On Thu, 2015-04-30 at 14:13 -0700, Jason Low wrote: > On Thu, 2015-04-30 at 14:42 -0400, Waiman Long wrote: > > > I do have a question of what kind of tearing you are talking about. Do > > you mean the tearing due to mm being changed in the middle of the > > access? The reason why I don't like this kind of construct is that I am > > not sure if > > the address translation p->mm->numa_scan_seq is being done once or > > twice. I looked at the compiled code and the translation is done only once. > > > > Anyway, the purpose of READ_ONCE and WRITE_ONCE is not for eliminating > > data tearing. They are to make sure that the compiler won't compile away > > data access and they are done in the order they appear in the program. I > > don't think it is a good idea to associate tearing elimination with > > those macros. So I would suggest removing the last sentence in your comment. > > Yes, I can remove the last sentence in the comment since the main goal > was to document that we're access this field without exclusive access.
--- Subject: [PATCH v3 2/5] sched, numa: Document usages of mm->numa_scan_seq 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.l...@hp.com> Suggested-by: Ingo Molnar <mi...@kernel.org> Acked-by: Rik van Riel <r...@redhat.com> --- kernel/sched/fair.c | 13 +++++++++++++ 1 files changed, 13 insertions(+), 0 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 5a44371..65a9a1dc 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 +2112,14 @@ 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 + * and the update is not guaranteed to be atomic. That's not + * much of an issue though, since this is just used for + * statistical sampling. Use READ_ONCE/WRITE_ONCE, which are not + * expensive, to avoid any form of compiler optimizations. + */ WRITE_ONCE(p->mm->numa_scan_seq, READ_ONCE(p->mm->numa_scan_seq) + 1); p->mm->numa_scan_offset = 0; } -- 1.7.2.5 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/