linux-kernel - [PATCH 5.9 496/757] um: time-travel: Fix IRQ handling in time_travel_handle

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Message-Id: <20201027135513.724343723@linuxfoundation.org>
Date:   Tue, 27 Oct 2020 14:52:26 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Johannes Berg <johannes.berg@...el.com>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Richard Weinberger <richard@....at>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.9 496/757] um: time-travel: Fix IRQ handling in time_travel_handle_message()

From: Johannes Berg <johannes.berg@...el.com>

[ Upstream commit ebef8ea2ba967026192a26f4529890893919bc57 ]

As the comment here indicates, we need to do the polling in the
idle loop without blocking interrupts, since interrupts can be
vhost-user messages that we must process even while in our idle
loop.

I don't know why I explained one thing and implemented another,
but we have indeed observed random hangs due to this, depending
on the timing of the messages.

Fixes: 88ce64249233 ("um: Implement time-travel=ext")
Signed-off-by: Johannes Berg <johannes.berg@...el.com>
Acked-By: Anton Ivanov <anton.ivanov@...bridgegreys.com>
Signed-off-by: Richard Weinberger <richard@....at>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/um/kernel/time.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/um/kernel/time.c b/arch/um/kernel/time.c
index 25eaa6a0c6583..c07436e89e599 100644
--- a/arch/um/kernel/time.c
+++ b/arch/um/kernel/time.c
@@ -70,13 +70,17 @@ static void time_travel_handle_message(struct um_timetravel_msg *msg,
 	 * read of the message and write of the ACK.
 	 */
 	if (mode != TTMH_READ) {
+		bool disabled = irqs_disabled();
+
+		BUG_ON(mode == TTMH_IDLE && !disabled);
+
+		if (disabled)
+			local_irq_enable();
 		while (os_poll(1, &time_travel_ext_fd) != 0) {
-			if (mode == TTMH_IDLE) {
-				BUG_ON(!irqs_disabled());
-				local_irq_enable();
-				local_irq_disable();
-			}
+			/* nothing */
 		}
+		if (disabled)
+			local_irq_disable();
 	}
 
 	ret = os_read_file(time_travel_ext_fd, msg, sizeof(*msg));
-- 
2.25.1