Bug #866

(resend) hwpmc [7/13]

Added by aoiko almost 7 years ago. Updated over 4 years ago.

Status:ClosedStart date:
Priority:NormalDue date:
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Description

Careful reading from other people is what I'm hoping for. Just because I'm
submitting it doesn't mean I believe the code (even the parts I've tested) is
bug-free!

Well, as for the 'well documented' part, I couldn't agree more. Would
something like this explanation (from the Oct/2000 commit message) be
sufficient?

"- Expand the set of atomic operations to optionally include memory barriers
in most of the atomic operations. Now for these operations, you can
use the normal atomic operation, you can use the operation with a read
barrier, or you can use the operation with a write barrier. The function
names follow the same semantics used in the ia64 instruction set. An
atomic operation with a read barrier has the extra suffix 'acq', due to
it having "acquire" semantics. An atomic operation with a write barrier
has the extra suffix 'rel'. These suffixes are inserted between the
name of the operation to perform and the typename. For example, the
atomic_add_int() function now has 3 variants:
- atomic_add_int() - this is the same as the previous function
- atomic_add_acq_int() - this function combines the add operation with a
read memory barrier
- atomic_add_rel_int() - this function combines the add operation with a
write memory barrier
- Add 'ptr' to the list of types that we can perform atomic operations
on. This allows one to do atomic operations on uintptr_t's. This is
useful in the mutex code, for example, because the actual mutex lock is
a pointer.
- Add two new operations for doing loads and stores with memory barriers.
The new load operations use a read barrier before the load, and the
new store operations use a write barrier after the load. For example,
atomic_load_acq_int() will atomically load an integer as well as
enforcing a read barrier."

Now, about them not being portable, I simply don't get what you mean. If
you're refering to memory-ordering semantics on other architectures, I think
they are adequate; indeed they seem to be written with ia64 in mind, which
according to Paul McKenney (see Table 1 in this article
http://www.linuxjournal.com/article/8212) are very relaxed. If anything, one
could argue that they are *too* portable and you're better of just
open-coding the memory barriers (although these macros, by emphasizing the
connection with the atomic op, may make the code clearer for some people).

That said, the names are not, IMHO, very intuitive.

Can you clarify?

Thanks,
Aggelos

History

#1 Updated by alexh over 4 years ago

See Issue1714.

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