@@ -81,18 +81,20 @@ it won't be able to actually examine the page until it acquires shared
8181or exclusive content lock.
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84- Rule #5 only affects VACUUM operations. Obtaining the
85- necessary lock is done by the bufmgr routine LockBufferForCleanup().
86- It first gets an exclusive lock and then checks to see if the shared pin
87- count is currently 1. If not, it releases the exclusive lock (but not the
88- caller's pin) and waits until signaled by another backend, whereupon it
89- tries again. The signal will occur when UnpinBuffer decrements the shared
90- pin count to 1. As indicated above, this operation might have to wait a
91- good while before it acquires lock, but that shouldn't matter much for
92- concurrent VACUUM. The current implementation only supports a single
93- waiter for pin-count-1 on any particular shared buffer. This is enough
94- for VACUUM's use, since we don't allow multiple VACUUMs concurrently on a
95- single relation anyway.
84+ Obtaining the lock needed under rule #5 is done by the bufmgr routines
85+ LockBufferForCleanup() or ConditionalLockBufferForCleanup(). They first get
86+ an exclusive lock and then check to see if the shared pin count is currently
87+ 1. If not, ConditionalLockBufferForCleanup() releases the exclusive lock and
88+ then returns false, while LockBufferForCleanup() releases the exclusive lock
89+ (but not the caller's pin) and waits until signaled by another backend,
90+ whereupon it tries again. The signal will occur when UnpinBuffer decrements
91+ the shared pin count to 1. As indicated above, this operation might have to
92+ wait a good while before it acquires lock, but that shouldn't matter much for
93+ concurrent VACUUM. The current implementation only supports a single waiter
94+ for pin-count-1 on any particular shared buffer. This is enough for VACUUM's
95+ use, since we don't allow multiple VACUUMs concurrently on a single relation
96+ anyway. Anyone wishing to obtain a cleanup lock outside of recovery or a
97+ VACUUM must use the conditional variant of the function.
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98100Buffer Manager's Internal Locking