Re: [PATCH 2/2] cgroup: explain the race between updater and flusher

[Paul E. McKenney]

On Thu, Jul 03, 2025 at 06:54:02PM -0700, Shakeel Butt wrote:
> On Thu, Jul 3, 2025 at 4:53 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >
> > On Thu, Jul 03, 2025 at 03:46:07PM -0700, Shakeel Butt wrote:
> [...]
> > > Let me answer this one first. The previous patch actually made
> > > init_llist_node() do WRITE_ONCE().
> > >
> > > So the actual question is why do we need
> > > data_race([READ|WRITE]_ONCE()) instead of just [READ|WRITE]_ONCE()?
> >
> > You should *almost* always use [READ|WRITE]_ONCE() instead of data_race().
> >
> > > Actually I had the similar question myself and found the following
> > > comment in include/linux/compiler.h:
> > >
> > > /**
> > >  * data_race - mark an expression as containing intentional data races
> > >  *
> > >  * This data_race() macro is useful for situations in which data races
> > >  * should be forgiven.  One example is diagnostic code that accesses
> > >  * shared variables but is not a part of the core synchronization design.
> > >  * For example, if accesses to a given variable are protected by a lock,
> > >  * except for diagnostic code, then the accesses under the lock should
> > >  * be plain C-language accesses and those in the diagnostic code should
> > >  * use data_race().  This way, KCSAN will complain if buggy lockless
> > >  * accesses to that variable are introduced, even if the buggy accesses
> > >  * are protected by READ_ONCE() or WRITE_ONCE().
> > >  *
> > >  * This macro *does not* affect normal code generation, but is a hint
> > >  * to tooling that data races here are to be ignored.  If the access must
> > >  * be atomic *and* KCSAN should ignore the access, use both data_race()
> > >  * and READ_ONCE(), for example, data_race(READ_ONCE(x)).
> > >  */
> > >
> > > IIUC correctly, I need to protect llist_node against tearing and as well
> > > as tell KCSAN to ignore the access for race then I should use both.
> > > Though I think KCSAN treat [READ|WRITE]_ONCE similar to data_race(), so
> > > it kind of seem redundant but I think at least I want to convey that we
> > > need protection against tearing and ignore KCSAN and using both conveys
> > > that. Let me know if you think otherwise.
> > >
> > > thanks a lot for taking a look.
> >
> > The thing to remember is that data_race() does not affect the
> > generated code (except of course when running KCSAN), and thus does
> > absolutely nothing to prevent load/store tearing.  You need things like
> > [READ|WRITE]_ONCE() to prevent tearing.
> >
> > So if it does not affect the generated code, what is the point of
> > data_race()?
> >
> > One answer to this question is for diagnostics where you want KCSAN
> > to check the main algorithm, but you don't want KCSAN to be confused
> > by the diagnostic accesses.  For example, you might use something like
> > ASSERT_EXCLUSIVE_ACCESS() as in __list_splice_init_rcu(), and not want
> > your diagnostic accesses to result in false-positive KCSAN reports
> > due to interactions with ASSERT_EXCLUSIVE_ACCESS() on some particular
> > memory location.  And if you were to use READ_ONCE() to access that same
> > memory location in your diagnostics, KCSAN would complain if they ran
> > concurrently with that ASSERT_EXCLUSIVE_ACCESS().  So you would instead
> > use data_race() to suppress such complaints.
> >
> > Does that make sense?
> 
> Thanks a lot Paul for the awesome explanation. Do you think keeping
> data_race() here would be harmful in a sense that it might cause
> confusion in future?

Yes, plus it might incorrectly suppress a KCSAN warning for a very
real bug.  So I strongly recommend removing the data_race() in this case.

							Thanx, Paul