Re: [PATCH] vsprintf: Fix memory barriers of ptr_key to have_filed_random_ptr_key

[Pavel Machek]

On Tue 2018-05-15 14:57:44, Steven Rostedt wrote:
> On Tue, 15 May 2018 09:55:13 -0700
> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Tue, May 15, 2018 at 7:06 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> > > -       smp_mb();
> > > +       smp_wmb();
> > >          WRITE_ONCE(have_filled_random_ptr_key, true);  
> > 
> > 
> > > +       /* Read ptr_key after reading have_filled_random_ptr_key */
> > > +       smp_rmb();
> > > +
> > >   #ifdef CONFIG_64BIT
> > >          hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);  
> > 
> > Hmm. smp_wmb/rmb are basically free on x86, but on some architectures
> > smp_rmb() in particular can be pretty expensive.
> > 
> > So when you have a "handoff" situation like this, it's _probably_ better to
> > use use "smp_store_release()" and "smp_load_acquire()". To some degree that
> > might also be better for documentation purposes, because that's exactly the
> > "release-acquire" pattern.
> > 
> > That said, I'm not convinced this really matters all that much for a
> > boot-time flag like this. The race is pretty theoretical.
> >
> 
> I was thinking the same. But since the smp_mb() is there, then it
> should be correct, which it currently isn't.
> 
> We could change this to a static key, and enable it after we set up
> the ptr_key. That would be a one time change at boot up, wont have
> races, and have no overhead.

OTOH... fixing theoretical races is nice, but probably should not go
to stable?
								Pavel
								
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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