Messages in this thread | | | Date | Mon, 14 May 2018 06:25:41 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Is read barrier missed in kfifo? |
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On Mon, May 14, 2018 at 08:57:55AM +0200, Peter Zijlstra wrote: > On Fri, May 11, 2018 at 09:20:53AM -0700, Paul E. McKenney wrote: > > On Fri, May 11, 2018 at 10:32:42AM +0200, Peter Zijlstra wrote: > > > On Fri, May 11, 2018 at 03:25:18PM +0800, Xiao Guangrong wrote: > > > > > > > > Hi, > > > > > > > > Currently, there is no read barrier between reading the index > > > > (kfifo.in) and fetching the real data from the fifo. > > > > > > > > I am afraid that will cause the vfifo is observed as not empty > > > > however the data is not actually ready for read. Right? > > > > > > That code is decidedly dodgy indeed. I can only see smp_wmb() but no > > > matching barriers at all -- therefore the code is almost certainly as > > > good as not having any barriers at all. > > > > > > I would suggest you try and convert the code to smp_store_release() and > > > smp_load_acquire() while you're at it. > > > > Isn't this one of the places where we rely on control dependencies? > > Then it bloody well should have a comment. But at least one side of the > fifo needs a read barrier I think. We can rely on a ctrl-dep on the > write side, where we read the head/tail values, compute space and then > conditionally allow writes to happen. > > But on the read side it's all reads and ctrl-dep doesn't help anything.
Agreed, for a control depdendency to help, there does need to be a control dependency. ;-)
Thanx, Paul
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