From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Keith Wiles <keith.wiles@intel.com>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Subject: [PATCH 5.15 155/177] xsk: Do not sleep in poll() when need_wakeup set
Date:   Mon, 20 Dec 2021 15:35:05 +0100
Message-Id: <20211220143045.289751892@linuxfoundation.org>
In-Reply-To: <20211220143040.058287525@linuxfoundation.org>
References: <20211220143040.058287525@linuxfoundation.org>
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From: Magnus Karlsson <magnus.karlsson@intel.com>

commit bd0687c18e635b63233dc87f38058cd728802ab4 upstream.

Do not sleep in poll() when the need_wakeup flag is set. When this
flag is set, the application needs to explicitly wake up the driver
with a syscall (poll, recvmsg, sendmsg, etc.) to guarantee that Rx
and/or Tx processing will be processed promptly. But the current code
in poll(), sleeps first then wakes up the driver. This means that no
driver processing will occur (baring any interrupts) until the timeout
has expired.

Fix this by checking the need_wakeup flag first and if set, wake the
driver and return to the application. Only if need_wakeup is not set
should the process sleep if there is a timeout set in the poll() call.

Fixes: 77cd0d7b3f25 ("xsk: add support for need_wakeup flag in AF_XDP rings")
Reported-by: Keith Wiles <keith.wiles@intel.com>
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20211214102607.7677-1-magnus.karlsson@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/xdp/xsk.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -692,8 +692,6 @@ static __poll_t xsk_poll(struct file *fi
 	struct xdp_sock *xs = xdp_sk(sk);
 	struct xsk_buff_pool *pool;
 
-	sock_poll_wait(file, sock, wait);
-
 	if (unlikely(!xsk_is_bound(xs)))
 		return mask;
 
@@ -705,6 +703,8 @@ static __poll_t xsk_poll(struct file *fi
 		else
 			/* Poll needs to drive Tx also in copy mode */
 			__xsk_sendmsg(sk);
+	} else {
+		sock_poll_wait(file, sock, wait);
 	}
 
 	if (xs->rx && !xskq_prod_is_empty(xs->rx))