On Sat, Oct 20, 2018 at 04:18:37PM -0400, Alan Stern wrote: > On Sat, 20 Oct 2018, Paul E. McKenney wrote: > > > The second (informal) litmus test has a more interesting Linux-kernel > > counterpart: > > > > void t1_interrupt(void) > > { > > r0 = READ_ONCE(y); > > smp_store_release(&x, 1); > > } > > > > void t1(void) > > { > > smp_store_release(&y, 1); > > } > > > > void t2(void) > > { > > r1 = smp_load_acquire(&x); > > r2 = smp_load_acquire(&y); > > } > > > > On store-reordering architectures that implement smp_store_release() > > as a memory-barrier instruction followed by a store, the interrupt could > > arrive betweentimes in t1(), so that there would be no ordering between > > t1_interrupt()'s store to x and t1()'s store to y. This could (again, > > in paranoid theory) result in the outcome r0==0 && r1==0 && r2==1. > > This is disconcerting only if we assume that t1_interrupt() has to be > executed by the same CPU as t1(). If the interrupt could be fielded by > a different CPU then the paranoid outcome is perfectly understandable, > even in an SC context. > > So the question really should be limited to situations where a handler > is forced to execute in the context of a particular thread. While > POSIX does allow such restrictions for user programs, I'm not aware of > any similar mechanism in the kernel. Good point, and I was in fact assuming that t1() and t1_interrupt() were executing on the same CPU. This sort of thing happens naturally in the kernel when both t1() and t1_interrupt() are accessing per-CPU variables. Thanx, Paul