. unpause() and revive() can race - revive() can run during
a device i/o unblock, causing two sendnb()s to occur, and the
2nd one to fail
. this can easily happen when a process is blocking on tty and
is then killed by a signal - tty cancels the i/o and then
kills the process by a signal
Change-Id: Ia319acaedfa336b78c030a2c4af7246959bdcf87