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authorTom Lane2007-07-16 21:20:36 +0000
committerTom Lane2007-07-16 21:20:36 +0000
commit113f402b1c46c960b44230609a803190d7f1f216 (patch)
tree42791a45e86ee26d181ee6b8aefd552efae361d0
parent56bcf425289b3534ed0be33c98491d1d7508a186 (diff)
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.
-rw-r--r--contrib/pg_buffercache/pg_buffercache_pages.c13
1 files changed, 10 insertions, 3 deletions
diff --git a/contrib/pg_buffercache/pg_buffercache_pages.c b/contrib/pg_buffercache/pg_buffercache_pages.c
index b28de718da..3942e615e9 100644
--- a/contrib/pg_buffercache/pg_buffercache_pages.c
+++ b/contrib/pg_buffercache/pg_buffercache_pages.c
@@ -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);
}