Re: [PATCH v2 1/2] tools/memory-model: Unify UNLOCK+LOCK pairings to po-unlock-lock-po

[Alan Stern]

On Thu, Jan 26, 2023 at 02:46:03PM +0100, Jonas Oberhauser wrote:
> LKMM uses two relations for talking about UNLOCK+LOCK pairings:
> 
> 	1) po-unlock-lock-po, which handles UNLOCK+LOCK pairings
> 	   on the same CPU or immediate lock handovers on the same
> 	   lock variable
> 
> 	2) po;[UL];(co|po);[LKW];po, which handles UNLOCK+LOCK pairs
> 	   literally as described in rcupdate.h#L1002, i.e., even
> 	   after a sequence of handovers on the same lock variable.
> 
> The latter relation is used only once, to provide the guarantee
> defined in rcupdate.h#L1002 by smp_mb__after_unlock_lock(), which
> makes any UNLOCK+LOCK pair followed by the fence behave like a full
> barrier.
> 
> This patch drops this use in favor of using po-unlock-lock-po
> everywhere, which unifies the way the model talks about UNLOCK+LOCK
> pairings.  At first glance this seems to weaken the guarantee given
> by LKMM: When considering a long sequence of lock handovers
> such as below, where P0 hands the lock to P1, which hands it to P2,
> which finally executes such an after_unlock_lock fence, the mb
> relation currently links any stores in the critical section of P0
> to instructions P2 executes after its fence, but not so after the
> patch.
> 
> P0(int *x, int *y, spinlock_t *mylock)
> {
>         spin_lock(mylock);
>         WRITE_ONCE(*x, 2);
>         spin_unlock(mylock);
>         WRITE_ONCE(*y, 1);
> }
> 
> P1(int *y, int *z, spinlock_t *mylock)
> {
>         int r0 = READ_ONCE(*y); // reads 1
>         spin_lock(mylock);
>         spin_unlock(mylock);
>         WRITE_ONCE(*z,1);
> }
> 
> P2(int *z, int *d, spinlock_t *mylock)
> {
>         int r1 = READ_ONCE(*z); // reads 1
>         spin_lock(mylock);
>         spin_unlock(mylock);
>         smp_mb__after_unlock_lock();
>         WRITE_ONCE(*d,1);
> }
> 
> P3(int *x, int *d)
> {
>         WRITE_ONCE(*d,2);
>         smp_mb();
>         WRITE_ONCE(*x,1);
> }
> 
> exists (1:r0=1 /\ 2:r1=1 /\ x=2 /\ d=2)
> 
> Nevertheless, the ordering guarantee given in rcupdate.h is actually
> not weakened.  This is because the unlock operations along the
> sequence of handovers are A-cumulative fences.  They ensure that any
> stores that propagate to the CPU performing the first unlock
> operation in the sequence must also propagate to every CPU that
> performs a subsequent lock operation in the sequence.  Therefore any
> such stores will also be ordered correctly by the fence even if only
> the final handover is considered a full barrier.
> 
> Indeed this patch does not affect the behaviors allowed by LKMM at
> all.  The mb relation is used to define ordering through:
> 1) mb/.../ppo/hb, where the ordering is subsumed by hb+ where the
>    lock-release, rfe, and unlock-acquire orderings each provide hb
> 2) mb/strong-fence/cumul-fence/prop, where the rfe and A-cumulative
>    lock-release orderings simply add more fine-grained cumul-fence
>    edges to substitute a single strong-fence edge provided by a long
>    lock handover sequence
> 3) mb/strong-fence/pb and various similar uses in the definition of
>    data races, where as discussed above any long handover sequence
>    can be turned into a sequence of cumul-fence edges that provide
>    the same ordering.
> 
> Signed-off-by: Jonas Oberhauser <jonas.oberhauser@xxxxxxxxxxxxxxx>
> ---

Reviewed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>

>  tools/memory-model/linux-kernel.cat | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
> index 07f884f9b2bf..6e531457bb73 100644
> --- a/tools/memory-model/linux-kernel.cat
> +++ b/tools/memory-model/linux-kernel.cat
> @@ -37,8 +37,19 @@ let mb = ([M] ; fencerel(Mb) ; [M]) |
>  	([M] ; fencerel(Before-atomic) ; [RMW] ; po? ; [M]) |
>  	([M] ; po? ; [RMW] ; fencerel(After-atomic) ; [M]) |
>  	([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) |
> -	([M] ; po ; [UL] ; (co | po) ; [LKW] ;
> -		fencerel(After-unlock-lock) ; [M])
> +(*
> + * Note: The po-unlock-lock-po relation only passes the lock to the direct
> + * successor, perhaps giving the impression that the ordering of the
> + * smp_mb__after_unlock_lock() fence only affects a single lock handover.
> + * However, in a longer sequence of lock handovers, the implicit
> + * A-cumulative release fences of lock-release ensure that any stores that
> + * propagate to one of the involved CPUs before it hands over the lock to
> + * the next CPU will also propagate to the final CPU handing over the lock
> + * to the CPU that executes the fence.  Therefore, all those stores are
> + * also affected by the fence.
> + *)
> +	([M] ; po-unlock-lock-po ;
> +		[After-unlock-lock] ; po ; [M])
>  let gp = po ; [Sync-rcu | Sync-srcu] ; po?
>  let strong-fence = mb | gp
>  
> -- 
> 2.17.1
>