Patch "xsk: Do not sleep in poll() when need_wakeup set" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    xsk: Do not sleep in poll() when need_wakeup set

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     xsk-do-not-sleep-in-poll-when-need_wakeup-set.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit c294ad4dbcde52cdc3e99b336e1369697074a768
Author: Magnus Karlsson <magnus.karlsson@xxxxxxxxx>
Date:   Tue Dec 14 11:26:07 2021 +0100

    xsk: Do not sleep in poll() when need_wakeup set
    
    [ Upstream commit bd0687c18e635b63233dc87f38058cd728802ab4 ]
    
    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@xxxxxxxxx>
    Signed-off-by: Magnus Karlsson <magnus.karlsson@xxxxxxxxx>
    Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
    Acked-by: Maciej Fijalkowski <maciej.fijalkowski@xxxxxxxxx>
    Link: https://lore.kernel.org/bpf/20211214102607.7677-1-magnus.karlsson@xxxxxxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index d6b500dc42084..d942e1bf443d9 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -692,8 +692,6 @@ static __poll_t xsk_poll(struct file *file, struct socket *sock,
 	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 *file, struct socket *sock,
 		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))