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Date: Wed, 21 Mar 2001 21:05:23 +1100
From: Andrew Morton <andrewm@uow.edu.au>
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To: "David S. Miller" <davem@redhat.com>
CC: Keith Owens <kaos@ocs.com.au>, nigel@nrg.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH for 2.5] preemptible kernel
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"David S. Miller" wrote:
> 
> Keith Owens writes:
>  > Or have I missed something?
> 
> Nope, it is a fundamental problem with such kernel pre-emption
> schemes.  As a result, it would also break our big-reader locks
> (see include/linux/brlock.h).
> 
> Basically, anything which uses smp_processor_id() would need to
> be holding some lock so as to not get pre-empted.
> 

It's a problem for uniprocessors as well.

Example:

#define current_cpu_data boot_cpu_data
#define pgd_quicklist (current_cpu_data.pgd_quick)

extern __inline__ void free_pgd_fast(pgd_t *pgd)
{
        *(unsigned long *)pgd = (unsigned long) pgd_quicklist;
        pgd_quicklist = (unsigned long *) pgd;
        pgtable_cache_size++;
}

Preemption could corrupt this list.

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