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STATUS_CALLBACK_RETURNED_PRI_BACK
The callback left the pooled worker in background mode
Windows background mode lowers CPU and I/O scheduling priorities for the calling thread. It is a thread state, so enabling it inside a callback affects whichever work later reuses that worker. The next callback can become unexpectedly slow even when its own priority configuration is normal.
Background mode should bracket only the intended work and be disabled before return. Restoring a numeric base priority alone is not sufficient because background mode also affects resource scheduling behavior beyond the ordinary thread priority value.
What to inspect
- Record entry and exit priority, background-mode transitions, I/O priority observations, and worker thread ID.
- Pair
THREAD_MODE_BACKGROUND_BEGINwithTHREAD_MODE_BACKGROUND_ENDon the same thread. - Place restoration in guaranteed cleanup rather than after the last expected statement.
- Check nested helpers so only the component that entered background mode exits it.
References
- Microsoft Open Specifications: NTSTATUS values
- Microsoft: Thread priority background processing mode
- Microsoft: SetThreadPriority
- Microsoft: pooled threads and callback environments
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