linux-kernel - [PATCH 4.19 274/276] vfio-ccw: Prevent quiesce function going into an infinite loop

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Date:   Wed, 29 May 2019 20:07:12 -0700
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Eric Farman <farman@...ux.ibm.com>,
        Farhan Ali <alifm@...ux.ibm.com>,
        Halil Pasic <pasic@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.19 274/276] vfio-ccw: Prevent quiesce function going into an infinite loop

[ Upstream commit d1ffa760d22aa1d8190478e5ef555c59a771db27 ]

The quiesce function calls cio_cancel_halt_clear() and if we
get an -EBUSY we go into a loop where we:
	- wait for any interrupts
	- flush all I/O in the workqueue
	- retry cio_cancel_halt_clear

During the period where we are waiting for interrupts or
flushing all I/O, the channel subsystem could have completed
a halt/clear action and turned off the corresponding activity
control bits in the subchannel status word. This means the next
time we call cio_cancel_halt_clear(), we will again start by
calling cancel subchannel and so we can be stuck between calling
cancel and halt forever.

Rather than calling cio_cancel_halt_clear() immediately after
waiting, let's try to disable the subchannel. If we succeed in
disabling the subchannel then we know nothing else can happen
with the device.

Suggested-by: Eric Farman <farman@...ux.ibm.com>
Signed-off-by: Farhan Ali <alifm@...ux.ibm.com>
Message-Id: <4d5a4b98ab1b41ac6131b5c36de18b76c5d66898.1555449329.git.alifm@...ux.ibm.com>
Reviewed-by: Eric Farman <farman@...ux.ibm.com>
Acked-by: Halil Pasic <pasic@...ux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@...hat.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/s390/cio/vfio_ccw_drv.c | 32 ++++++++++++++++++--------------
 1 file changed, 18 insertions(+), 14 deletions(-)

diff --git a/drivers/s390/cio/vfio_ccw_drv.c b/drivers/s390/cio/vfio_ccw_drv.c
index 0e0a743aeaf69..7a06cdff6572d 100644
--- a/drivers/s390/cio/vfio_ccw_drv.c
+++ b/drivers/s390/cio/vfio_ccw_drv.c
@@ -40,26 +40,30 @@ int vfio_ccw_sch_quiesce(struct subchannel *sch)
 	if (ret != -EBUSY)
 		goto out_unlock;
 
+	iretry = 255;
 	do {
-		iretry = 255;
 
 		ret = cio_cancel_halt_clear(sch, &iretry);
-		while (ret == -EBUSY) {
-			/*
-			 * Flush all I/O and wait for
-			 * cancel/halt/clear completion.
-			 */
-			private->completion = &completion;
-			spin_unlock_irq(sch->lock);
 
-			wait_for_completion_timeout(&completion, 3*HZ);
+		if (ret == -EIO) {
+			pr_err("vfio_ccw: could not quiesce subchannel 0.%x.%04x!\n",
+			       sch->schid.ssid, sch->schid.sch_no);
+			break;
+		}
+
+		/*
+		 * Flush all I/O and wait for
+		 * cancel/halt/clear completion.
+		 */
+		private->completion = &completion;
+		spin_unlock_irq(sch->lock);
 
-			private->completion = NULL;
-			flush_workqueue(vfio_ccw_work_q);
-			spin_lock_irq(sch->lock);
-			ret = cio_cancel_halt_clear(sch, &iretry);
-		};
+		if (ret == -EBUSY)
+			wait_for_completion_timeout(&completion, 3*HZ);
 
+		private->completion = NULL;
+		flush_workqueue(vfio_ccw_work_q);
+		spin_lock_irq(sch->lock);
 		ret = cio_disable_subchannel(sch);
 	} while (ret == -EBUSY);
 out_unlock:
-- 
2.20.1