The patch below does not apply to the 5.15-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.
Possible dependencies:
2f3893437a4e ("io_uring: cmpxchg for poll arm refs release") 49f1c68e048f ("io_uring: optimise submission side poll_refs") de08356f4858 ("io_uring: refactor poll arm error handling") 063a007996bf ("io_uring: change arm poll return values") 13a99017ff19 ("io_uring: remove events caching atavisms") 0638cd7be212 ("io_uring: clean poll ->private flagging") 48863ffd3e81 ("io_uring: clean up tracing events") 9ca9fb24d5fe ("io_uring: mutex locked poll hashing") e6f89be61410 ("io_uring: introduce a struct for hash table") a2cdd5193218 ("io_uring: pass hash table into poll_find") 0ec6dca22319 ("io_uring: use state completion infra for poll reqs") 8b1dfd343ae6 ("io_uring: clean up io_ring_ctx_alloc") 4a07723fb4bb ("io_uring: limit the number of cancellation buckets") 1ab1edb0a104 ("io_uring: pass poll_find lock back") 38513c464d3d ("io_uring: switch cancel_hash to use per entry spinlock") aff5b2df9e8b ("io_uring: better caching for ctx timeout fields") b9ba8a4463cd ("io_uring: add support for level triggered poll") 735729844819 ("io_uring: move rsrc related data, core, and commands") 3b77495a9723 ("io_uring: split provided buffers handling into its own file") 7aaff708a768 ("io_uring: move cancelation into its own file")
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 2f3893437a4ebf2e892ca172e9e122841319d675 Mon Sep 17 00:00:00 2001 From: Pavel Begunkov asml.silence@gmail.com Date: Sun, 20 Nov 2022 16:57:41 +0000 Subject: [PATCH] io_uring: cmpxchg for poll arm refs release
Replace atomically substracting the ownership reference at the end of arming a poll with a cmpxchg. We try to release ownership by setting 0 assuming that poll_refs didn't change while we were arming. If it did change, we keep the ownership and use it to queue a tw, which is fully capable to process all events and (even tolerates spurious wake ups).
It's a bit more elegant as we reduce races b/w setting the cancellation flag and getting refs with this release, and with that we don't have to worry about any kinds of underflows. It's not the fastest path for polling. The performance difference b/w cmpxchg and atomic dec is usually negligible and it's not the fastest path.
Cc: stable@vger.kernel.org Fixes: aa43477b04025 ("io_uring: poll rework") Signed-off-by: Pavel Begunkov asml.silence@gmail.com Link: https://lore.kernel.org/r/0c95251624397ea6def568ff040cad2d7926fd51.166896305... Signed-off-by: Jens Axboe axboe@kernel.dk
diff --git a/io_uring/poll.c b/io_uring/poll.c index 055632e9092a..1b78b527075d 100644 --- a/io_uring/poll.c +++ b/io_uring/poll.c @@ -518,7 +518,6 @@ static int __io_arm_poll_handler(struct io_kiocb *req, unsigned issue_flags) { struct io_ring_ctx *ctx = req->ctx; - int v;
INIT_HLIST_NODE(&req->hash_node); req->work.cancel_seq = atomic_read(&ctx->cancel_seq); @@ -586,11 +585,10 @@ static int __io_arm_poll_handler(struct io_kiocb *req,
if (ipt->owning) { /* - * Release ownership. If someone tried to queue a tw while it was - * locked, kick it off for them. + * Try to release ownership. If we see a change of state, e.g. + * poll was waken up, queue up a tw, it'll deal with it. */ - v = atomic_dec_return(&req->poll_refs); - if (unlikely(v & IO_POLL_REF_MASK)) + if (atomic_cmpxchg(&req->poll_refs, 1, 0) != 1) __io_poll_execute(req, 0); } return 0;