[sheepdog] [PATCH] bs_sheepdog.c: fix io errors during writing to a snapshot

[nia dev]

Works now :-) Thanks !!!

On Tue, Jan 7, 2014 at 8:21 AM, FUJITA Tomonori
<fujita.tomonori at lab.ntt.co.jp> wrote:
> On Tue, 07 Jan 2014 22:23:00 +0900
> Hitoshi Mitake <mitake.hitoshi at gmail.com> wrote:
>
>> At Tue,  7 Jan 2014 21:59:31 +0900,
>> Ryusuke Konishi wrote:
>>>
>>> The sheepdog driver fails with io errors when write access is
>>> requested for a snapshot vdi.  The failure happens in create_branch
>>> function:
>>>
>>>  tgtd: read_write_object(684) No object found (oid: 8000000
>>>  000000000, old_oid: 0)
>>>  tgtd: create_branch(1160) reloading new inode object failed
>>>  tgtd: bs_sheepdog_request(1197) creating writable VDI from
>>>  snapshot failed
>>>
>>>  sd 12:0:0:1: [sdb] Unhandled sense code
>>>  sd 12:0:0:1: [sdb] Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
>>>  sd 12:0:0:1: [sdb] Sense Key : Medium Error [current]
>>>  sd 12:0:0:1: [sdb] Add. Sense: Unrecovered read error
>>>  sd 12:0:0:1: [sdb] CDB: Write(10): 2a 00 00 00 20 a8 00 00 08 00
>>>  Buffer I/O error on device sdb1, logical block 1041
>>>  lost page write due to I/O error on sdb1
>>>
>>> This turned out to be caused by a race condition among multiple write
>>> requests.  When bs_sheepdog_request() receives a write request for the
>>> snapshot vdi, it tries to change the snapshot to a writable vdi with
>>> the create_branch function.  However, the current implementation of
>>> create_branch() cannot handle concurrent requests exclusively nor
>>> protected from regular io routine (sd_io).
>>>
>>> This fixes the above io-error issue by serializing create_branch()
>>> with a pthread reader/writer lock, and also fixes the race condition
>>> between create_branch() and sd_io() with the lock.
>>>
>>> Signed-off-by: Ryusuke Konishi <konishi.ryusuke at lab.ntt.co.jp>
>>> Cc: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
>>> ---
>>>  usr/bs_sheepdog.c |   22 +++++++++++++++++-----
>>>  1 file changed, 17 insertions(+), 5 deletions(-)
>>
>> Ooops, thanks a lot for your fix. It must be a very hard debug and
>> sorry for annoying!
>>
>> Reviewed-by: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
>
> Applied, thanks a lot, guys.
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