On Mon, Nov 11, 2019 at 04:10:20PM +0100, Marco Elver wrote: > On Mon, 11 Nov 2019 at 15:31, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > On Mon, Nov 11, 2019 at 03:17:51PM +0100, Marco Elver wrote: > > > On Sun, 10 Nov 2019 at 21:44, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > > > > > On Sun, Nov 10, 2019 at 11:20:53AM -0800, Linus Torvalds wrote: > > > > > On Sun, Nov 10, 2019 at 11:12 AM Linus Torvalds > > > > > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > And this is where WRITE_IDEMPOTENT would make a possible difference. > > > > > > In particular, if we make the optimization to do the "read and only > > > > > > write if changed" > > > > > > > > > > It might be useful for checking too. IOW, something like KCSAN could > > > > > actually check that if a field has an idempotent write to it, all > > > > > writes always have the same value. > > > > > > > > > > Again, there's the issue with lifetime. > > > > > > > > > > Part of that is "initialization is different". Those writes would not > > > > > be marked idempotent, of course, and they'd write another value. > > > > > > > > > > There's also the issue of lifetime at the _end_ of the use, of course. > > > > > There _are_ interesting data races at the end of the lifetime, both > > > > > reads and writes. > > > > > > > > > > In particular, if it's a sticky flag, in order for there to not be any > > > > > races, all the writes have to happen with a refcount held, and the > > > > > final read has to happen after the final refcount is dropped (and the > > > > > refcounts have to have atomicity and ordering, of course). I'm not > > > > > sure how easy something like that is model in KSAN. Maybe it already > > > > > does things like that for all the other refcount stuff we do. > > > > > > > > > > But the lifetime can be problematic for other reasons too - in this > > > > > particular case we have a union for that sticky flag (which is used > > > > > under the refcount), and then when the final refcount is released we > > > > > read that value (thus no data race) but because of the union we will > > > > > now start using that field with *different* data. It becomes that RCU > > > > > list head instead. > > > > > > > > > > That kind of "it used to be a sticky flag, but now the lifetime of the > > > > > flag is over, and it's something entirely different" might be a > > > > > nightmare for something like KCSAN. It sounds complicated to check > > > > > for, but I have no idea what KCSAN really considers complicated or > > > > > not. > > > > > > > > But will "one size fits all" be practical and useful? > > > > > > > > For my code, I would be happy to accept a significant "false positive" > > > > rate to get even a probabilistic warning of other-task accesses to some > > > > of RCU's fields. Even if the accesses were perfect from a functional > > > > viewpoint, they could be problematic from a performance and scalability > > > > viewpoint. And for something like RCU, real bugs, even those that are > > > > very improbable, need to be fixed. > > > > > > > > But other code (and thus other developers and maintainers) are going to > > > > have different needs. For all I know, some might have good reasons to > > > > exclude their code from KCSAN analysis entirely. > > > > > > > > Would it make sense for KCSAN to have per-file/subsystem/whatever flags > > > > specifying the depth of the analysis? > > > > > > Just to answer this: we already have this, and disable certain files > > > already. So it's an option if required. Just need maintainers to add > > > KCSAN_SANITIZE := n, or KCSAN_SANITIZE_file.o := n to Makefiles, and > > > KCSAN will simply ignore those. > > > > > > FWIW we now also have a config option to "ignore repeated writes with > > > the same value". It may be a little overaggressive/imprecise in > > > filtering data races, but anything else like the super precise > > > analysis involving tracking lifetimes and values (and whatever else > > > the rules would require) is simply too complex. So, the current > > > solution will avoid reporting cases like the original report here > > > (__alloc_file), but at the cost of maybe being a little imprecise. > > > It's probably a reasonable trade-off, given that we have too many data > > > races to deal with on syzbot anyway. > > > > Nice! > > > > Is this added repeated-writes analysis something that can be disabled? > > I would prefer that the analysis of RCU complain in this case as a > > probabilistic cache-locality warning. If it can be disabled, please > > let me know if there is anything that I need to do to make this happen. > > It's hidden behind a Kconfig config option, and actually disabled by > default. We can't enable/disable this on a per-file basis. > > Right now, we'll just enable it on the public syzbot instance, which > will use the most conservative config. Of course you can still run > your own fuzzer/stress test of choice with KCSAN and the option > disabled. Is that enough? > > Otherwise I could also just say if the symbolized top stack frame > contains "rcu_", don't ignore -- which would be a little hacky and > imprecise though. What do you prefer? Tough question. ;-) We could try the "rcu_" trick, and if it doesn't cause problems for others, why not? Thanx, Paul
