diff options
author | Tom Lane | 2007-07-16 21:20:36 +0000 |
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committer | Tom Lane | 2007-07-16 21:20:36 +0000 |
commit | 113f402b1c46c960b44230609a803190d7f1f216 (patch) | |
tree | 42791a45e86ee26d181ee6b8aefd552efae361d0 | |
parent | 56bcf425289b3534ed0be33c98491d1d7508a186 (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.c | 13 |
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); } |