[BUG][RFC] Miscalculated inflight counter in io_uring

[Pavel Begunkov <asml.silence@gmail.com>]

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In case of IORING_SETUP_SQPOLL | IORING_SETUP_IOPOLL:

@inflight count returned by io_submit_sqes() is the number of entries
picked up from a sqring including already completed/failed. And
completed but not failed requests will be placed into @poll_list.

Then io_sq_thread() tries to poll @inflight events, even though failed
won't appear in @poll_list. Thus, it will think that there are always
something to poll (i.e. @inflight > 0)

There are several issues with this:
1. io_sq_thread won't ever sleep
2. io_sq_thread() may be left running and actively polling even after
user process is destroyed
3. the same goes for mm_struct with all vmas of the user process
TL;DR;
awhile @inflight > 0, io_sq_thread won't put @cur_mm, so locking
recycling of vmas used for rings' mapping, which hold refcount of
io_uring's struct file. Thus, io_uring_release() won't be called, as
well as kthread_{park,stop}(). That's all in case when the user process
haven't unmapped rings.


I'm not sure how to fix it better:
1. try to put failed into poll_list (grabbing mutex).

2. test for zero-inflight case with comparing sq and cq. something like
```
if (nr_polled == 0) {
	lock(comp_lock);
	if (cached_cq_head == cached_sq_tail)
		inflight = 0;
	unlock(comp_lock);
}
```
But that's adds extra spinlock locking in fast-path. And that's unsafe
to use non-cached heads/tails, as it could be maliciously changed by
userspace.

3. Do some counting of failed (probably needs atomic or synchronisation)

4. something else?


-- 
Yours sincerely,
Pavel Begunkov


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