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

[Hitoshi Mitake]

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>

Thanks,
Hitoshi