On Sat, Feb 17, 2018 at 04:25:35AM +0100, Andrea Parri wrote: > 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).
Please also notice that 5a8897cc7631f only touched alpha's atomic.h: I see no corresponding commit/change on {,cmp}xchg.h (where the "mb" is currently conditionally executed). Andrea > > Andrea > > > > > >