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From:   Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>
CC:     Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>,
        "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Oleksii Moisieiev <Oleksii_Moisieiev@epam.com>
Subject: [PATCH] xen/pvcalls-back: fix permanently masked event channel
Thread-Topic: [PATCH] xen/pvcalls-back: fix permanently masked event channel
Thread-Index: AQHZLEqQuRGgM2P0bEiSEaENUdLo/Q==
Date:   Thu, 19 Jan 2023 21:11:15 +0000
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There is a sequence of events that can lead to a permanently masked
event channel, because xen_irq_lateeoi() is newer called. This happens
when a backend receives spurious write event from a frontend. In this
case pvcalls_conn_back_write() returns early and it does not clears the
map->write counter. As map->write > 0, pvcalls_back_ioworker() returns
without calling xen_irq_lateeoi(). This leaves the event channel in
masked state, a backend does not receive any new events from a
frontend and the whole communication stops.

Move atomic_set(&map->write, 0) to the very beginning of
pvcalls_conn_back_write() to fix this issue.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Reported-by: Oleksii Moisieiev <oleksii_moisieiev@epam.com>
---
 drivers/xen/pvcalls-back.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/xen/pvcalls-back.c b/drivers/xen/pvcalls-back.c
index a7d293fa8d14..60f5cd70d770 100644
--- a/drivers/xen/pvcalls-back.c
+++ b/drivers/xen/pvcalls-back.c
@@ -173,6 +173,8 @@ static bool pvcalls_conn_back_write(struct sock_mapping=
 *map)
 	RING_IDX cons, prod, size, array_size;
 	int ret;
=20
+	atomic_set(&map->write, 0);
+
 	cons =3D intf->out_cons;
 	prod =3D intf->out_prod;
 	/* read the indexes before dealing with the data */
@@ -197,7 +199,6 @@ static bool pvcalls_conn_back_write(struct sock_mapping=
 *map)
 		iov_iter_kvec(&msg.msg_iter, READ, vec, 2, size);
 	}
=20
-	atomic_set(&map->write, 0);
 	ret =3D inet_sendmsg(map->sock, &msg, size);
 	if (ret =3D=3D -EAGAIN) {
 		atomic_inc(&map->write);
--=20
2.38.1