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Fix typo and outdated information in README.barrier

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README.barrier didn't seem to get the memo when atomics were added. Fix
that.

Author: Tatsuo Ishii, David Rowley
Discussion: https://postgr.es/m/20210516.211133.2159010194908437625.t-ishii%40sraoss.co.jp
Backpatch-through: 9.6, oldest supported release
This commit is contained in:
David Rowley 2021-05-18 09:55:54 +12:00
parent c3cc73e144
commit 010429eb1f
1 changed files with 5 additions and 7 deletions
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12
src/backend/storage/lmgr/README.barrier
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@ -103,7 +103,7 @@ performed before the barrier, and vice-versa.
Although this code will work, it is needlessly inefficient. On systems with Although this code will work, it is needlessly inefficient. On systems with
strong memory ordering (such as x86), the CPU never reorders loads with other strong memory ordering (such as x86), the CPU never reorders loads with other
loads, nor stores with other stores. It can, however, allow a load to loads, nor stores with other stores. It can, however, allow a load to be
performed before a subsequent store. To avoid emitting unnecessary memory performed before a subsequent store. To avoid emitting unnecessary memory
instructions, we provide two additional primitives: pg_read_barrier(), and instructions, we provide two additional primitives: pg_read_barrier(), and
pg_write_barrier(). When a memory barrier is being used to separate two pg_write_barrier(). When a memory barrier is being used to separate two
@ -155,18 +155,16 @@ Although this may compile down to a single machine-language instruction,
the CPU will execute that instruction by reading the current value of foo, the CPU will execute that instruction by reading the current value of foo,
adding one to it, and then storing the result back to the original address. adding one to it, and then storing the result back to the original address.
If two CPUs try to do this simultaneously, both may do their reads before If two CPUs try to do this simultaneously, both may do their reads before
either one does their writes. Eventually we might be able to use an atomic either one does their writes. Such a case could be made safe by using an
fetch-and-add instruction for this specific case on architectures that support atomic variable and an atomic add. See port/atomics.h.
it, but we can't rely on that being available everywhere, and we currently
have no support for it at all. Use a lock.
2. Eight-byte loads and stores aren't necessarily atomic. We assume in 2. Eight-byte loads and stores aren't necessarily atomic. We assume in
various places in the source code that an aligned four-byte load or store is various places in the source code that an aligned four-byte load or store is
atomic, and that other processes therefore won't see a half-set value. atomic, and that other processes therefore won't see a half-set value.
Sadly, the same can't be said for eight-byte value: on some platforms, an Sadly, the same can't be said for eight-byte value: on some platforms, an
aligned eight-byte load or store will generate two four-byte operations. If aligned eight-byte load or store will generate two four-byte operations. If
you need an atomic eight-byte read or write, you must make it atomic with a you need an atomic eight-byte read or write, you must either serialize access
lock. with a lock or use an atomic variable.
3. No ordering guarantees. While memory barriers ensure that any given 3. No ordering guarantees. While memory barriers ensure that any given
process performs loads and stores to shared memory in order, they don't process performs loads and stores to shared memory in order, they don't