CA-395093: sync between tapback slave process and xenopsd #435
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Signed-off-by: Chunjie Zhu <[email protected]>
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The xenopsd side code update, xapi-project/xen-api#6825 |
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add @MarkSymsCtx @TimSmithCtx @LunfanZhang to review |
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Jan 9, 2026
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This all feels like it is adding complexity and risk to an already fragile part of the VM start process. I can easily see the tapback process getting stuck waiting for the response. I think we need to find a better way of achieving the aims of this pair of PRs.
| err = errno; | ||
| goto fail; | ||
| } | ||
| err = fprintf(fp, "ping"); |
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This should follow the precendent set by the PV ring handshake and use integer state values.
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Independent of the correct solution, the explanation that is part of this PR should be part of the commit message. |
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In some corner case, physical-device-path xenstore watch event is fired before slave tapback process ready to process xenstore watch event, thus, slave tapback process would miss xenstore watch event, then blktap io datapath fails to establish.
In xenopsd side, the vbd-script waits for tapback slave process ready by checking /var/run/tapback..statefile, if the file is present and file contains "ping" string, then vbd-script updates the file, writes "pong" to the file and continues to update xenstore, otherwise, just wait.
In tapback slave process side, once it get prepared to process xenstore watch event, it writes "ping" string to /var/run/tapback..statefile, then waits for acknowledge by checking if the file contains "pong" string, after seeing "pong" string, it removes /var/run/tapback..statefile and continues to work.