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Remove cautions about using volatile from spin.h.

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Commit 0709b7ee72 obsoleted this comment
but neglected to update it.

Thomas Munro
This commit is contained in:
Robert Haas 2015-10-16 14:06:22 -04:00
parent a53c06a13e
commit 78652a3332
1 changed files with 4 additions and 9 deletions
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13
src/include/storage/spin.h
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@ -25,15 +25,10 @@
* Callers must beware that the macro argument may be evaluated multiple
* times!
*
* CAUTION: Care must be taken to ensure that loads and stores of
* shared memory values are not rearranged around spinlock acquire
* and release. This is done using the "volatile" qualifier: the C
* standard states that loads and stores of volatile objects cannot
* be rearranged *with respect to other volatile objects*. The
* spinlock is always written through a volatile pointer by the
* spinlock macros, but this is not sufficient by itself: code that
* protects shared data with a spinlock MUST reference that shared
* data through a volatile pointer.
* Load and store operations in calling code are guaranteed not to be
* reordered with respect to these operations, because they include a
* compiler barrier. (Before PostgreSQL 9.5, callers needed to use a
* volatile qualifier to access data protected by spinlocks.)
*
* Keep in mind the coding rule that spinlocks must not be held for more
* than a few instructions. In particular, we assume it is not possible