Re: [PATCH v2] Call EvtchnUnmask within the BlkifRing lock

[Owen Smith <owen.smith@xxxxxxxxxx>]

Reviewed-by: Owen Smith <owen.smith@xxxxxxxxxx>

________________________________________
From: Tu Dinh <ngoc-tu.dinh@xxxxxxxxxx>
Sent: 30 March 2026 9:32 AM
To: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
Cc: Tu Dinh; Owen Smith
Subject: [PATCH v2] Call EvtchnUnmask within the BlkifRing lock

The call to EvtchnUnmask accesses Channel outside of the blkif ring
lock. Therefore, it can access a stale channel if the DPC is still
running after the channel has been closed in BlkifRingDisconnect. Since
BlkifRingDisconnect runs at DISPATCH_LEVEL, we cannot use
KeFlushQueuedDpcs and have to guard against the event channel's closure
via the Enabled flag instead.

Signed-off-by: Tu Dinh <ngoc-tu.dinh@xxxxxxxxxx>
---
v2: Invert the check conditions before calling EvtchnUnmask
---
 src/xenvbd/ring.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/src/xenvbd/ring.c b/src/xenvbd/ring.c
index 50f8d58..af8a2fc 100644
--- a/src/xenvbd/ring.c
+++ b/src/xenvbd/ring.c
@@ -1612,18 +1612,21 @@ BlkifRingDpc(
         KeRaiseIrql(DISPATCH_LEVEL, &Irql);
         __BlkifRingAcquireLock(BlkifRing);
         Retry = BlkifRingPoll(BlkifRing);
+
+        if (!Retry && BlkifRing->Enabled) {
+            (VOID) XENBUS_EVTCHN(Unmask,
+                                 &Ring->EvtchnInterface,
+                                 BlkifRing->Channel,
+                                 FALSE,
+                                 TRUE);
+        }
+
         __BlkifRingReleaseLock(BlkifRing);
         KeLowerIrql(Irql);

         if (!Retry)
             break;
     }
-
-    XENBUS_EVTCHN(Unmask,
-                  &Ring->EvtchnInterface,
-                  BlkifRing->Channel,
-                  FALSE,
-                  TRUE);
 }

 #define TIME_US(_us)        ((_us) * 10)
--
2.53.0.windows.2


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