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SubjectRe: question about spinlocks on UP
On Wed, Jul 15, 1998 at 04:19:33PM +0200, Andi Kleen wrote:
> Oliver Neukum <neukum@fachschaft.org.chemie.uni-muenchen.de> writes:
>
> > is the first parameter of spin_lock_irqsave relevant on UP ?
> > In other words is
> >
> > #ifdef SMP
> > spinlock_t lock_xxx ...
> > #endif
> >
> > #ifdef SMP
> > spin_lock_irqsave(&lock_xxx,flags);
> > #else
> > spin_lock_irqsave(NULL,flags);
> > #endif
> >
> > without side effects and is that worth it ?
> > I don't like declaring a global variable unnecessary.
>
> spinlocks are generally a nop on UP. Because they're macros you can
> just write
>
>
> #ifdef SMP
> spinlock_t bla_lock;
> #endif
>
> ....
>
> spin_lock_irqsave(&bla_lock, flags);
>
> The preprocessor will remove the &bla_lock in the UP case.

I would not recommend the technique for the common use.
The similar code was a pain for me when I debugged deadlocks
and turned on spinlock emulation on UP kernel :-)

I don't think that a certain number of spinlock_t variables are
worth the time needed to fix all missing declarations when
you decide to debug something.

Best wishes
Andrey V.
Savochkin

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