Re: [PATCH] rcu: Use READ_ONCE() for rdp->gpwrap access in __note_gp_changes()

[Zilin Guan]

On Wed, Nov 06, 2024 at 12:18:25PM -0800, Paul E. McKenney wrote:
> Good eyes!!!
> 
> But did you find this with KCSAN, or by visual inspection?
> 
> The reason that I ask is that the __note_gp_changes() should be
> invoked with the leaf rnp->lock held, which should exclude writes to
> the rdp->gpwrap fields for all CPUs corresponding to that leaf rcu_node
> structure.
> 
> Note the raw_lockdep_assert_held_rcu_node(rnp) call at the beginning of
> this function.
> 
> So I believe that the proper fix is to *remove* READ_ONCE() from accesses
> to rdp->gpwrap in this function.
> 
> Or am I missing something here?
> 
>                                                         Thanx, Paul

I found this by visual inspection.

When reviewing the function __note_gp_changes(), I noticed that other 
accesses to rdp->gpwrap are protected with either READ_ONCE() or 
WRITE_ONCE(), which led me to suspect a potential data race at line 1305.

However, I am not certain whether holding rnp->lock protects access to 
rdp->gpwrap in this case. If it indeed ensures that no concurrent writes
can occur, then I agree that the correct approach would be to remove 
READ_ONCE() from those accesses.

Thanks,
Zilin