On 7/9/19 2:02 AM, Damien Le Moal wrote:
Simultaneously writing to a sequential zone of a zoned block device
from multiple contexts requires mutual exclusion for BIO issuing to
ensure that writes happen sequentially. However, even for a well
behaved user correctly implementing such synchronization, BIO plugging
may interfere and result in BIOs from the different contextx to be
reordered if plugging is done outside of the mutual exclusion section,
e.g. the plug was started by a function higher in the call chain than
the function issuing BIOs.
Context A Context B
| blk_start_plug()
| ...
| seq_write_zone()
| mutex_lock(zone)
| submit_bio(bio-0)
| submit_bio(bio-1)
| mutex_unlock(zone)
| return
| ------------------------------> | seq_write_zone()
| mutex_lock(zone)
| submit_bio(bio-2)
| mutex_unlock(zone)
| <------------------------------ |
| blk_finish_plug()
In the above example, despite the mutex synchronization resulting in the
correct BIO issuing order 0, 1, 2, context A BIOs 0 and 1 end up being
issued after BIO 2 when the plug is released with blk_finish_plug().
To fix this problem, introduce the internal helper function
blk_mq_plug() to access the current context plug, return the current
plug only if the target device is not a zoned block device or if the
BIO to be plugged not a write operation. Otherwise, ignore the plug and
return NULL, resulting is all writes to zoned block device to never be
plugged.
Are there classes of zoned devices for which the plug list is useful? If
so, have you considered any other approaches, e.g. one plug list per
request queue instead of one plug list per task in case of zoned devices?
Thanks,
Bart.
