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Date: Thu, 30 Apr 2015 16:58:24 -0400 From: Waiman Long <waiman.long@...com> To: Davidlohr Bueso <dave@...olabs.net> CC: Jason Low <jason.low2@...com>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Andrew Morton <akpm@...ux-foundation.org>, Oleg Nesterov <oleg@...hat.com>, Frederic Weisbecker <fweisbec@...il.com>, Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>, Steven Rostedt <rostedt@...dmis.org>, Preeti U Murthy <preeti@...ux.vnet.ibm.com>, Mike Galbraith <umgwanakikbuti@...il.com>, Aswin Chandramouleeswaran <aswin@...com>, Scott J Norton <scott.norton@...com> Subject: Re: [PATCH v2 2/5] sched, numa: Document usages of mm->numa_scan_seq On 04/30/2015 02:54 PM, Davidlohr Bueso wrote: > On Thu, 2015-04-30 at 14:42 -0400, Waiman Long wrote: >> On 04/29/2015 02:45 PM, Jason Low wrote: >>> 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@...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. >> 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. > I agree. Related, Linus also had some thoughts about the _very specific_ > purposes of these macros: > http://www.spinics.net/lists/linux-next/msg32494.html Actually, I don't think modern compiler will reload a read value unless it runs out of usable registers. It is more likely that it will reuse a previously read value within the same function if READ_ONCE() isn't there. Cheers, Longman -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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