On 05/09, Paul E. McKenney wrote:
>
> On Thu, May 09, 2024 at 05:13:12PM +0200, Oleg Nesterov wrote:
> >
> > We can move these WARN_ON()'s into the ->rss_lock protected section.
> >
> > Or perhaps we can use data_race(rsp->gp_count) ? To be honest I thought
> > that READ_ONCE() should be enough...
> >
> > Or we can simply kill these WARN_ON_ONCE()'s.
>
> Or we could move those WARN_ON_ONCE() under the lock.
Sure, see above.
But could you help me to understand this magic? I naively thought
that READ_ONCE() is always "safe"...
So, unless I am totally confused it turns out that, say,
CPU 0 CPU 1
----- -----
spin_lock(LOCK);
++X; READ_ONCE(X); // data race
spin_unlock(LOCK);
is data-racy accoring to KCSAN, while
CPU 0 CPU 1
----- -----
spin_lock(LOCK);
WRITE_ONCE(X, X+1); READ_ONCE(X); // no data race
spin_unlock(LOCK);
is not.
Why is that?
Trying to read Documentation/dev-tools/kcsan.rst... it says
KCSAN is aware of *marked atomic operations* (``READ_ONCE``,
WRITE_ONCE``,
...
if all accesses to a variable that is accessed concurrently are properly
marked, KCSAN will never trigger a watchpoint
but how can KCSAN detect that all accesses to X are properly marked? I see
nothing
KCSAN-related in the definition of WRITE_ONCE() or READ_ONCE().
And what does the "all accesses" above actually mean? The 2nd version does
WRITE_ONCE(X, X+1);
but "X + 1" is the plain/unmarked access?
Thanks,
Oleg.
