linux-kernel - Re: [PATCH] sched: Avoid that __wait_on_bit

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Date:	Wed, 10 Aug 2016 18:27:19 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Bart Van Assche <bart.vanassche@...disk.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Neil Brown <neilb@...e.de>,
	Michael Shaver <jmshaver@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [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.