Re: [patch 2/2] fix SMP data race in pagetable setup vs walking

[Nick Piggin <npiggin@suse.de>]

On Mon, May 05, 2008 at 05:57:20PM +0100, Hugh Dickins wrote:
> On Mon, 5 May 2008, Nick Piggin wrote:
> > Index: linux-2.6/mm/memory.c
> > ===================================================================
> > --- linux-2.6.orig/mm/memory.c
> > +++ linux-2.6/mm/memory.c
> > @@ -311,6 +311,37 @@ int __pte_alloc(struct mm_struct *mm, pm
> >  	if (!new)
> >  		return -ENOMEM;
> >  
> > +	/*
> > +	 * Ensure all pte setup (eg. pte page lock and page clearing) are
> > +	 * visible before the pte is made visible to other CPUs by being
> > +	 * put into page tables.
> > +	 *
> > +	 * The other side of the story is the pointer chasing in the page
> > +	 * table walking code (when walking the page table without locking;
> > +	 * ie. most of the time). Fortunately, these data accesses consist
> > +	 * of a chain of data-dependent loads, meaning most CPUs (alpha
> > +	 * being the notable exception) will already guarantee loads are
> > +	 * seen in-order. x86 has a "reference" implementation of
> > +	 * smp_read_barrier_depends() barriers in its page table walking
> > +	 * code, even though that barrier is a simple noop on that architecture.
> > +	 * Alpha obviously also has the required barriers.
> > +	 *
> > +	 * It is debatable whether or not the smp_read_barrier_depends()
> > +	 * barriers are required for kernel page tables; it could be that
> > +	 * nowhere in the kernel do we walk those pagetables without taking
> > +	 * init_mm's page_table_lock.
> 
> Just delete "; it could be that ... init_mm's page_table_lock":
> in general (if not everywhere) the architectures do not nowadays
> take init_mm's page_table_lock to walk down those pagetables (blame
> me for removing it, if anyone thinks that was wrong: I stand by it).

OK. That's fine IMO and I'll remove that line.

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