Messages in this thread | | | Date | Wed, 29 Apr 2015 14:14:41 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v2 2/5] sched, numa: Document usages of mm->numa_scan_seq |
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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.
Cheers, Longman
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