These can occur in the following scenario:
- thread 1 has blocked signal A
- thread 2 has not blocked signal A
- signal A is sent to a process with thread 1 and thread 2
- thread 1 is simultaneously unblocking signal A, and thus jumps to the
trampoline
- thread 2 is awoken, but thread 1 won the "fetch_andn" to clear the
signal bit.
If the signal asm is jumped to automatically after each timer interrupt,
this scenario will also be relatively common due to that.
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