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. Regards, Xiaoguang Wang
