If tf_trapno contains garbage which appears to be equal to T_NMI,
e.g. due to thread previously entered kernel due to NMI, doreti
sequence skips ast, and does so until a trap or hardware interrupt
occur.
The visible effects of the issue are quite confusing. First, signals
delivery is postponed in observable ways. In particular, the
guarantee that unblocked async signals queue is flushed before a
return from syscall, is broken. Second, if there are pending signals,
all interruptible sleeps of the stuck thread are aborted immediately.
Since modern CPUs are relatively fast and tickless kernel generates
low interrupt rate, the faulty condition might exist for long time (in
an application time scale).
In collaboration with: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
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