On Thu, Jul 16, 2020 at 03:47:17AM +0100, Matthew Wilcox wrote: > On Thu, Jul 16, 2020 at 11:46:56AM +1000, Dave Chinner wrote: > > And why should we compromise performance on hundreds of millions of > > modern systems to fix an extremely rare race on an extremely rare > > platform that maybe only a hundred people world-wide might still > > use? > > I thought that wasn't the argument here. It was that some future > compiler might choose to do something absolutely awful that no current > compiler does, and that rather than disable the stupid "optimisation", > we'd be glad that we'd already stuffed the source code up so that it > lay within some tortuous reading of the C spec. There are actually many reasons to avoid data races; see https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE > > The memory model is just too complicated. Look at the recent exchange > between myself & Dan Williams. I spent literally _hours_ trying to > figure out what rules to follow. > > https://lore.kernel.org/linux-mm/CAPcyv4jgjoLqsV+aHGJwGXbCSwbTnWLmog5-rxD2i31vZ2rDNQ@xxxxxxxxxxxxxx/ > https://lore.kernel.org/linux-mm/CAPcyv4j2+7XiJ9BXQ4mj_XN0N+rCyxch5QkuZ6UsOBsOO1+2Vg@xxxxxxxxxxxxxx/ > > Neither Dan nor I are exactly "new" to Linux kernel development. As Dave > is saying here, having to understand the memory model is too high a bar. > > Hell, I don't know if what we ended up with for v4 is actually correct. > It lokos good to me, but *shrug* > > https://lore.kernel.org/linux-mm/159009507306.847224.8502634072429766747.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ Yes, it's too complicated. I'm not sure there's much of a solution, though. Of course, we also have easy-to-use synchronization primitives like mutex, spinlock, rw_semaphore, etc. The problems arise when people think they know better and try to write something more "optimized". We need to have a higher bar for accepting changes where the memory model is a concern at all. - Eric
