This is a note to let you know that I've just added the patch titled rcu: Mark accesses to rcu_state.n_force_qs to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: rcu-mark-accesses-to-rcu_state.n_force_qs.patch and it can be found in the queue-5.15 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 2431774f04d1050292054c763070021bade7b151 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" <paulmck@xxxxxxxxxx> Date: Tue, 20 Jul 2021 06:16:27 -0700 Subject: rcu: Mark accesses to rcu_state.n_force_qs From: Paul E. McKenney <paulmck@xxxxxxxxxx> commit 2431774f04d1050292054c763070021bade7b151 upstream. This commit marks accesses to the rcu_state.n_force_qs. These data races are hard to make happen, but syzkaller was equal to the task. Reported-by: syzbot+e08a83a1940ec3846cd5@xxxxxxxxxxxxxxxxxxxxxxxxx Acked-by: Marco Elver <elver@xxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- kernel/rcu/tree.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1907,7 +1907,7 @@ static void rcu_gp_fqs(bool first_time) struct rcu_node *rnp = rcu_get_root(); WRITE_ONCE(rcu_state.gp_activity, jiffies); - rcu_state.n_force_qs++; + WRITE_ONCE(rcu_state.n_force_qs, rcu_state.n_force_qs + 1); if (first_time) { /* Collect dyntick-idle snapshots. */ force_qs_rnp(dyntick_save_progress_counter); @@ -2550,7 +2550,7 @@ static void rcu_do_batch(struct rcu_data /* Reset ->qlen_last_fqs_check trigger if enough CBs have drained. */ if (count == 0 && rdp->qlen_last_fqs_check != 0) { rdp->qlen_last_fqs_check = 0; - rdp->n_force_qs_snap = rcu_state.n_force_qs; + rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs); } else if (count < rdp->qlen_last_fqs_check - qhimark) rdp->qlen_last_fqs_check = count; @@ -2898,10 +2898,10 @@ static void __call_rcu_core(struct rcu_d } else { /* Give the grace period a kick. */ rdp->blimit = DEFAULT_MAX_RCU_BLIMIT; - if (rcu_state.n_force_qs == rdp->n_force_qs_snap && + if (READ_ONCE(rcu_state.n_force_qs) == rdp->n_force_qs_snap && rcu_segcblist_first_pend_cb(&rdp->cblist) != head) rcu_force_quiescent_state(); - rdp->n_force_qs_snap = rcu_state.n_force_qs; + rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs); rdp->qlen_last_fqs_check = rcu_segcblist_n_cbs(&rdp->cblist); } } @@ -4128,7 +4128,7 @@ int rcutree_prepare_cpu(unsigned int cpu /* Set up local state, ensuring consistent view of global state. */ raw_spin_lock_irqsave_rcu_node(rnp, flags); rdp->qlen_last_fqs_check = 0; - rdp->n_force_qs_snap = rcu_state.n_force_qs; + rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs); rdp->blimit = blimit; rdp->dynticks_nesting = 1; /* CPU not up, no tearing. */ rcu_dynticks_eqs_online(); Patches currently in stable-queue which might be from paulmck@xxxxxxxxxx are queue-5.15/rcu-mark-accesses-to-rcu_state.n_force_qs.patch