Messages in this thread | | | Date | Fri, 18 Jan 2019 16:56:38 +0100 | From | Andrea Parri <> | Subject | Re: Plain accesses and data races in the Linux Kernel Memory Model |
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On Fri, Jan 18, 2019 at 10:10:22AM -0500, Alan Stern wrote: > On Thu, 17 Jan 2019, Andrea Parri wrote: > > > > Can the compiler (maybe, it does?) transform, at the C or at the "asm" > > > level, LB1's P0 in LB2's P0 (LB1 and LB2 are reported below)? > > > > > > C LB1 > > > > > > { > > > int *x = &a; > > > } > > > > > > P0(int **x, int *y) > > > { > > > int *r0; > > > > > > r0 = rcu_dereference(*x); > > > *r0 = 0; > > > smp_wmb(); > > > WRITE_ONCE(*y, 1); > > > } > > > > > > P1(int **x, int *y, int *b) > > > { > > > int r0; > > > > > > r0 = READ_ONCE(*y); > > > rcu_assign_pointer(*x, b); > > > } > > > > > > exists (0:r0=b /\ 1:r0=1) > > > > > > > > > C LB2 > > > > > > { > > > int *x = &a; > > > } > > > > > > P0(int **x, int *y) > > > { > > > int *r0; > > > > > > r0 = rcu_dereference(*x); > > > if (*r0) > > > *r0 = 0; > > > smp_wmb(); > > > WRITE_ONCE(*y, 1); > > > } > > > > > > P1(int **x, int *y, int *b) > > > { > > > int r0; > > > > > > r0 = READ_ONCE(*y); > > > rcu_assign_pointer(*x, b); > > > } > > > > > > exists (0:r0=b /\ 1:r0=1) > > > > > > LB1 and LB2 are data-race free, according to the patch; LB1's "exists" > > > clause is not satisfiable, while LB2's "exists" clause is satisfiable. > > A relatively simple solution to this problem would be to say that > smp_wmb doesn't order plain writes.
It seems so; I don't have other solutions to suggest ATM. (But, TBH, I'm still in the process of reviewing/testing these changes... )
And yes, this is a pain! : I don't have the exact statistics, but I'm willing to believe that removing this order will take us back ~99% of the current (~500!) uses of smp_wmb() ;-/
Oh, well, maybe we'll find a better solution one day: after all, that one doesn't seem worse than what the current LKMM has to say! ;-)
> > I think the rest of the memory model would then be okay. However, I am > open to arguments that this approach is too complex and we should > insist on the same kind of strict ordering for race avoidance that the > C11 standard uses (i.e., conflicting accesses separated by full memory > barriers or release & acquire barriers or locking).
Indeed; maybe, we've just found another reason to obsolete smp_wmb()! ;-)
Andrea
> > Alan >
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