From 118e0d4a1bc85d4ecea0427e440a72d21ffbfa6a Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Mon, 11 Oct 2021 16:51:30 +0200 Subject: [PATCH] rcu/nocb: Make local rcu_nocb_lock_irqsave() safe against concurrent deoffloading rcu_nocb_lock_irqsave() can be preempted between the call to rcu_segcblist_is_offloaded() and the actual locking. This matters now that rcu_core() is preemptible on PREEMPT_RT and the (de-)offloading process can interrupt the softirq or the rcuc kthread. As a result we may locklessly call into code that requires nocb locking. In practice this is a problem while we accelerate callbacks on rcu_core(). Simply disabling interrupts before (instead of after) checking the NOCB offload state fixes the issue. Reported-and-tested-by: Valentin Schneider Tested-by: Sebastian Andrzej Siewior Signed-off-by: Frederic Weisbecker Cc: Valentin Schneider Cc: Peter Zijlstra Cc: Sebastian Andrzej Siewior Cc: Josh Triplett Cc: Joel Fernandes Cc: Boqun Feng Cc: Neeraj Upadhyay Cc: Uladzislau Rezki Cc: Thomas Gleixner Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.h | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-) diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index 305cf6a..4f6c67b 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -447,12 +447,16 @@ static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp, static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp); #ifdef CONFIG_RCU_NOCB_CPU static void __init rcu_organize_nocb_kthreads(void); -#define rcu_nocb_lock_irqsave(rdp, flags) \ -do { \ - if (!rcu_segcblist_is_offloaded(&(rdp)->cblist)) \ - local_irq_save(flags); \ - else \ - raw_spin_lock_irqsave(&(rdp)->nocb_lock, (flags)); \ + +/* + * Disable IRQs before checking offloaded state so that local + * locking is safe against concurrent de-offloading. + */ +#define rcu_nocb_lock_irqsave(rdp, flags) \ +do { \ + local_irq_save(flags); \ + if (rcu_segcblist_is_offloaded(&(rdp)->cblist)) \ + raw_spin_lock(&(rdp)->nocb_lock); \ } while (0) #else /* #ifdef CONFIG_RCU_NOCB_CPU */ #define rcu_nocb_lock_irqsave(rdp, flags) local_irq_save(flags) -- 2.7.4