srcu: Hold srcu_struct ->lock when updating ->srcu_gp_seq

A read of the srcu_struct structure's ->srcu_gp_seq field should not
need READ_ONCE() when that structure's ->lock is held.  Except that this
lock is not always held when updating this field.  This commit therefore
acquires the lock around updates and removes a now-unneeded READ_ONCE().

This data race was reported by KCSAN.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
[ paulmck: Switch from READ_ONCE() to lock per Peter Zilstra question. ]
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 119a373..c19c1df 100644 (file)
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -450,7 +450,7 @@ static void srcu_gp_start(struct srcu_struct *ssp)
        spin_unlock_rcu_node(sdp);  /* Interrupts remain disabled. */
        smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */
        rcu_seq_start(&ssp->srcu_gp_seq);
-       state = rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq));
+       state = rcu_seq_state(ssp->srcu_gp_seq);
        WARN_ON_ONCE(state != SRCU_STATE_SCAN1);
 }
 
@@ -1130,7 +1130,9 @@ static void srcu_advance_state(struct srcu_struct *ssp)
                        return; /* readers present, retry later. */
                }
                srcu_flip(ssp);
+               spin_lock_irq_rcu_node(ssp);
                rcu_seq_set_state(&ssp->srcu_gp_seq, SRCU_STATE_SCAN2);
+               spin_unlock_irq_rcu_node(ssp);
        }
 
        if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) == SRCU_STATE_SCAN2) {