On Fri, Feb 16, 2018 at 05:22:55PM -0500, Alan Stern wrote:
> Since commit 76ebbe78f739 ("locking/barriers: Add implicit
> smp_read_barrier_depends() to READ_ONCE()") was merged for the 4.15
> kernel, it has not been necessary to use smp_read_barrier_depends().
> Similarly, commit 59ecbbe7b31c ("locking/barriers: Kill
> lockless_dereference()") removed lockless_dereference() from the
> kernel.
>
> Since these primitives are no longer part of the kernel, they do not
> belong in the Linux Kernel Memory Consistency Model. This patch
> removes them, along with the internal rb-dep relation, and updates the
> revelant documentation.
>
> Signed-off-by: Alan Stern <st...@rowland.harvard.edu>
>
> ---
[...]
> Index: usb-4.x/tools/memory-model/linux-kernel.def
> ===================================================================
> --- usb-4.x/tools/memory-model.orig/linux-kernel.def
> +++ usb-4.x/tools/memory-model/linux-kernel.def
> @@ -13,14 +13,12 @@ WRITE_ONCE(X,V) { __store{once}(X,V); }
> smp_store_release(X,V) { __store{release}(*X,V); }
> smp_load_acquire(X) __load{acquire}(*X)
> rcu_assign_pointer(X,V) { __store{release}(X,V); }
> -lockless_dereference(X) __load{lderef}(X)
> rcu_dereference(X) __load{deref}(X)
^^^ __load{once}
>
> // Fences
> smp_mb() { __fence{mb} ; }
> smp_rmb() { __fence{rmb} ; }
> smp_wmb() { __fence{wmb} ; }
> -smp_read_barrier_depends() { __fence{rb_dep}; }
> smp_mb__before_atomic() { __fence{before-atomic} ; }
> smp_mb__after_atomic() { __fence{after-atomic} ; }
> smp_mb__after_spinlock() { __fence{after-spinlock} ; }
> Index: usb-4.x/tools/memory-model/Documentation/cheatsheet.txt
> ===================================================================
> --- usb-4.x/tools/memory-model.orig/Documentation/cheatsheet.txt
> +++ usb-4.x/tools/memory-model/Documentation/cheatsheet.txt
> @@ -6,8 +6,7 @@
> Store, e.g., WRITE_ONCE() Y
> Y
> Load, e.g., READ_ONCE() Y Y
> Y
> Unsuccessful RMW operation Y Y
> Y
> -smp_read_barrier_depends() Y Y Y
> -*_dereference() Y Y Y
> Y
> +rcu_dereference() Y Y Y
> Y
> Successful *_acquire() R Y Y Y Y Y
> Y
> Successful *_release() C Y Y Y W
> Y
> smp_rmb() Y R Y Y R
Akira's observation about READ_ONCE extends to all (annotated) loads. In
fact, it also applies to loads corresponding to unsuccessful RMW operations;
consider, for example, the following variation of MP+onceassign+derefonce:
C T
{
y=z;
z=0;
}
P0(int *x, int **y)
{
WRITE_ONCE(*x, 1);
smp_store_release(y, x);
}
P1(int **y, int *z)
{
int *r0;
int r1;
r0 = cmpxchg_relaxed(y, z, z);
r1 = READ_ONCE(*r0);
}
exists (1:r0=x /\ 1:r1=0)
The final state is allowed w/o the patch, and forbidden w/ the patch.
This also reminds me of
5a8897cc7631fa544d079c443800f4420d1b173f
("locking/atomics/alpha: Add smp_read_barrier_depends() to
_release()/_relaxed() atomics")
(that we probably want to mention in the commit message).
Andrea
>
>
