On 7/10/19 11:55 AM, Jens Axboe wrote: > On 7/9/19 8:29 AM, Ming Lei wrote: >> On Tue, Jul 09, 2019 at 06:02:19PM +0900, 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(). >> >> I am wondering how you guarantee that context B is always run after >> context A. >> >>> >>> 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. >> >> Another candidate approach is to run the following code before >> releasing 'zone' lock: >> >> if (current->plug) >> blk_finish_plug(context->plug) >> >> Then we can fix zone specific issue in zone code only, and avoid generic >> blk-core change for zone issue. > > I prefer that to the existing solution as well. My apologies, you lost me: do you mean that you prefer Ming's suggestion and force FS or dm users to manually unplug in the case of zoned block devices ? Thanks. -- Damien Le Moal Western Digital Research
