Re: BUG #19792: BUG: Deadlock between WaitIO and read-stream look-ahead under io_method=io_uring (18.6)

Hi Thomas,

Thanks for the report.

I'm starting to learn AIO and have no Linux machine for io_uring, so I
could not
reproduce this with a real workload. From the code, I think it goes
like this.

t1 A: an INSERT splitting leaf 627 holds its lock.
t2 B: autovacuum's btree scan starts a look-ahead read of 628.
BM_IO_IN_PROGRESS is set, but io_wref is not yet.
t3 A: gets 628 from the FSM. WaitIO() sees no io_wref, so it
sleeps on the buffer CV.
t4 B: stages the IO and sets io_wref, but does not wake A.
t5 B: waits for the lock on 627 that A holds.
t6 A and B wait on each other forever.

With io_uring, only the owner or a backend that waits through io_wref
reaps the completion, so nobody reaps it. [1] only fixes waits
through io_wref, and A never gets there.

0001 is Thomas Munro's patch from [1]. A woken A can find the IO
still staged, and waits on submit_cv.
0002 wakes the buffer CV once io_wref is set. It also wakes
submit_cv after synchronous execution, which 0001 misses.

Draft attached. Could someone check them?

[1]
https://www.postgresql.org/message-id/CA%2BhUKG%2BmZYrSdnhk-XrBYO18H829K77S9gMKUsykOiTJtqB43g%40mail.gmail.com

Thanks,
Shihao