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Description: kernel: fix rwlock implementation
Symptom: Kernel hangs, due to deadlock on an rwlock.
Problem: With upstream commit 94232a4332de ("s390/rwlock: improve writer
fairness") rwlock writer fairness was supposed to be
implemented. If a writer tries to take an rwlock it sets
unconditionally the writer bit within the lock word and waits
until all readers have released the lock. This however can lead
to a deadlock since rwlocks can be taken recursively by readers.
If e.g. CPU 0 holds the lock as a reader, and CPU 1 wants to
write-lock the lock, then CPU 1 sets the writer bit and
afterwards busy waits for CPU 0 to release the lock. If now CPU 0
tries to read-lock the lock again (recursively) it will also busy
wait until CPU 1 removes the writer bit, which will never happen,
since it waits for the first reader on CPU 0 to release the lock.
Solution: Revert the rwlock writer fairness semantics again.
** Affects: linux (Ubuntu)
Importance: Undecided
Assignee: Skipper Bug Screeners (skipper-screen-team)
Status: New
** Tags: architecture-s39064 bugnameltc-166421 severity-high
targetmilestone-inin16041
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[Ubuntu 16.04] kernel: fix rwlock implementation
https://bugs.launchpad.net/bugs/1761674
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