Re: Semantics of smp_mb() [was : Re: [PATCH] Fix RCU race in access of nohz_cpu_mask ]

[Andrew James Wade]

On Sunday 11 December 2005 12:41, Srivatsa Vaddagiri wrote:
> [Changed the subject line to be more generic in the interest of wider audience]
> 
> We seem to be having some confusion over the exact semantics of smp_mb().
> 
> Specifically, are all stores preceding smp_mb() guaranteed to have finished
> (committed to memory/corresponding cache-lines on other CPUs invalidated) 
> *before* successive loads are issued?

I doubt it. That's definitely not true of smp_wmb(), which boils down to
__asm__ __volatile__ ("": : :"memory") on SMP i386 (which the constrains
how the compiler orders write instructions, but is otherwise a nop. i386
has in-order writes.).

And it makes sense that wmb() wouldn't wait for writes: RCU needs
constraints on the order in which writes become visible, but has very week
constraints on when they do. Waiting for writes to flush would hurt
performance.

Andrew Wade
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