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.
