Re: [PATCH v2 2/5] sched, numa: Document usages of mm->numa_scan_seq

[Jason Low <jason.low2@hp.com>]

On Wed, 2015-04-29 at 14:14 -0400, Waiman Long wrote:
> On 04/28/2015 04:00 PM, Jason Low wrote:
> > The p->mm->numa_scan_seq is accessed using READ_ONCE/WRITE_ONCE
> > and modified without exclusive access. It is not clear why it is
> > accessed this way. This patch provides some documentation on that.
> >
> > Signed-off-by: Jason Low<jason.low2@hp.com>
> > ---
> >   kernel/sched/fair.c |   12 ++++++++++++
> >   1 files changed, 12 insertions(+), 0 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 5a44371..794f7d7 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -1794,6 +1794,11 @@ static void task_numa_placement(struct task_struct *p)
> >   	u64 runtime, period;
> >   	spinlock_t *group_lock = NULL;
> >
> > +	/*
> > +	 * The p->mm->numa_scan_seq gets updated without
> > +	 * exclusive access. Use READ_ONCE() here to ensure
> > +	 * that the field is read in a single access.
> > +	 */
> >   	seq = READ_ONCE(p->mm->numa_scan_seq);
> >   	if (p->numa_scan_seq == seq)
> >   		return;
> > @@ -2107,6 +2112,13 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)
> >
> >   static void reset_ptenuma_scan(struct task_struct *p)
> >   {
> > +	/*
> > +	 * We only did a read acquisition of the mmap sem, so
> > +	 * p->mm->numa_scan_seq is written to without exclusive access.
> > +	 * That's not much of an issue though, since this is just used
> > +	 * for statistical sampling. Use WRITE_ONCE and READ_ONCE, which
> > +	 * are not expensive, to avoid load/store tearing.
> > +	 */
> >   	WRITE_ONCE(p->mm->numa_scan_seq, READ_ONCE(p->mm->numa_scan_seq) + 1);
> >   	p->mm->numa_scan_offset = 0;
> >   }
> 
> READ_ONCE followed by a WRITE_ONCE won't stop load/store tearing from 
> happening unless you use an atomic instruction to do the increment. So I 
> think your comment may be a bit misleading.

Right, the READ and WRITE operations will still be done separately and
won't be atomic. Here, we're saying that this prevents load/store
tearing on each of those individual write/read operations. Please let me
know if you prefer this to be worded differently.