Writers waiting on readers to finish can set the RW_LOCK_WRITE_SPINNER
bit. This prevents most new readers from coming on. However, the last
reader to unlock also clears the bit which means new readers can sneak
in and the cycle starts over.
Change the code to keep the bit after last unlock.
Note that starvation potential is still there: no matter how many write
spinners are there, there is one bit. After the writer unlocks, the lock
is free to get raided by readers again. It is good enough for the time
being.
The real fix would include counting writers.
This runs into a caveat: the writer which set the bit may now be preempted.
In order to get rid of the problem all attempts to set the bit are preceeded
with critical_enter.
The bit gets cleared when the thread which set it goes to sleep. This way
an invariant holds that if the bit is set, someone is actively spinning and
will grab the lock soon. In particular this means that readers which find
the lock in this transient state can safely spin until the lock finds itself
an owner (i.e. they don't need to block nor speculate how long to spin
speculatively).
Tested by: pho
9feec7ef69