On 3/23/20 10:45 AM, Xiaoguang Wang wrote: > hi, > >> On 3/23/20 5:50 AM, Xiaoguang Wang wrote: >>> While diving into iouring fileset resigster/unregister/update codes, >>> we found one bug in fileset update codes. Iouring fileset update codes >>> use a percpu_ref variable to check whether can put previous registered >>> file, only when the refcnt of the perfcpu_ref variable reachs zero, can >>> we safely put these files, but this do not work well. If applications >>> always issue requests continually, this perfcpu_ref will never have an >>> chance to reach zero, and it'll always be in atomic mode, also will >>> defeat the gains introduced by fileset register/unresiger/update feature, >>> which are used to reduce the atomic operation overhead of fput/fget. >>> >>> To fix this issue, we remove the percpu_ref related codes, and add two new >>> counter: sq_seq and cq_seq to struct io_ring_ctx: >>> sq_seq: the most recent issued requset sequence number, which is >>> protected uring_lock. >>> cq_seq: the most recent completed request sequence number, which is >>> protected completion_lock. >>> >>> When we update fileset(under uring_lock), we record the current sq_seq, >>> and when cq_seq is greater or equal to recorded sq_seq, we know we can >>> put previous registered file safely. >> >> Maybe I'm misunderstanding the idea here, but what if you have the >> following: >> >> - sq_seq 200, cq_seq 100 >> >> We have 100 inflight, and an unregister request comes in. I then >> issue 100 nops, which complete. cq_seq is now 200, but none of the >> original requests that used the file have completed. >> >> What am I missing? > No, you're right. I had thought requests will be completed in the order > they are issued, thanks for pointing this. > As for not using per percpu_ref per registered file, I also worry about > the memory consume, because the max allowed registered files are 32768. Yeah, I think we have to be a bit creative here with the solution... Please continue to think about it, would be great to have a better solution for this! -- Jens Axboe
