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kernel/condition_variable: Unblock earlier and simplify the code more.

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Unblocking after unsetting fVariable just causes too many headaches
and corner cases to deal with; the code as-is did not actually handle
all of them, as it missed the case where the entry thread had called
thread_prepare_to_block but had not yet actually blocked.

Hopefully the last fix for #17444.
This commit is contained in:
Augustin Cavalier 2021-12-07 14:15:54 -05:00
parent acdd232ad5
commit 7ce3c9283e
1 changed files with 3 additions and 11 deletions
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14
src/system/kernel/condition_variable.cpp
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@ -400,23 +400,15 @@ ConditionVariable::_NotifyLocked(bool all, status_t result)
} else {
const status_t waitStatus = atomic_get_and_set(&entry->fWaitStatus, result);
// Prevent the thread from changing status after we unset its fVariable,
// as otherwise it could become unblocked (or may already be so) and then
// re-block itself on something else before we call thread_unblock.
SpinLocker threadLocker(thread->scheduler_lock);
// We need not waste time trying to unblock the thread if it already unblocked.
if (waitStatus == STATUS_WAITING && thread_is_blocked(thread))
thread_unblock(thread, result);
// No matter what the thread is doing, as we were the ones to clear its
// fThread, so we are the ones responsible for decrementing fEntriesCount.
// (We may not validly access the entry once we unset its fVariable.)
atomic_pointer_set(&entry->fVariable, (ConditionVariable*)NULL);
atomic_add(&fEntriesCount, -1);
// Do this after unsetting fVariable, as in case the entry wakes up
// and tries to remove itself, it need not not have to wait for us.
// (We check thread->state here as it cannot change while we hold
// the scheduler lock, unlike thread_is_blocked, which can!)
if (waitStatus == STATUS_WAITING && thread->state == B_THREAD_WAITING)
thread_unblock_locked(thread, result);
}
if (!all)