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From: Cornelia Huck <cornelia.huck@de.ibm.com>
To: peter.maydell@linaro.org
Cc: qemu-devel@nongnu.org, agraf@suse.de, borntraeger@de.ibm.com,
	jfrei@linux.vnet.ibm.com,
	Cornelia Huck <cornelia.huck@de.ibm.com>,
	Sascha Silbe <silbe@linux.vnet.ibm.com>
Subject: [Qemu-devel] [PULL 05/14] watchdog/diag288: avoid race condition on expired watchdog
Date: Tue,  1 Mar 2016 12:52:54 +0100	[thread overview]
Message-ID: <1456833183-27244-6-git-send-email-cornelia.huck@de.ibm.com> (raw)
In-Reply-To: <1456833183-27244-1-git-send-email-cornelia.huck@de.ibm.com>

From: Sascha Silbe <silbe@linux.vnet.ibm.com>

When configured to inject an NMI, watchdog_perform_action() may cause
the BQL to be temporarily relinquished (inject_nmi() → ... →
s390_nmi() → s390_cpu_restart() → run_on_cpu()). When the guest issues
diag 288 again in response to the NMI, the diag 288 operation will
race against wdt_diag288_reset(). Depending on scheduler behaviour,
wdt_diag288_reset() may be run after the guest issued a diag 288
Init. As a result, we will cancel the timer the guest just set up. The
effect observed by the guest is that a second expiry does not trigger
the watchdog action and diag 288 Change operations fail.

Fix this by resetting the timer _before_ invoking the action.

Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Acked-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
---
 hw/watchdog/wdt_diag288.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/hw/watchdog/wdt_diag288.c b/hw/watchdog/wdt_diag288.c
index 5eb5b94..1c3658e 100644
--- a/hw/watchdog/wdt_diag288.c
+++ b/hw/watchdog/wdt_diag288.c
@@ -51,15 +51,19 @@ static void diag288_reset(void *opaque)
 static void diag288_timer_expired(void *dev)
 {
     qemu_log_mask(CPU_LOG_RESET, "Watchdog timer expired.\n");
-    watchdog_perform_action();
-    /* Reset the watchdog only if the guest was notified about expiry. */
+    /* Reset the watchdog only if the guest gets notified about
+     * expiry. watchdog_perform_action() may temporarily relinquish
+     * the BQL; reset before triggering the action to avoid races with
+     * diag288 instructions. */
     switch (get_watchdog_action()) {
     case WDT_DEBUG:
     case WDT_NONE:
     case WDT_PAUSE:
-         return;
+        break;
+    default:
+        wdt_diag288_reset(dev);
     }
-    wdt_diag288_reset(dev);
+    watchdog_perform_action();
 }
 
 static int wdt_diag288_handle_timer(DIAG288State *diag288,
-- 
2.7.2

  parent reply	other threads:[~2016-03-01 11:53 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-03-01 11:52 [Qemu-devel] [PULL 00/14] s390x patches Cornelia Huck
2016-03-01 11:52 ` [Qemu-devel] [PULL 01/14] linux-headers: update against kvm/next Cornelia Huck
2016-03-01 11:52 ` [Qemu-devel] [PULL 02/14] s390x/kvm: sync fprs via kvm_run Cornelia Huck
2016-03-01 11:52 ` [Qemu-devel] [PULL 03/14] s390x: fix debug statement in trigger_page_fault() Cornelia Huck
2016-03-01 11:52 ` [Qemu-devel] [PULL 04/14] s390x: remove {kvm_}s390_virtio_irq() Cornelia Huck
2016-03-01 11:52 ` Cornelia Huck [this message]
2016-03-01 11:52 ` [Qemu-devel] [PULL 06/14] s390x/virtio: old machine leftovers Cornelia Huck
2016-03-01 11:52 ` [Qemu-devel] [PULL 07/14] s390x/css: introduce indicator refcounting interfaces Cornelia Huck
2016-03-01 11:52 ` [Qemu-devel] [PULL 08/14] s390x/pci: fix reg/dereg irq functions Cornelia Huck
2016-03-01 11:52 ` [Qemu-devel] [PULL 09/14] s390x/css: Allocate channel_subsys statically Cornelia Huck
2016-03-01 11:52 ` [Qemu-devel] [PULL 10/14] s390x/css: Use static initialization for channel_subsys fields Cornelia Huck
2016-03-01 11:53 ` [Qemu-devel] [PULL 11/14] s390x/pci: use PCI_MSIX_FLAGS on retrieving the MSIX entries Cornelia Huck
2016-03-01 11:53 ` [Qemu-devel] [PULL 12/14] MAINTAINERS: Remove the old s390-virtio machine Cornelia Huck
2016-03-01 11:53 ` [Qemu-devel] [PULL 13/14] MAINTAINERS: Remove entry for hw/s390x/s390-virtio-bus.[ch] Cornelia Huck
2016-03-01 11:53 ` [Qemu-devel] [PULL 14/14] s390x/css: only suspend when enabled by orb Cornelia Huck
2016-03-01 13:45 ` [Qemu-devel] [PULL 00/14] s390x patches Peter Maydell

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