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SubjectRe: [PATCH 00/21] mm: Preemptibility -v6
On 01/24, Peter Zijlstra wrote:
>
> On Fri, 2011-01-21 at 16:33 +0100, Peter Zijlstra wrote:
>
> > Index: linux-2.6/mm/rmap.c
> > ===================================================================
> > --- linux-2.6.orig/mm/rmap.c
> > +++ linux-2.6/mm/rmap.c
> > @@ -1559,9 +1559,20 @@ void __put_anon_vma(struct anon_vma *ano
> > * Synchronize against page_lock_anon_vma() such that
> > * we can safely hold the lock without the anon_vma getting
> > * freed.
> > + *
> > + * Relies on the full mb implied by the atomic_dec_and_test() from
> > + * put_anon_vma() against the full mb implied by mutex_trylock() from
> > + * page_lock_anon_vma(). This orders:
> > + *
> > + * page_lock_anon_vma() VS put_anon_vma()
> > + * mutex_trylock() atomic_dec_and_test()
> > + * smp_mb() smp_mb()
> > + * atomic_read() mutex_is_locked()
>
> Bah!, I thought all mutex_trylock() implementations used an atomic op
> with return value (which implies a mb), but it looks like (at least*)
> PPC doesn't and only provides a LOCK barrier.

But, mutex_trylock() must imply the one-way barrier, otherwise it
is buggy, no?

IOW, page_lock_anon_vma() does:

if (mutex_trylock(anon_vma->root->lock)) {
...
atomic_read(&anon_vma->refcount);
...
}
If this atomic_read() can leak out of the critical section, then
I think mutex_trylock() should be fixed. Or I misunderstood the
problem completely...

BTW, from https://lkml.org/lkml/2010/11/26/213

+ * Similar to page_get_anon_vma() except it locks the anon_vma.
...
- struct anon_vma *anon_vma = page_get_anon_vma(page);

looks like, page_get_anon_vma() becomes unused.

Oleg.



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