Re: [win-pv-devel] [PATCH 2/2] Fix race calculating SrbExt->Count

[Andreas Kinzler <ml-ak@xxxxxxxxx>]

In the analysis of my hangs it also turned out that an incorrect value
of SrbExt->Count caused the loss of the SRB. In PdoCompleteResponse I
added the following debug code

-    if (InterlockedDecrement(&SrbExt->ReqCount) == 0) {
+       c = (ULONG) InterlockedDecrement(&SrbExt->Count);
+       if (c + 1 > Pdo->maxReqsPerSrb)
+       {
+               Pdo->maxReqsPerSrb = c + 1;
+               Warning("new maximum reqs/SRB = %08x, PDO = %I64x, SRB = %I64x\n", 
Pdo->maxReqsPerSrb, (ULONGLONG) Pdo, (ULONGLONG) Srb);
+       }
+    if (c == 0) {

Then in the debug log I saw the following line

XENVBD|PdoCompleteResponse:new maximum reqs/SRB = ffffffff, PDO = 
ffffe001ff1b2020, SRB = ffffe00200d439f0

just seconds before the PDO resets started. That means the count had
dropped to -2. However I did not (yet) find the reason for that. Thanks
for finding the race. Since the issue is quite clear now, I don't think
the debug code is needed. Attached is my patch (only for the count
issue) with a renaming of the "Count" field to make it more clear what
kind of count it is.

Regards Andreas


On 16.02.2017 11:11, owen.smith@xxxxxxxxxx wrote:
> From: Owen Smith <owen.smith@xxxxxxxxxx>
>
> It is possible under heavy loads for the backend to start completing
> sub-requests of a Srb before the SrbExt Count is set. This would
> leave the count unable to reach 0 (as 1 or more requests are skipped
> by count being overridden)

Attachment: srbext-count.patch
Description: Text document

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