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Restore lock level to update statusFlags

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Reverts 27838981be (some comments are kept).  Per discussion, it does
not seem safe to relax the lock level used for this; in order for it to
be safe, there would have to be memory barriers between the point we set
the flag and the point we set the trasaction Xid, which perhaps would
not be so bad; but there would also have to be barriers at the readers'
side, which from a performance perspective might be bad.

Now maybe this analysis is wrong and it *is* safe for some reason, but
proof of that is not trivial.

Discussion: https://postgr.es/m/20201118190928.vnztes7c2sldu43a@alap3.anarazel.de
This commit is contained in:
Alvaro Herrera
2020-11-26 12:30:48 -03:00
parent 9fbc3f318d
commit dcfff74fb1
4 changed files with 18 additions and 18 deletions
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6
src/include/storage/proc.h
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@@ -102,9 +102,9 @@ typedef enum
* but its myProcLocks[] lists are valid.
*
* We allow many fields of this struct to be accessed without locks, such as
* statusFlags or delayChkpt. However, keep in mind that writing mirrored ones
* (see below) requires holding ProcArrayLock or XidGenLock in at least shared
* mode, so that pgxactoff does not change concurrently.
* delayChkpt and isBackgroundWorker. However, keep in mind that writing
* mirrored ones (see below) requires holding ProcArrayLock or XidGenLock in
* at least shared mode, so that pgxactoff does not change concurrently.
*
* Mirrored fields:
*