Messages in this thread Patch in this message | | | Subject | Re: [PATCH 1/3] sched, timer: Remove usages of ACCESS_ONCE in the scheduler | From | Jason Low <> | Date | Thu, 16 Apr 2015 20:25:48 -0700 |
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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; }
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