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-$Id: Design,v 11.2 1999/11/21 23:08:27 bostic Exp $
-
-There are three ways we do locking in the mpool code:
-
-Locking a handle mutex to provide concurrency for DB_THREAD operations.
-Locking the region mutex to provide mutual exclusion while reading and
-    writing structures in the shared region.
-Locking buffer header mutexes during I/O.
-
-The first will not be further described here.  We use the shared mpool
-region lock to provide mutual exclusion while reading/modifying all of
-the data structures, including the buffer headers.  We use a per-buffer
-header lock to wait on buffer I/O.  The order of locking is as follows:
-
-Searching for a buffer:
-    Acquire the region lock.
-    Find the buffer header.
-    Increment the reference count (guarantee the buffer stays).
-    While the BH_LOCKED flag is set (I/O is going on) {
-	Release the region lock.
-	    Explicitly yield the processor if it's not the first pass
-	    through this loop, otherwise, we can simply spin because
-	    we'll be simply switching between the two locks.
-	Request the buffer lock.
-	The I/O will complete...
-	Acquire the buffer lock.
-	Release the buffer lock.
-	Acquire the region lock.
-    }
-    Return the buffer.
-
-Reading/writing a buffer:
-    Acquire the region lock.
-    Find/create the buffer header.
-    If reading, increment the reference count (guarantee the buffer stays).
-    Set the BH_LOCKED flag.
-    Acquire the buffer lock (guaranteed not to block).
-    Release the region lock.
-    Do the I/O and/or initialize the buffer contents.
-    Release the buffer lock.
-	At this point, the buffer lock is available, but the logical
-	operation (flagged by BH_LOCKED) is not yet completed.  For
-	this reason, among others, threads checking the BH_LOCKED flag
-	must loop around their test.
-    Acquire the region lock.
-    Clear the BH_LOCKED flag.
-    Release the region lock.
-    Return/discard the buffer.
-
-Pointers to DB_MPOOL, MPOOL, DB_MPOOLFILE and MPOOLFILE structures are
-not reacquired when a region lock is reacquired because they couldn't
-have been closed/discarded and because they never move in memory.