On Sat, Apr 20, 2019 at 12:06:58AM +0900, Akira Yokosawa wrote: > Hi Paul, > > Please find inline comments below. > > On Fri, 19 Apr 2019 05:47:20 -0700, Paul E. McKenney wrote: > > On Fri, Apr 19, 2019 at 02:53:02AM +0200, Andrea Parri wrote: > >>> Are you saying that on x86, atomic_inc() acts as a full memory barrier > >>> but not as a compiler barrier, and vice versa for > >>> smp_mb__after_atomic()? Or that neither atomic_inc() nor > >>> smp_mb__after_atomic() implements a full memory barrier? > >> > >> I'd say the former; AFAICT, these boil down to: > >> > >> > >> https://elixir.bootlin.com/linux/v5.1-rc5/source/arch/x86/include/asm/atomic.h#L95 > >> > >> https://elixir.bootlin.com/linux/v5.1-rc5/source/arch/x86/include/asm/barrier.h#L84 > > > > OK, how about the following? > > > > Thanx, Paul > > > > ------------------------------------------------------------------------ > > > > commit 19d166dadc4e1bba4b248fb46d32ca4f2d10896b > > Author: Paul E. McKenney <paul...@linux.ibm.com> > > Date: Fri Apr 19 05:20:30 2019 -0700 > > > > tools/memory-model: Make smp_mb__{before,after}_atomic() match x86 > > > > Read-modify-write atomic operations that do not return values need not > > provide any ordering guarantees, and this means that both the compiler > > and the CPU are free to reorder accesses across things like atomic_inc() > > and atomic_dec(). The stronger systems such as x86 allow the compiler > > to do the reordering, but prevent the CPU from so doing, and these > > systems implement smp_mb__{before,after}_atomic() as compiler barriers. > > The weaker systems such as Power allow both the compiler and the CPU > > to reorder accesses across things like atomic_inc() and atomic_dec(), > > and implement smp_mb__{before,after}_atomic() as full memory barriers. > > > > This means that smp_mb__before_atomic() only orders the atomic operation > > itself with accesses preceding the smp_mb__before_atomic(), and does > > not necessarily provide any ordering whatsoever against accesses > > folowing the atomic operation. Similarly, smp_mb__after_atomic() > > s/folowing/following/
Good eyes, fixed! > > only orders the atomic operation itself with accesses following the > > smp_mb__after_atomic(), and does not necessarily provide any ordering > > whatsoever against accesses preceding the atomic operation. Full > > ordering > > therefore requires both an smp_mb__before_atomic() before the atomic > > operation and an smp_mb__after_atomic() after the atomic operation. > > > > Therefore, linux-kernel.cat's current model of Before-atomic > > and After-atomic is too strong, as it guarantees ordering of > > accesses on the other side of the atomic operation from the > > smp_mb__{before,after}_atomic(). This commit therefore weakens > > the guarantee to match the semantics called out above. > > > > Reported-by: Andrea Parri <andrea.pa...@amarulasolutions.com> > > Suggested-by: Alan Stern <st...@rowland.harvard.edu> > > Signed-off-by: Paul E. McKenney <paul...@linux.ibm.com> > > > > diff --git a/Documentation/memory-barriers.txt > > b/Documentation/memory-barriers.txt > > index 169d938c0b53..e5b97c3e8e39 100644 > > --- a/Documentation/memory-barriers.txt > > +++ b/Documentation/memory-barriers.txt > > @@ -1888,7 +1888,37 @@ There are some more advanced barrier functions: > > atomic_dec(&obj->ref_count); > > > > This makes sure that the death mark on the object is perceived to be > > set > > - *before* the reference counter is decremented. > > + *before* the reference counter is decremented. However, please note > > + that smp_mb__before_atomic()'s ordering guarantee does not necessarily > > + extend beyond the atomic operation. For example: > > + > > + obj->dead = 1; > > + smp_mb__before_atomic(); > > + atomic_dec(&obj->ref_count); > > + r1 = a; > > + > > + Here the store to obj->dead is not guaranteed to be ordered with > > + with the load from a. This reordering can happen on x86 as follows: > > s/with// Fixed fixed. ;-) > And I beg you to avoid using the single letter variable "a". > It's confusing. Good point! I changed it to "x". > > + (1) The compiler can reorder the load from a to precede the > > + atomic_dec(), (2) Because x86 smp_mb__before_atomic() is only a > > + compiler barrier, the CPU can reorder the preceding store to > > + obj->dead with the later load from a. > > + > > + This could be avoided by using READ_ONCE(), which would prevent the > > + compiler from reordering due to both atomic_dec() and READ_ONCE() > > + being volatile accesses, and is usually preferable for loads from > > + shared variables. However, weakly ordered CPUs would still be > > + free to reorder the atomic_dec() with the load from a, so a more > > + readable option is to also use smp_mb__after_atomic() as follows: > > The point here is not just "readability", but also the portability of the > code, isn't it? As Andrea noted, in this particular case, the guarantee that the store to obj->dead precedes the load from x is portable. Either the smp_mb__before_atomic() or the atomic_dec() must provide the ordering. However, you are right that there is some non-portability. But this non-portability involves the order of the atomic_dec() and the store to x. So what I did was ... > Thanks, Akira > > > + > > + WRITE_ONCE(obj->dead, 1); > > + smp_mb__before_atomic(); > > + atomic_dec(&obj->ref_count); > > + smp_mb__after_atomic(); > > + r1 = READ_ONCE(a); > > + > > + This orders all three accesses against each other, and also makes > > + the intent quite clear. ... change the above paragraph to read as follows: In addition, the example without the smp_mb__after_atomic() does not necessarily order the atomic_dec() with the load from x. In contrast, the example with both smp_mb__before_atomic() and smp_mb__after_atomic() orders all three accesses against each other, and also makes the intent quite clear. Does that help? Thanx, Paul > > See Documentation/atomic_{t,bitops}.txt for more information. > > > > diff --git a/tools/memory-model/linux-kernel.cat > > b/tools/memory-model/linux-kernel.cat > > index 8dcb37835b61..b6866f93abb8 100644 > > --- a/tools/memory-model/linux-kernel.cat > > +++ b/tools/memory-model/linux-kernel.cat > > @@ -28,8 +28,8 @@ include "lock.cat" > > let rmb = [R \ Noreturn] ; fencerel(Rmb) ; [R \ Noreturn] > > let wmb = [W] ; fencerel(Wmb) ; [W] > > let mb = ([M] ; fencerel(Mb) ; [M]) | > > - ([M] ; fencerel(Before-atomic) ; [RMW] ; po? ; [M]) | > > - ([M] ; po? ; [RMW] ; fencerel(After-atomic) ; [M]) | > > + ([M] ; fencerel(Before-atomic) ; [RMW]) | > > + ([RMW] ; fencerel(After-atomic) ; [M]) | > > ([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) | > > ([M] ; po ; [UL] ; (co | po) ; [LKW] ; > > fencerel(After-unlock-lock) ; [M]) > > >