On Sep 10, 2007, at 12:46:33, Denys Vlasenko wrote:
My point is that people are confused as to what atomic_read()
exactly means, and this is bad. Same for cpu_relax(). First one
says "read", and second one doesn't say "barrier".
Q&A:
Q: When is it OK to use atomic_read()?
A: You are asking the question, so never.
Q: But I need to check the value of the atomic at this point in time...
A: Your code is buggy if it needs to do that on an atomic_t for
anything other than debugging or optimization. Use either
atomic_*_return() or a lock and some normal integers.
Q: "So why can't the atomic_read DTRT magically?"
A: Because "the right thing" depends on the situation and is usually
best done with something other than atomic_t.
If somebody can post some non-buggy code which is correctly using
atomic_read() *and* depends on the compiler generating extra
nonsensical loads due to "volatile" then the issue *might* be
reconsidered. This also includes samples of code which uses
atomic_read() and needs memory barriers (so that we can fix the buggy
code, not so we can change atomic_read()). So far the only code
samples anybody has posted are buggy regardless of whether or not the
value and/or accessors are flagged "volatile" or not. And hey, maybe
the volatile ops *should* be implemented in inline ASM for future-
proof-ness, but that's a separate issue.
Cheers,
Kyle Moffett
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