Fix pg_buffercache to release buffer partition locks in reverse order,

and add a note about why.  This is not tremendously important right now,
probably, but it will get more urgent if NUM_BUFFER_PARTITIONS is increased
as much as proposed.

diff --git a/contrib/pg_buffercache/pg_buffercache_pages.c b/contrib/pg_buffercache/pg_buffercache_pages.c
index 991c3db8e251daba873a451ec594361b48f3e943..e7c5b06a563bfcc62ae236203dfab2a0133abb0e 100644 (file)
--- a/contrib/pg_buffercache/pg_buffercache_pages.c
+++ b/contrib/pg_buffercache/pg_buffercache_pages.c
@@ -3,7 +3,7 @@
  * pg_buffercache_pages.c
  *   display some contents of the buffer cache
  *
- *   $PostgreSQL: pgsql/contrib/pg_buffercache/pg_buffercache_pages.c,v 1.12 2007/04/07 16:09:14 momjian Exp $
+ *   $PostgreSQL: pgsql/contrib/pg_buffercache/pg_buffercache_pages.c,v 1.13 2007/07/16 21:20:36 tgl Exp $
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
@@ -110,7 +110,8 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
        /*
         * To get a consistent picture of the buffer state, we must lock all
         * partitions of the buffer map.  Needless to say, this is horrible
-        * for concurrency...
+        * for concurrency.  Must grab locks in increasing order to avoid
+        * possible deadlocks.
         */
        for (i = 0; i < NUM_BUFFER_PARTITIONS; i++)
            LWLockAcquire(FirstBufMappingLock + i, LW_SHARED);
@@ -145,8 +146,14 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
            UnlockBufHdr(bufHdr);
        }
 
-       /* Release Buffer map. */
-       for (i = 0; i < NUM_BUFFER_PARTITIONS; i++)
+       /*
+        * And release locks.  We do this in reverse order for two reasons:
+        * (1) Anyone else who needs more than one of the locks will be trying
+        * to lock them in increasing order; we don't want to release the other
+        * process until it can get all the locks it needs.
+        * (2) This avoids O(N^2) behavior inside LWLockRelease.
+        */
+       for (i = NUM_BUFFER_PARTITIONS; --i >= 0;)
            LWLockRelease(FirstBufMappingLock + i);
    }