Re: [BUG][RFC] Miscalculated inflight counter in io_uring

[Jens Axboe]

On 10/25/19 3:48 AM, Pavel Begunkov wrote:
> 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?

Can we just look at the completion count? Ala:

prev_tail = ctx->cached_cq_tail;
inflight += io_submit_sqes(ctx, to_submit, cur_mm != NULL,      
                                             mm_fault);
if (prev_tail != ctx->cached_cq_tail)
	inflight -= (ctx->cached_cq_tail - prev_tail);

or something like that.

-- 
Jens Axboe