Messages in this thread | | | Date | Thu, 19 Jan 2023 11:07:52 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
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On Wed, Jan 18, 2023 at 09:41:38AM -0800, Paul E. McKenney wrote: > On Wed, Jan 18, 2023 at 11:03:35AM -0500, Alan Stern wrote: > > On Tue, Jan 17, 2023 at 09:17:04PM -0800, Paul E. McKenney wrote: > > > On Tue, Jan 17, 2023 at 09:15:15PM -0500, Alan Stern wrote: > > > > Maybe we don't. Please test the patch below; I think it will do what > > > > you want -- and it doesn't rule out nesting. > > > > > > It works like a champ on manual/kernel/C-srcu*.litmus in the litmus > > > repository on github, good show and thank you!!! > > > > > > I will make more tests, and am checking this against the rest of the > > > litmus tests in the repo, but in the meantime would you be willing to > > > have me add your Signed-off-by? > > > > I'll email a real patch submission in the not-too-distant future, > > assuming you don't find any problems with the new code. > > Sounds good! > > The current state is that last night's testing found a difference only > for C-srcu-nest-5.litmus, in which case your version gives the correct > answer and mainline is wrong. There were a couple of broken tests, which > I fixed and a test involving spin_unlock_wait(), which is at this point > perma-broken due to the Linux kernel no longer having such a thing. > (Other than its re-introduction into i915, but they define it as a > spin_lock_irq() followed by a spin_unlock_irq(), so why worry?) > There were also a few timeouts. > > I intend to run the longer tests overnight.
Except that I had an episode of pilot error. :-/
But here are a couple of litmus tests illustrating how SRCU read-side critical sections do not flatten/fuse the way that RCU read-side critical sections do.
Why the difference? Because such a Linux-kernel SRCU implementation would require additional storage of the order (T * S), where T is the number of tasks and S is the number of srcu_struct structures. That just won't be happening.
Here they are, and both behave the way that I would expect given your unofficial patch:
https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-7.litmus https://github.com/paulmckrcu/litmus/blob/master/manual/kernel/C-srcu-nest-8.litmus
Whew! ;-)
Thanx, Paul
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