Messages in this thread | | | Date | Fri, 17 Apr 2015 10:19:41 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 1/3] sched, timer: Remove usages of ACCESS_ONCE in the scheduler |
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* Jason Low <jason.low2@hp.com> wrote:
> On Thu, 2015-04-16 at 20:24 +0200, Ingo Molnar wrote: > > Would it make sense to add a few comments to the seq field definition > > site(s), about how it's supposed to be accessed - or to the > > READ_ONCE()/WRITE_ONCE() sites, to keep people from wondering? > > How about this: > > --- > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 5a44371..63fa87f 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; > }
Perfect! It just needs a changelog and a SOB.
Thanks,
Ingo
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