On 5/7/07, Erik Mouw <mouw@xxxxxxxxxxxx> wrote:
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On Mon, May 07, 2007 at 09:25:02PM +0530, Linux Learner wrote:
> Is following code SMP safe?
>
> #define InterlockedIncrement(x) ({ asm volatile ("lock; incl %0" : :
> "m" (*x) ); })
> #define InterlockedDecrement(x) ({ asm volatile ("lock; decl %0" : :
> "m" (*x) ); })
Why are you reinventing the wheel? The kernel has a couple of well
tested and debugged locking primitives with the additional benefit that
they also work on non-x86 architectures.
> Does -O2 affect SMP safeness of above code?
They're not safe with any optimisation level because the compiler is
free to reorder instructions around your own lock. Use the kernel
supplied locking primitives.
Eric, but the lock primitive acts as a memory barrier , isn't it?
AFAIK gcc still optimises the code even if it encounters a volatile
after asm but that may be the case when gcc is absolutely sure that
the code inside is unreachable. Isn't it?
Will this optimization effect the memory barrier act of the lock prefix here?
Please CMIIW
Thank you
~psr
Erik
- --
They're all fools. Don't worry. Darwin may be slow, but he'll
eventually get them. -- Matthew Lammers in alt.sysadmin.recovery
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