Commit d1ffa760 authored Apr 16, 2019 by Farhan Ali Committed by Cornelia Huck Apr 24, 2019

vfio-ccw: Prevent quiesce function going into an infinite loop



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@linux.ibm.com>
Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
Message-Id: <4d5a4b98ab1b41ac6131b5c36de18b76c5d66898.1555449329.git.alifm@linux.ibm.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

parent b49bdc86

drivers/s390/cio/vfio_ccw_drv.c

+18 −14

Original line number	Diff line number	Diff line
		@@ -43,11 +43,17 @@ int vfio_ccw_sch_quiesce(struct subchannel *sch)
		if (ret != -EBUSY)
		goto out_unlock;

		do {
		iretry = 255;
		do {

		ret = cio_cancel_halt_clear(sch, &iretry);
		while (ret == -EBUSY) {

		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.
		@@ -55,14 +61,12 @@ int vfio_ccw_sch_quiesce(struct subchannel *sch)
		private->completion = &completion;
		spin_unlock_irq(sch->lock);

		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_cancel_halt_clear(sch, &iretry);
		};

		ret = cio_disable_subchannel(sch);
		} while (ret == -EBUSY);
		out_unlock:

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