Re: why are some atomic_t's not volatile, while most are?

From: Chris Snook
Date: Tue Aug 07 2007 - 21:32:47 EST


Zan Lynx wrote:
On Tue, 2007-08-07 at 15:38 -0600, Chris Friesen wrote:
Chris Snook wrote:

That's why we define atomic_read like so:

#define atomic_read(v) ((v)->counter)

This avoids the aliasing problem, because the compiler must de-reference the pointer every time, which requires a memory fetch.
Can you guarantee that the pointer dereference cannot be optimised away on any architecture? Without other restrictions, a suficiently intelligent optimiser could notice that the address of v doesn't change in the loop and the destination is never written within the loop, so the read could be hoisted out of the loop.

Even now, powerpc (as an example) defines atomic_t as:

typedef struct { volatile int counter; } atomic_t


That volatile is there precisely to force the compiler to dereference it every single time.

I just tried this with GCC 4.2 on x86_64 because I was curious.

struct counter_t { volatile int counter; } test;
struct counter_t *tptr = &test;

int main() {
int i;

tptr->counter = 0;
i = 0;
while(tptr->counter < 100) {
i++;
}
return 0;
}

$ gcc -O3 -S t.c

a snippet of t.s:
main:
.LFB2:
movq tptr(%rip), %rdx
movl $0, (%rdx)
.p2align 4,,7
.L2:
movl (%rdx), %eax
cmpl $99, %eax
jle .L2


Now with the volatile removed:
main:
.LFB2:
movq tptr(%rip), %rax
movl $0, (%rax)
.L2:
jmp .L2

If the compiler can see it clearly, it will optimize out the load
without the volatile.

This is not a problem, since indirect references will cause the CPU to fetch the data from memory/cache anyway.

-- Chris
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/