[Qemu-devel] thread-pool.c race condition?

[Stefan Hajnoczi]

John Snow has reported that qemu-io can hang when the host is under
heavy load.  He made the following observations in gdb:

1. The program is sitting in aio_poll() (called by bdrv_prwv_co())
waiting for request completion.

2. The thread pool has a ThreadPoolElement with ->state == THREAD_DONE.

The ThreadPoolElement should have been reaped by
thread_pool_completion_bh() and its callback invoked.  For some reason
this didn't happen and the program is blocked in poll(2) waiting.

This suggests a race condition in thread-pool.c or qemu_bh_schedule()
(used to complete ThreadPoolElement from a QEMU event loop).

I don't have a good theory why this happens yet.  Just wanted to share
in case someone else hits this problem.

Stefan