"ipig" <ipig(at)ercist(dot)iscas(dot)ac(dot)cn> writes:
> That is to say, if p0 wants to lock A again, then p0 will be put before p1, and p0 will be at the head of the queue. Why do we need to find the first waiter which conflicts p0? I think that p0 must be added at the head of the wait queue.
Your analysis is assuming that there are only two kinds of lock, which
is not so. Process A might hold a weak lock and process B a slightly
stronger lock that doesn't conflict with A's. In the wait queue there
might be process C wanting a lock that conflicts with B's but not A's,
followed by process D wanting a strong lock that conflicts with all three.
Now suppose A wants to get a lock of the same type D wants. Since this
conflicts with B's existing lock, A must wait. A must go into the queue
before D (else deadlock) but if possible it should go after C, on
A concrete example here is
A has AccessShareLock (reader's lock)
B has RowExclusiveLock (writer's lock)
C wants ShareLock (hence blocked by B but not A)
D wants AccessExclusiveLock (must wait for all three)
If A wants to upgrade to AccessExclusiveLock, it *must* queue in front
of D, and we'd prefer that it queue behind C not in front of C.
regards, tom lane
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