Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 28 Oct 2011 04:37:53 -0700 | Subject | Re: >Re: [RFC] should VM_BUG_ON(cond) really evaluate cond |
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On Thu, Oct 27, 2011 at 9:43 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote: > > The only requirement of atomic_read() is that it must return value > before or after an atomic_write(), not a garbled value.
The problem is that gcc *can* return a garbled value.
> In fact, if a compiler is stupid enough to issue two reads on following > code :
The compiler really *can* be that "stupid". Except the code tends to look like this:
int value = atomic_read(&atomic_var); if (value > 10) return; .. do something with value ..
and gcc may decide - under register pressure, and in the absense of a 'volatile' - to read 'value' first once for that "> 10" check, and then it drops the registers and instead of saving it on the stack frame, it can decide to re-load it from atomic_var.
IOW, "value" could be two or more different values: one value when testing, and *another* value in "do something with value".
This is why we have "ACCESS_ONCE()".
Whether atomics guarantee ACCESS_ONCE() semantics or not is not entirely clear. But afaik, there is no way to tell gcc "access at *most* once, and never ever reload".
Linus
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