On 2020/09/07 20:24, Kanchan Joshi wrote: > On Mon, Sep 7, 2020 at 1:52 PM Damien Le Moal <[email protected]> wrote: >> >> On 2020/09/07 16:01, Kanchan Joshi wrote: >>>> Even for SMR, the user is free to set the elevator to none, which disables >>>> zone >>>> write locking. Issuing writes correctly then becomes the responsibility of >>>> the >>>> application. This can be useful for settings that for instance use NCQ I/O >>>> priorities, which give better results when "none" is used. >>> >>> Was it not a problem that even if the application is sending writes >>> correctly, scheduler may not preserve the order. >>> And even when none is being used, re-queue can happen which may lead >>> to different ordering. >> >> "Issuing writes correctly" means doing small writes, one per zone at most. In >> that case, it does not matter if the block layer reorders writes. Per zone, >> they >> will still be sequential. >> >>>> As far as I know, zoned drives are always used in tightly controlled >>>> environments. Problems like "does not know what other applications would be >>>> doing" are non-existent. Setting up the drive correctly for the use case >>>> at hand >>>> is a sysadmin/server setup problem, based on *the* application (singular) >>>> requirements. >>> >>> Fine. >>> But what about the null-block-zone which sets MQ-deadline but does not >>> actually use write-lock to avoid race among multiple appends on a >>> zone. >>> Does that deserve a fix? >> >> In nullblk, commands are executed under a spinlock. So there is no >> concurrency >> problem. The spinlock serializes the execution of all commands. null_blk zone >> append emulation thus does not need to take the scheduler level zone write >> lock >> like scsi does. > > I do not see spinlock for that. There is one "nullb->lock", but its > scope is limited to memory-backed handling. > For concurrent zone-appends on a zone, multiple threads may set the > "same" write-pointer into incoming request(s). > Are you referring to any other spinlock that can avoid "same wp being > returned to multiple threads".
Checking again, it looks like you are correct. nullb->lock is indeed only used for processing read/write with memory backing turned on. We either need to extend that spinlock use, or add one to protect the zone array when doing zoned commands and checks of read/write against a zone wp. Care to send a patch ? I can send one too. -- Damien Le Moal Western Digital Research
