mirror of
https://git.postgresql.org/git/postgresql.git
synced 2024-08-29 21:17:18 +02:00
Fix typo and outdated information in README.barrier
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:
parent
c3cc73e144
commit
010429eb1f
@ -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
|
||||||
|
Loading…
Reference in New Issue
Block a user