On Mon, Nov 04, 2024 at 03:12:30PM +0000, Zilin Guan wrote: > In function __note_gp_changes(), rdp->gpwrap is read using READ_ONCE() > in line 1307: > > 1307 if (IS_ENABLED(CONFIG_PROVE_RCU) && READ_ONCE(rdp->gpwrap)) > 1308 WRITE_ONCE(rdp->last_sched_clock, jiffies); > > while read directly in line 1305: > > 1305 if (ULONG_CMP_LT(rdp->gp_seq_needed, rnp->gp_seq_needed) || > rdp->gpwrap) > 1306 WRITE_ONCE(rdp->gp_seq_needed, rnp->gp_seq_needed); > > In the same environment, reads in two places should have the same > protection. > > Signed-off-by: Zilin Guan <zilinguan811@xxxxxxxxx> 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 > --- > kernel/rcu/tree.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index b1f883fcd918..d3e2b420dce5 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -1302,7 +1302,7 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp) > zero_cpu_stall_ticks(rdp); > } > rdp->gp_seq = rnp->gp_seq; /* Remember new grace-period state. */ > - if (ULONG_CMP_LT(rdp->gp_seq_needed, rnp->gp_seq_needed) || rdp->gpwrap) > + if (ULONG_CMP_LT(rdp->gp_seq_needed, rnp->gp_seq_needed) || READ_ONCE(rdp->gpwrap)) > WRITE_ONCE(rdp->gp_seq_needed, rnp->gp_seq_needed); > if (IS_ENABLED(CONFIG_PROVE_RCU) && READ_ONCE(rdp->gpwrap)) > WRITE_ONCE(rdp->last_sched_clock, jiffies); > -- > 2.34.1 >
