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SubjectRe: [PATCH] sched: Avoid that __wait_on_bit_lock() hangs
On 08/10, Bart Van Assche wrote:
>
> On 08/10/2016 03:46 AM, Oleg Nesterov wrote:
> > OK. Could you try another debugging patch below?
> >
> > Oleg.
> > ---
> >
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index e5a3244..9d5f892 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -711,6 +711,15 @@ static inline int page_has_private(struct page *page)
> > return !!(page->flags & PAGE_FLAGS_PRIVATE);
> > }
> >
> > +void unlock_page(struct page *page);
> > +static inline void __ClearPageLocked_x(struct page *page)
> > +{
> > + if (PageLocked(compound_head(page)))
> > + unlock_page(page);
> > +}
> > +
> > +#define __ClearPageLocked(page) __ClearPageLocked_x(page)
> > +
> > #undef PF_ANY
> > #undef PF_HEAD
> > #undef PF_NO_TAIL
>
> Hi Oleg,
>
> Are you sure that all __ClearPageLocked() users pass the compound head
> to that macro?

Hmm. it obviously should... which kernel version do you use for testing?

From include/linux/page-flags.h

__PAGEFLAG(Locked, locked, PF_NO_TAIL)

and

#define PF_NO_TAIL(page, enforce) ({ \
VM_BUG_ON_PGFLAGS(enforce && PageTail(page), page); \
compound_head(page);})

and this matches compound_head() in lock/unlock_page().

> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -711,6 +711,17 @@ static inline int page_has_private(struct page *page)
> return !!(page->flags & PAGE_FLAGS_PRIVATE);
> }
>
> +void unlock_page(struct page *page);
> +static inline void __ClearPageLocked_x(struct page *page)
> +{
> + if (PageLocked(compound_head(page)))
> + unlock_page(page);
> + else
> + __ClearPageLocked(page);
> +}

No, no. If you use an old kernel (which doesn't call compound_head() in
lock_page()), then just remove compound_head() from __ClearPageLocked_x()
above:

static inline void __ClearPageLocked_x(struct page *page)
{
if (PageLocked(page))
unlock_page(page);
}

even if this shouldn't make any difference afaics, note the
VM_BUG_ON_PGFLAGS() above.

Oleg.

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