From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 03F2979C0 for ; Wed, 30 Nov 2022 18:51:21 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7B5A5C433D7; Wed, 30 Nov 2022 18:51:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1669834280; bh=f469Eyl9iTgGdzilUrmfkK6gyjtcEXnM/7qfFyjJtCI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=k2LGn5QEBXoUzTtqtAGj8mfpGy1SG5Ycscw+IiT3gwJ0oC5EED9p+xFYpg2cJJgzN Q73QxZ3dsEReLrRKarJ6Yqk3/UKJVSQgE3+QlGX39wj2+f70by9DORwF3LYrsZDHVi CdFT/nNUsUxuU7HXGwxTj98eiVcgiYFD59ymo2DM= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Pavel Begunkov , Jens Axboe Subject: [PATCH 6.0 195/289] io_uring: cmpxchg for poll arm refs release Date: Wed, 30 Nov 2022 19:23:00 +0100 Message-Id: <20221130180548.542665171@linuxfoundation.org> X-Mailer: git-send-email 2.38.1 In-Reply-To: <20221130180544.105550592@linuxfoundation.org> References: <20221130180544.105550592@linuxfoundation.org> User-Agent: quilt/0.67 Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit From: Pavel Begunkov commit 2f3893437a4ebf2e892ca172e9e122841319d675 upstream. 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 Link: https://lore.kernel.org/r/0c95251624397ea6def568ff040cad2d7926fd51.1668963050.git.asml.silence@gmail.com Signed-off-by: Jens Axboe Signed-off-by: Greg Kroah-Hartman --- io_uring/poll.c | 8 +++----- 1 file changed, 3 insertions(+), 5 deletions(-) --- a/io_uring/poll.c +++ b/io_uring/poll.c @@ -519,7 +519,6 @@ static int __io_arm_poll_handler(struct 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); @@ -587,11 +586,10 @@ static int __io_arm_poll_handler(struct 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;