Hi Eric, On 5/8/07, Erik Mouw <mouw@xxxxxxxxxxxx> wrote:
On Tue, May 08, 2007 at 08:51:22AM +0530, pradeep singh wrote: > On 5/7/07, Erik Mouw <mouw@xxxxxxxxxxxx> wrote: > >On Mon, May 07, 2007 at 09:25:02PM +0530, Linux Learner wrote: > >> 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? For the CPU, yes. But unless you tell the compiler that your inline assembly has certain side effects the compiler can still reorder it with other instructions. > 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? Volatile isn't enough, you really need to tell the compiler the instruction affects memory.
In other words, I hope you mean i should rather include "memory"(*x) , isn't it? Can you please help me with understanding if memory is the correct cloberring directive to be used in such cases to fake a side effect to the gcc ?
Yes, you are right. I tried it with memory clobbering directive and then it worked as expected (without reordering).
-Mohan
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