Messages in this thread | | | Date | Tue, 14 Mar 2000 13:44:48 +0100 (CET) | From | Ingo Molnar <> | Subject | Re: [patch] preemptive kernel, preemptive-2.3.52-A7 |
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On Tue, 14 Mar 2000, Andrea Arcangeli wrote:
> >i've also introduced a new light-weight 'IRQ-atomic' increment and > >decrement method in atomic.h, because ->may_preempt has to be > >local-IRQ-safe. > > Could you describe which race you are talking about? I can't see any > race on such field.
i've introduced atomic_inc_local() and atomic_dec_local() to implement 'local-CPU-IRQ atomic' increments and decrements. The following increment which you are using:
current->blah--;
is not theoretically local-IRQ-safe, because the compiler is free to reorder this with other lines. (and thus those other lines could get possibly moved into the 'freely preemptable' area, causing subtle small-window bugs.)
the following is safe as well:
barrier(); current->blah--; barrier();
but i just felt we want to make it IRQ-atomic as well (although this is not strictly necessery now as IRQ contexts never change current->blah.) If current->blah is modified from IRQ contexts as well then 'current->blah--' is not IRQ-safe as it might get compiled into several instructions:
movl blah, %eax decl %eax movl %eax, blah
although this does not happen on x86 currently, it could theoretically. But your version is definitely buggy because it lacks a compiler-level barrier. (which atomic_dec_local() automatically is)
Ingo
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