Re: [RFC v3 01/11] eventfd: track eventfd_signal() recursion depth separately in different cases

On 1/27/21 8:04 PM, Jason Wang wrote:

On 2021/1/27 下午5:11, Yongji Xie wrote:

quoted

On Wed, Jan 27, 2021 at 11:38 AM Jason Wang [off-list ref] wrote:

quoted

On 2021/1/20 下午2:52, Yongji Xie wrote:

quoted

On Wed, Jan 20, 2021 at 12:24 PM Jason Wang [off-list ref] wrote:

quoted

On 2021/1/19 下午12:59, Xie Yongji wrote:

quoted

Now we have a global percpu counter to limit the recursion depth
of eventfd_signal(). This can avoid deadlock or stack overflow.
But in stack overflow case, it should be OK to increase the
recursion depth if needed. So we add a percpu counter in eventfd_ctx
to limit the recursion depth for deadlock case. Then it could be
fine to increase the global percpu counter later.

I wonder whether or not it's worth to introduce percpu for each eventfd.

How about simply check if eventfd_signal_count() is greater than 2?

It can't avoid deadlock in this way.

I may miss something but the count is to avoid recursive eventfd call.
So for VDUSE what we suffers is e.g the interrupt injection path:

userspace write IRQFD -> vq->cb() -> another IRQFD.

It looks like increasing EVENTFD_WAKEUP_DEPTH should be sufficient?

Actually I mean the deadlock described in commit f0b493e ("io_uring:
prevent potential eventfd recursion on poll"). It can break this bug
fix if we just increase EVENTFD_WAKEUP_DEPTH.

Ok, so can wait do something similar in that commit? (using async stuffs 
like wq).

io_uring should be fine in current kernels, but aio would still be
affected by this. But just in terms of recursion, bumping it one more
should probably still be fine.

-- 
Jens Axboe