------- Comment From h.carst...@de.ibm.com 2018-04-07 04:22 EDT-------
The attached patch applies to all kernel versions including v4.14.
With v4.15 the rwlock code has been rewritten. See upstream git commit
eb3b7b848fb3 ("s390/rwlock: introduce rwlock wait queueing").
Since the upstream code has been rewritten there also won't be an
upstream git commit id available which contains the attached fix.
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https://bugs.launchpad.net/bugs/1761674
Title:
[Ubuntu 16.04] kernel: fix rwlock implementation
Status in Ubuntu on IBM z Systems:
Triaged
Status in linux package in Ubuntu:
Triaged
Bug description:
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.
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