Messages in this thread | | | Subject | Re: [PATCH v2 2/5] sched, numa: Document usages of mm->numa_scan_seq | From | Jason Low <> | Date | Fri, 01 May 2015 10:40:08 -0700 |
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On Fri, 2015-05-01 at 08:21 -0700, Paul E. McKenney wrote: > On Thu, Apr 30, 2015 at 02:13:07PM -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. > > > > In terms of data tearing, an example would be the write operation gets > > split into multiple stores (though this is architecture dependent). The > > idea was that since we're modifying a seq variable without the write > > lock, we want to remove any forms of optimizations as mentioned above or > > unpredictable behavior, since READ_ONCE/WRITE_ONCE adds no overhead. > > Just to be clear... READ_ONCE() and WRITE_ONCE() do not avoid data tearing > in cases where the thing read or written is too big for a machine word.
Right, that makes sense. I've updated the comment to instead mention that it's used to avoid "compiler optimizations".
> If the thing read/written does fit into a machine word and if the location > read/written is properly aligned, I would be quite surprised if either > READ_ONCE() or WRITE_ONCE() resulted in any sort of tearing.
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