Re: [PATCH v2 1/3] Capture Context->Request once per SyncWorker iteration

[Owen Smith <owen.smith@xxxxxxxxxx>]

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

________________________________________
From: win-pv-devel <win-pv-devel-bounces@xxxxxxxxxxxxxxxxxxxx> on behalf of Tu 
Dinh <ngoc-tu.dinh@xxxxxxxxxx>
Sent: 16 September 2026 1:34 PM
To: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
Cc: Tu Dinh
Subject: [PATCH v2 1/3] Capture Context->Request once per SyncWorker iteration

Context->Request is a shared variable and so we must capture it instead
of rereading every time. Otherwise, this sequence will be possible:
* A synchronized action (e.g. __SyncProcessorDisableInterrupts)
  completes from the owner's viewpoint, but its worker has not finished
  updating Request = Context->Request yet;
* The owner sets Context->Request to something else;
* Request = Context->Request executes and sets Request to the wrong
  value.

Assisted-by: LLM
Signed-off-by: Tu Dinh <ngoc-tu.dinh@xxxxxxxxxx>
---
 src/xenbus/sync.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/src/xenbus/sync.c b/src/xenbus/sync.c
index eefa2fe..b416dbb 100644
--- a/src/xenbus/sync.c
+++ b/src/xenbus/sync.c
@@ -284,21 +284,22 @@ SyncWorker(

     Request = SYNC_REQUEST_NONE;
     for (;;) {
-        NTSTATUS status;
+        SYNC_REQUEST    Next;
+        NTSTATUS        status;

-        KeMemoryBarrier();
+        Next = ReadAcquire((LONG *)&Context->Request);

-        if (Context->Request == SYNC_REQUEST_EXIT)
+        if (Next == SYNC_REQUEST_EXIT)
             break;

-        if (Context->Request == Request) {
+        if (Next == Request) {
             _mm_pause();
             continue;
         }

         status = STATUS_SUCCESS;

-        switch (Context->Request) {
+        switch (Next) {
         case SYNC_REQUEST_DISABLE_INTERRUPTS:
             status = __SyncProcessorDisableInterrupts(&Irql);
             break;
@@ -321,7 +322,7 @@ SyncWorker(
         }

         if (NT_SUCCESS(status))
-            Request = Context->Request;
+            Request = Next;
     }

     ASSERT3U(KeGetCurrentIrql(), ==, DISPATCH_LEVEL);
--
2.55.0.windows.3



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