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Commit7718a99

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Fix race in dsm_unpin_segment() when handles are reused.
Teach dsm_unpin_segment() to skip segments that are in the processof being destroyed by another backend, when searching for a handle.Such a segment cannot possibly be the one we are looking for, evenif its handle matches. Another slot might hold a recently createdsegment that has the same handle value by coincidence, and we needto keep searching for that one.The bug caused rare "cannot unpin a segment that is not pinned"errors on 10 and 11. Similar to commit6c0fb94 for dsm_attach().Back-patch to 10, where dsm_unpin_segment() landed.Author: Thomas MunroReported-by: Justin PryzbyTested-by: Justin Pryzby (along with other recent DSA/DSM fixes)Discussion:https://postgr.es/m/20190216023854.GF30291@telsasoft.com
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  • src/backend/storage/ipc

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‎src/backend/storage/ipc/dsm.c

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -904,8 +904,8 @@ dsm_unpin_segment(dsm_handle handle)
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LWLockAcquire(DynamicSharedMemoryControlLock,LW_EXCLUSIVE);
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for (i=0;i<dsm_control->nitems;++i)
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{
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/* Skip unused slots. */
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if (dsm_control->item[i].refcnt==0)
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/* Skip unused slots and segments that are concurrently going away. */
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if (dsm_control->item[i].refcnt<=1)
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continue;
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/* If we've found our handle, we can stop searching. */

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