rcu/tree: Keep kfree_rcu() awake during lock contention

On PREEMPT_RT kernels, the krcp spinlock gets converted to an rt-mutex
and causes kfree_rcu() callers to sleep. This makes it unusable for
callers in purely atomic sections such as non-threaded IRQ handlers and
raw spinlock sections. Fix it by converting the spinlock to a raw
spinlock.

Vetting all code paths, there is no reason to believe that the raw
spinlock will hurt RT latencies as it is not held for a long time.

Cc: bigeasy@linutronix.de
Cc: Uladzislau Rezki <urezki@gmail.com>
Reviewed-by: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index ba4c477..c5de5ad 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3016,7 +3016,7 @@ struct kfree_rcu_cpu {
        struct kfree_rcu_bulk_data *bhead;
        struct kfree_rcu_bulk_data *bcached;
        struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
-       spinlock_t lock;
+       raw_spinlock_t lock;
        struct delayed_work monitor_work;
        bool monitor_todo;
        bool initialized;
@@ -3049,12 +3049,12 @@ static void kfree_rcu_work(struct work_struct *work)
        krwp = container_of(to_rcu_work(work),
                            struct kfree_rcu_cpu_work, rcu_work);
        krcp = krwp->krcp;
-       spin_lock_irqsave(&krcp->lock, flags);
+       raw_spin_lock_irqsave(&krcp->lock, flags);
        head = krwp->head_free;
        krwp->head_free = NULL;
        bhead = krwp->bhead_free;
        krwp->bhead_free = NULL;
-       spin_unlock_irqrestore(&krcp->lock, flags);
+       raw_spin_unlock_irqrestore(&krcp->lock, flags);
 
        /* "bhead" is now private, so traverse locklessly. */
        for (; bhead; bhead = bnext) {
@@ -3157,14 +3157,14 @@ static inline void kfree_rcu_drain_unlock(struct kfree_rcu_cpu *krcp,
        krcp->monitor_todo = false;
        if (queue_kfree_rcu_work(krcp)) {
                // Success! Our job is done here.
-               spin_unlock_irqrestore(&krcp->lock, flags);
+               raw_spin_unlock_irqrestore(&krcp->lock, flags);
                return;
        }
 
        // Previous RCU batch still in progress, try again later.
        krcp->monitor_todo = true;
        schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
-       spin_unlock_irqrestore(&krcp->lock, flags);
+       raw_spin_unlock_irqrestore(&krcp->lock, flags);
 }
 
 /*
@@ -3177,11 +3177,11 @@ static void kfree_rcu_monitor(struct work_struct *work)
        struct kfree_rcu_cpu *krcp = container_of(work, struct kfree_rcu_cpu,
                                                 monitor_work.work);
 
-       spin_lock_irqsave(&krcp->lock, flags);
+       raw_spin_lock_irqsave(&krcp->lock, flags);
        if (krcp->monitor_todo)
                kfree_rcu_drain_unlock(krcp, flags);
        else
-               spin_unlock_irqrestore(&krcp->lock, flags);
+               raw_spin_unlock_irqrestore(&krcp->lock, flags);
 }
 
 static inline bool
@@ -3252,7 +3252,7 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
        local_irq_save(flags);  // For safely calling this_cpu_ptr().
        krcp = this_cpu_ptr(&krc);
        if (krcp->initialized)
-               spin_lock(&krcp->lock);
+               raw_spin_lock(&krcp->lock);
 
        // Queue the object but don't yet schedule the batch.
        if (debug_rcu_head_queue(head)) {
@@ -3283,7 +3283,7 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
 
 unlock_return:
        if (krcp->initialized)
-               spin_unlock(&krcp->lock);
+               raw_spin_unlock(&krcp->lock);
        local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(kfree_call_rcu);
@@ -3315,11 +3315,11 @@ kfree_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
                struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
 
                count = krcp->count;
-               spin_lock_irqsave(&krcp->lock, flags);
+               raw_spin_lock_irqsave(&krcp->lock, flags);
                if (krcp->monitor_todo)
                        kfree_rcu_drain_unlock(krcp, flags);
                else
-                       spin_unlock_irqrestore(&krcp->lock, flags);
+                       raw_spin_unlock_irqrestore(&krcp->lock, flags);
 
                sc->nr_to_scan -= count;
                freed += count;
@@ -3346,15 +3346,15 @@ void __init kfree_rcu_scheduler_running(void)
        for_each_online_cpu(cpu) {
                struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
 
-               spin_lock_irqsave(&krcp->lock, flags);
+               raw_spin_lock_irqsave(&krcp->lock, flags);
                if (!krcp->head || krcp->monitor_todo) {
-                       spin_unlock_irqrestore(&krcp->lock, flags);
+                       raw_spin_unlock_irqrestore(&krcp->lock, flags);
                        continue;
                }
                krcp->monitor_todo = true;
                schedule_delayed_work_on(cpu, &krcp->monitor_work,
                                         KFREE_DRAIN_JIFFIES);
-               spin_unlock_irqrestore(&krcp->lock, flags);
+               raw_spin_unlock_irqrestore(&krcp->lock, flags);
        }
 }
 
@@ -4250,7 +4250,7 @@ static void __init kfree_rcu_batch_init(void)
        for_each_possible_cpu(cpu) {
                struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
 
-               spin_lock_init(&krcp->lock);
+               raw_spin_lock_init(&krcp->lock);
                for (i = 0; i < KFREE_N_BATCHES; i++) {
                        INIT_RCU_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work);
                        krcp->krw_arr[i].krcp = krcp;