Re: FAILED: patch "[PATCH] io_uring: prevent potential eventfd recursion on poll" failed to apply to 5.5-stable tree

[Sasha Levin <sashal@kernel.org>]

On Sun, Feb 09, 2020 at 12:52:51PM +0100, gregkh@linuxfoundation.org wrote:
>
>The patch below does not apply to the 5.5-stable tree.
>If someone wants it applied there, or to any other stable or longterm
>tree, then please email the backport, including the original git commit
>id to <stable@vger.kernel.org>.
>
>thanks,
>
>greg k-h
>
>------------------ original commit in Linus's tree ------------------
>
>From f0b493e6b9a8959356983f57112229e69c2f7b8c Mon Sep 17 00:00:00 2001
>From: Jens Axboe <axboe@kernel.dk>
>Date: Sat, 1 Feb 2020 21:30:11 -0700
>Subject: [PATCH] io_uring: prevent potential eventfd recursion on poll
>
>If we have nested or circular eventfd wakeups, then we can deadlock if
>we run them inline from our poll waitqueue wakeup handler. It's also
>possible to have very long chains of notifications, to the extent where
>we could risk blowing the stack.
>
>Check the eventfd recursion count before calling eventfd_signal(). If
>it's non-zero, then punt the signaling to async context. This is always
>safe, as it takes us out-of-line in terms of stack and locking context.
>
>Cc: stable@vger.kernel.org # 5.1+
>Signed-off-by: Jens Axboe <axboe@kernel.dk>

I queued it back to 5.5 by taking f2842ab5b72d ("io_uring: enable option
to only trigger eventfd for async completions") and working around
missing commit 69b3e546139a ("io_uring: change io_ring_ctx bool fields
into bit fields"). However, 5.4 is a bit more complex than what I can
tackle without a test suite.

Jens, is there something I can run to validate io_uring on older
kernels?

-- 
Thanks,
Sasha