On Sun, Aug 04, 2024 at 04:02:51PM +0800, Yafang Shao wrote:
> One solution we're currently exploring is leveraging the preadv2(2)
> syscall. By using the RWF_NOWAIT flag, preadv2(2) can avoid the XFS inode
> lock hung task. This can be illustrated as follows:
>
> retry:
> if (preadv2(fd, iovec, cnt, offset, RWF_NOWAIT) < 0) {
> sleep(n)
> goto retry;
But that's not how you're supposed to use RWF_NOWAIT! You're supposed
to try it _once_ in the thread that can't block, then hand the I/O off
to another thread which _can_ block. Because that single thread is the
one which does all the blocking I/O, there's no lock contention.
So this is a kernel workaround for bad application design, and should
be rejected.
