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Commitcd3ccf8

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Base LWLock limits directly on MAX_BACKENDS
Jacob reported that comments for LW_SHARED_MASK referenced a MAX_BACKENDSlimit of 2^23-1, but that MAX_BACKENDS is actually limited to 2^18-1. Thelimit was lowered in4835458, but the comment in lwlock.c wasn't updated.Instead of just fixing the comment, it seems better to directly base thelwlock defines on MAX_BACKENDS and add static assertions to ensure that thereis enough space. That way there's no comment that can go out of sync in thefuture.As part of that change I noticed that for some reason the high bit wasn't usedfor flags, which seems somewhat odd. Redefine the flag values to start at thehighest bit.Reported-by: Jacob Brazeal <jacob.brazeal@gmail.com>Reviewed-by: Jacob Brazeal <jacob.brazeal@gmail.com>Discussion:https://postgr.es/m/CA+COZaBO_s3LfALq=b+HcBHFSOEGiApVjrRacCe4VP9m7CJsNQ@mail.gmail.com
1 parent6394a3a commitcd3ccf8

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‎src/backend/storage/lmgr/lwlock.c

Lines changed: 20 additions & 10 deletions
Original file line numberDiff line numberDiff line change
@@ -91,19 +91,29 @@
9191
#endif
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9393

94-
#defineLW_FLAG_HAS_WAITERS((uint32) 1 << 30)
95-
#defineLW_FLAG_RELEASE_OK((uint32) 1 << 29)
96-
#defineLW_FLAG_LOCKED((uint32) 1 << 28)
97-
98-
#defineLW_VAL_EXCLUSIVE((uint32) 1 << 24)
94+
#defineLW_FLAG_HAS_WAITERS((uint32) 1 << 31)
95+
#defineLW_FLAG_RELEASE_OK((uint32) 1 << 30)
96+
#defineLW_FLAG_LOCKED((uint32) 1 << 29)
97+
#defineLW_FLAG_BITS3
98+
#defineLW_FLAG_MASK(((1<<LW_FLAG_BITS)-1)<<(32-LW_FLAG_BITS))
99+
100+
/* assumes MAX_BACKENDS is a (power of 2) - 1, checked below */
101+
#defineLW_VAL_EXCLUSIVE(MAX_BACKENDS + 1)
99102
#defineLW_VAL_SHARED1
100103

101-
#defineLW_LOCK_MASK((uint32) ((1 << 25)-1))
102-
/* Must be greater than MAX_BACKENDS - which is 2^23-1, so we're fine. */
103-
#defineLW_SHARED_MASK((uint32) ((1 << 24)-1))
104+
/* already (power of 2)-1, i.e. suitable for a mask */
105+
#defineLW_SHARED_MASKMAX_BACKENDS
106+
#defineLW_LOCK_MASK(MAX_BACKENDS | LW_VAL_EXCLUSIVE)
107+
108+
109+
StaticAssertDecl(((MAX_BACKENDS+1)&MAX_BACKENDS)==0,
110+
"MAX_BACKENDS + 1 needs to be a power of 2");
111+
112+
StaticAssertDecl((MAX_BACKENDS&LW_FLAG_MASK)==0,
113+
"MAX_BACKENDS and LW_FLAG_MASK overlap");
104114

105-
StaticAssertDecl(LW_VAL_EXCLUSIVE> (uint32)MAX_BACKENDS,
106-
"MAX_BACKENDS too big for lwlock.c");
115+
StaticAssertDecl((LW_VAL_EXCLUSIVE&LW_FLAG_MASK)==0,
116+
"LW_VAL_EXCLUSIVE and LW_FLAG_MASK overlap");
107117

108118
/*
109119
* There are three sorts of LWLock "tranches":

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