Was sent before, I am including it here, as its about uuid_mutex.
>>
>> patch 2-9: Are cleanup and or preparatory patches.
>>
>> patch 10-14: Drops the uuid_mutex and makes sure there is enough lock,
>> as discussed in the patch change log.
>>
>> patch 15: A generic cleanup patch around functions in the same context.
>>
>> These patches are on top of
>>    https://github.com/kdave/btrfs-devel.git remove-volume-mutex
>> And it will be a good idea to go along with the kill-volume-mutex patches.
>>
>> This is tested with xfstests and there are no _new_ regression. And I am
>> trying to understand the old regressions, and notice that they are
>> inconsistent.
>>
>> Anand Jain (15):
>>    btrfs: optimize move uuid_mutex closer to the critical section
>>    btrfs: rename struct btrfs_fs_devices::list
>>    btrfs: cleanup __btrfs_open_devices() drop head pointer
>>    btrfs: rename __btrfs_close_devices to close_fs_devices
>>    btrfs: rename __btrfs_open_devices to open_fs_devices
>>    btrfs: cleanup find_device() drop list_head pointer
>>    btrfs: cleanup btrfs_rm_device() promote fs_devices pointer
>>    btrfs: cleanup btrfs_rm_device() use cur_devices
>>    btrfs: uuid_mutex in read_chunk_tree, add a comment
>>    btrfs: drop uuid_mutex in btrfs_free_extra_devids()
>>    btrfs: drop uuid_mutex in btrfs_open_devices()
>>    btrfs: drop uuid_mutex in close_fs_devices()
>>    btrfs: drop uuid_mutex in btrfs_dev_replace_finishing()
>>    btrfs: drop uuid_mutex in btrfs_destroy_dev_replace_tgtdev()
>>    btrfs: cleanup btrfs_destroy_dev_replace_tgtdev() localize
>>      btrfs_fs_devices
> 
> Patches 10 and 12 haven't been merged, the rest is now in misc-next.
> Testing hasn't revealed any problems related to the uuid/device locks
> but as said before we don't have stress tests.

  Our test cases are sequential. We need same device related test cases
  in a concurrent manner.

  Just for experiment, I ran two instances of xfstests concurrently (on a
  separate set of test and scratch devices), they ran fine with these
  patches, not a perfect solution though. We need to think of better
  ways.

  The main challenge with testing concurrency / racing is how to
  synchronize racing threads.

Thanks, Anand


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