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the QLp structure used to occupy 24 bytes on amd64.
with some rearranging the fields we can get it to 16 bytes,
saving 8K in the data section for the 1024 preallocated
structs in the ql arena.
the rest of the changes are of cosmetic nature:
- getqlp() zeros the next pointer, so there is no need to set
it when queueing the entry.
- always explicitely compare pointers to nil.
- delete unused code from ape's qlock.c
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the initial issue was that wunlock() would wakeup readers while
holding the spinlock causing deadlock in libthread programs where
rendezvous() would do a thread switch within the same process
which then can acquire the RWLock again.
the first fix tried to prevent holding the spinlock, waking up
one reader at a time with releasing an re-acquiering the spinlock.
this violates the invariant that readers can only wakup writers
in runlock() when multiple readers where queued at the time of
wunlock(). at the first wakeup, q->head != nil so runlock() would
find a reader queued on runlock() when it expected a writer.
this (hopefully last) fix unlinks *all* the reader QLp's atomically
and in order while holding the spinlock and then traverses the
dequeued chain of QLp structures again to call rendezvous() so
the invariant described above holds.
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when wunlock() was used by threads running within the same proc,
the wunlock() can deadlock as it keeps holding the RWLock.lock
spinlock while indirectly calling _threadrendezvous(). when
_threadrendezvous() switches to another thread in the same proc,
then that thread can hang at rlock()/wlock()/runlock() again
waiting for wunlock() to release the spinlock which will never
happen as lock() does not schedule threads.
wunlock() is changed to release the spinlock during rendezvous
wakeup of readers. note that this is a bit dangerous as more
readers might queue concurrently now which means that if
we cannot keep up with the wakeups, we might keep on waking
readers forever. that will be another patch for the future.
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