On 04/12/2019 04:10 AM, Cornelia Huck wrote:
On Thu, 11 Apr 2019 16:30:44 -0400
Farhan Ali <alifm@xxxxxxxxxxxxx> wrote:
On 04/11/2019 12:24 PM, Cornelia Huck wrote:
On Mon, 8 Apr 2019 17:05:32 -0400
Farhan Ali <alifm@xxxxxxxxxxxxx> wrote:
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@xxxxxxxxxxxxx>
Signed-off-by: Farhan Ali <alifm@xxxxxxxxxxxxx>
---
drivers/s390/cio/vfio_ccw_drv.c | 27 ++++++++++++---------------
1 file changed, 12 insertions(+), 15 deletions(-)
diff --git a/drivers/s390/cio/vfio_ccw_drv.c b/drivers/s390/cio/vfio_ccw_drv.c
index 5aca475..4405f2a 100644
--- a/drivers/s390/cio/vfio_ccw_drv.c
+++ b/drivers/s390/cio/vfio_ccw_drv.c
@@ -43,26 +43,23 @@ 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);
-
+ /*
+ * Flush all I/O and wait for
+ * cancel/halt/clear completion.
+ */
+ private->completion = &completion;
+ spin_unlock_irq(sch->lock);
+
+ if (ret == -EBUSY)
I don't think you need to do the unlock/lock and change
private->completion if you don't actually wait, no?
If we don't end up waiting, then changing private->completion would not
be needed. But we would still need to release the spinlock due to [1].
Looking at the possible return codes:
* -ENODEV -> device is not operational anyway, in theory you should even
not need to bother with disabling the subchannel
* -EIO -> we've run out of retries, and the subchannel still is not
idle; I'm not sure if we could do anything here, as disable is
unlikely to work, either
We could break out of the loop early for these cases. My thinking was I
wanted to depend on the result of trying to disable, because ultimately
that's what we want.
I can add the cases to break out of the loop early.
The -ENODEV case does not really hurt, as it will get us out of the
loop anyway. But for the -EIO case, I think we'll get -EBUSY from the
disable and stay within the loop endlessly?
* -EBUSY -> we expect an interrupt (or a timeout), the loop looks fine
for that
* 0 -> the one thing that might happen is that we get an unsolicited
interrupt between the successful cancel_halt_clear and the disable;
not even giving up the lock here might even be better here?
I didn't think of this case, but if cancel_halt_clear succeeds with 0
then we should wait, no?
For 0 I don't expect a solicited interrupt (documentation for the
functions says that the subchannel is idle in that case); it's just the
unsolicited interrupt that might get into the way.
I think this loop will probably work as it is after this patch, but
giving up the lock when not really needed makes me a bit queasy... what
do others think?
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);
- };
-
+ private->completion = NULL;
[1] flush_workqueue can go to sleep so we would still need to release
spinlock and reacquire it again to try disabling the subchannel.
Grr, I thought we could skip the flush in the !-EBUSY case, but I think
we can't due to the possibility of an unsolicited interrupt... what
simply adding handling for -EIO (although I'm not sure what we can
sensibly do in that case) and leave the other cases as they are now?
Thinking a little bit more about EIO, if the return code is EIO then it
means we have exhausted all our options with cancel_halt_clear and the
subchannel/device is still status pending, right?
I think we should still continue to try and disable the subchannel,
because if not then the subchannel/device could in some point of time
come back and bite us. So we really should protect the system from this
behavior.
I think for EIO we should log an error message, but still try to
continue with disabling the subchannel. What do you or others think?
+ flush_workqueue(vfio_ccw_work_q);
+ spin_lock_irq(sch->lock);
ret = cio_disable_subchannel(sch);
} while (ret == -EBUSY);
out_unlock:
