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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 process
of being destroyed by another backend, when searching for a handle.
Such a segment cannot possibly be the one we are looking for, even
if its handle matches. Another slot might hold a recently created
segment that has the same handle value by coincidence, and we need
to keep searching for that one.
The bug caused rare "cannot unpin a segment that is not pinned"
errors on 10 and 11. Similar to commit 6c0fb941
for dsm_attach().
Back-patch to 10, where dsm_unpin_segment() landed.
Author: Thomas Munro
Reported-by: Justin Pryzby
Tested-by: Justin Pryzby (along with other recent DSA/DSM fixes)
Discussion: https://postgr.es/m/20190216023854.GF30291@telsasoft.com
This commit is contained in:
parent
7f39f03441
commit
1d93d18045
@ -891,8 +891,8 @@ dsm_unpin_segment(dsm_handle handle)
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LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
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LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
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for (i = 0; i < dsm_control->nitems; ++i)
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for (i = 0; i < dsm_control->nitems; ++i)
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{
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{
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/* Skip unused slots. */
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/* Skip unused slots and segments that are concurrently going away. */
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if (dsm_control->item[i].refcnt == 0)
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if (dsm_control->item[i].refcnt <= 1)
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continue;
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continue;
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/* If we've found our handle, we can stop searching. */
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/* If we've found our handle, we can stop searching. */
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