Re: [PATCH] seqlock: Use WRITE_ONCE() when updating sequence

[Waiman Long]

On 12/18/24 10:45 AM, Paul E. McKenney wrote:
> On Tue, Dec 17, 2024 at 10:30:39PM -0500, Waiman Long wrote:
> > On 12/17/24 6:17 PM, Daniel Xu wrote:
> > > `sequence` is a concurrently accessed shared variable on the reader
> > > side. Therefore, it needs to be wrapped in WRITE_ONCE() in order to
> > > prevent unwanted compiler optimizations like store tearing.
> > >
> > > Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> > > ---
> > >    include/linux/seqlock.h | 14 +++++++-------
> > >    1 file changed, 7 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> > > index 5298765d6ca4..f4c6f2507742 100644
> > > --- a/include/linux/seqlock.h
> > > +++ b/include/linux/seqlock.h
> > > @@ -45,7 +45,7 @@ static inline void __seqcount_init(seqcount_t *s, const char *name,
> > >    	 * Make sure we are not reinitializing a held lock:
> > >    	 */
> > >    	lockdep_init_map(&s->dep_map, name, key, 0);
> > > -	s->sequence = 0;
> > > +	WRITE_ONCE(s->sequence, 0);
> > >    }
> > The init function certainly doesn't need to use WRITE_ONCE().
> > >    #ifdef CONFIG_DEBUG_LOCK_ALLOC
> > > @@ -405,7 +405,7 @@ do {									\
> > >    static inline void do_raw_write_seqcount_begin(seqcount_t *s)
> > >    {
> > >    	kcsan_nestable_atomic_begin();
> > > -	s->sequence++;
> > > +	WRITE_ONCE(s->sequence, READ_ONCE(s->sequence) + 1);
> > >    	smp_wmb();
> > >    }
> > > @@ -426,7 +426,7 @@ do {									\
> > >    static inline void do_raw_write_seqcount_end(seqcount_t *s)
> > >    {
> > >    	smp_wmb();
> > > -	s->sequence++;
> > > +	WRITE_ONCE(s->sequence, READ_ONCE(s->sequence) + 1);
> > >    	kcsan_nestable_atomic_end();
> > >    }
> > > @@ -548,9 +548,9 @@ static inline void do_write_seqcount_end(seqcount_t *s)
> > >    static inline void do_raw_write_seqcount_barrier(seqcount_t *s)
> > >    {
> > >    	kcsan_nestable_atomic_begin();
> > > -	s->sequence++;
> > > +	WRITE_ONCE(s->sequence, READ_ONCE(s->sequence) + 1);
> > >    	smp_wmb();
> > > -	s->sequence++;
> > > +	WRITE_ONCE(s->sequence, READ_ONCE(s->sequence) + 1);
> > >    	kcsan_nestable_atomic_end();
> > >    }
> > > @@ -569,7 +569,7 @@ static inline void do_write_seqcount_invalidate(seqcount_t *s)
> > >    {
> > >    	smp_wmb();
> > >    	kcsan_nestable_atomic_begin();
> > > -	s->sequence+=2;
> > > +	WRITE_ONCE(s->sequence, READ_ONCE(s->sequence) + 2);
> > >    	kcsan_nestable_atomic_end();
> > >    }
> > > @@ -673,7 +673,7 @@ read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
> > >    static __always_inline void raw_write_seqcount_latch(seqcount_latch_t *s)
> > >    {
> > >    	smp_wmb();	/* prior stores before incrementing "sequence" */
> > > -	s->seqcount.sequence++;
> > > +	WRITE_ONCE(s->seqcount.sequence, READ_ONCE(s->seqcount.sequence) + 1);
> > >    	smp_wmb();      /* increment "sequence" before following stores */
> > >    }
> > For seqcount, its actual value isn't important. What is important is whether
> > the value changes and whether it is even or odd. So even if store tearing is
> > happening, it shouldn't affect its operation. I doubt we need to use
> > WRITE_ONCE() here. Could you come up with a scenario where store tearing
> > will make it behave incorrectly?
> But why expand the state space?
I don't quite get what you mean by "state space".
> Also, there are potentially "optimizations" other than store tearing.
> No, I haven't seen them yet, but then again, there were a great many
> optimizations that were not being used back when I started coding C.

All the inc's are bounded by smp_wmb()'s which should limit the kind of 
optimizations that can be done by the compiler. That is why I don't see 
a need to use WRITE_ONCE() here. Of course, there may be some corner 
cases that I may miss. So I ask for whether such a case exists.

Cheers,
Longman