Cc: Christoph Lameter <clameter@...>, Chris Snook <csnook@...>, Linux Kernel Mailing List <linux-kernel@...>, <linux-arch@...>, Linus Torvalds <torvalds@...>, <netdev@...>, Andrew Morton <akpm@...>, <ak@...>, <heiko.carstens@...>, <davem@...>, <schwidefsky@...>, <wensong@...>, <horms@...>, <wjiang@...>, <cfriesen@...>, <zlynx@...>, <rpjday@...>, <jesper.juhl@...>, <segher@...>, Herbert Xu <herbert@...>, Paul E. McKenney <paulmck@...>
No, it definitely doesn't. Why should it?
"Atomic w.r.t. all processors" is just your normal, simple "atomicity"
for SMP systems (ensure that that object is modified / set / replaced
in main memory atomically) and has nothing to do with "volatile"
behaviour.
"Volatile behaviour" itself isn't consistently defined (at least
definitely not consistently implemented in various gcc versions across
platforms), but it is /expected/ to mean something like: "ensure that
every such access actually goes all the way to memory, and is not
re-ordered w.r.t. to other accesses, as far as the compiler can take
care of these". The last "as far as compiler can take care" disclaimer
comes about due to CPUs doing their own re-ordering nowadays.
For example (say on i386):
(A)
$ cat tp1.c
int a;
void func(void)
{
a = 10;
a = 20;
}
$ gcc -Os -S tp1.c
$ cat tp1.s
...
movl $20, a
...
(B)
$ cat tp2.c
volatile int a;
void func(void)
{
a = 10;
a = 20;
}
$ gcc -Os -S tp2.c
$ cat tp2.s
...
movl $10, a
movl $20, a
...
(C)
$ cat tp3.c
int a;
void func(void)
{
*(volatile int *)&a = 10;
*(volatile int *)&a = 20;
}
$ gcc -Os -S tp3.c
$ cat tp3.s
...
movl $10, a
movl $20, a
...
In (A) the compiler optimized "a = 10;" away, but the actual store
of the final value "20" to "a" was still "atomic". (B) and (C) also
exhibit "volatile" behaviour apart from the "atomicity".
But as others replied, it seems some callers out there depend upon
atomic ops exhibiting "volatile" behaviour as well, so that answers
my initial question, actually. I haven't looked at the code Paul
pointed me at, but I wonder if that "forget(x)" macro would help
those cases. I'd wish to avoid the "volatile" primitive, personally.
Satyam
-
