Messages in this thread | | | Date | Tue, 27 Mar 2018 12:25:21 +0200 | From | Andrea Parri <> | Subject | Re: [PATCH for-4.17 2/2] powerpc: Remove smp_mb() from arch_spin_is_locked() |
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On Tue, Mar 27, 2018 at 11:06:56AM +1100, Benjamin Herrenschmidt wrote: > On Mon, 2018-03-26 at 12:37 +0200, Andrea Parri wrote: > > Commit 51d7d5205d338 ("powerpc: Add smp_mb() to arch_spin_is_locked()") > > added an smp_mb() to arch_spin_is_locked(), in order to ensure that > > > > Thread 0 Thread 1 > > > > spin_lock(A); spin_lock(B); > > r0 = spin_is_locked(B) r1 = spin_is_locked(A); > > > > never ends up with r0 = r1 = 0, and reported one example (in ipc/sem.c) > > relying on such guarantee. > > > > It's however understood (and undocumented) that spin_is_locked() is not > > required to ensure such ordering guarantee, > > Shouldn't we start by documenting it ?
I do sympathize with your concern about the documentation! ;) The patch in [1] was my (re)action to this concern; the sort of the patch is unclear to me by this time (and I'm not aware of other proposals in this respect).
> > > guarantee that is currently > > _not_ provided by all implementations/arch, and that callers relying on > > such ordering should instead use suitable memory barriers before acting > > on the result of spin_is_locked(). > > > > Following a recent auditing[1] of the callers of {,raw_}spin_is_locked() > > revealing that none of them are relying on this guarantee anymore, this > > commit removes the leading smp_mb() from the primitive thus effectively > > reverting 51d7d5205d338. > > I would rather wait until it is properly documented. Debugging that IPC > problem took a *LOT* of time and energy, I wouldn't want these issues > to come and bite us again.
I understand. And I'm grateful for this debugging as well as for the (IMO) excellent account of it you provided in 51d7d5205d338.
Said this ;) I cannot except myself from saying that I would probably have resisted that solution (adding an smp_mb() in my arch_spin_is_locked), and instead "blamed"/suggested that caller to fix his memory ordering...
Andrea
> > > [1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2 > > > > Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com> > > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> > > Cc: Paul Mackerras <paulus@samba.org> > > Cc: Michael Ellerman <mpe@ellerman.id.au> > > Cc: Peter Zijlstra <peterz@infradead.org> > > Cc: Ingo Molnar <mingo@redhat.com> > > Cc: Linus Torvalds <torvalds@linux-foundation.org> > > --- > > arch/powerpc/include/asm/spinlock.h | 1 - > > 1 file changed, 1 deletion(-) > > > > diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h > > index b9ebc3085fb79..ecc141e3f1a73 100644 > > --- a/arch/powerpc/include/asm/spinlock.h > > +++ b/arch/powerpc/include/asm/spinlock.h > > @@ -67,7 +67,6 @@ static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock) > > > > static inline int arch_spin_is_locked(arch_spinlock_t *lock) > > { > > - smp_mb(); > > return !arch_spin_value_unlocked(*lock); > > } > >
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