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Don't let protected variable access to be reordered after spinlock release.

LWLockAcquireWithVar needs to set the protected variable while holdingthe spinlock. Need to use a volatile pointer to make sure it doesn't getreordered by the compiler. The other functions that accessed the protectedvariable already got this right.9.4 only. Earlier releases didn't have this code, and in master, spinlockrelease acts as a compiler barrier.

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src/backend/storage/lmgr
- lwlock.c

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`‎src/backend/storage/lmgr/lwlock.c`

Lines changed: 3 additions & 2 deletions

Original file line number	Diff line number	Diff line change
`@@ -482,6 +482,7 @@ static inline bool`
`482`	`482`	`LWLockAcquireCommon(LWLockl,LWLockModemode,uint64valptr,uint64val)`
`483`	`483`	`{`
`484`	`484`	`volatileLWLock*lock=l;`
	`485`	`+volatileuint64*valp=valptr;`
`485`	`486`	`PGPROC*proc=MyProc;`
`486`	`487`	`boolretry= false;`
`487`	`488`	`boolresult= true;`
`@@ -637,8 +638,8 @@ LWLockAcquireCommon(LWLock l, LWLockMode mode, uint64 valptr, uint64 val)`
`637`	`638`	`}`
`638`	`639`
`639`	`640`	`/* If there's a variable associated with this lock, initialize it */`
`640`		`-if (valptr)`
`641`		`-*valptr=val;`
	`641`	`+if (valp)`
	`642`	`+*valp=val;`
`642`	`643`
`643`	`644`	`/* We are done updating shared state of the lock itself. */`
`644`	`645`	`SpinLockRelease(&lock->mutex);`

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Commit4971c36

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`‎src/backend/storage/lmgr/lwlock.c`

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