On Thu, Apr 16, 2020 at 11:34:44PM +0200, Sebastian Andrzej Siewior wrote:
> On 2020-04-16 14:00:57 [-0700], Paul E. McKenney wrote:
> >
> > We might need different calling-context restrictions for the two variants
> > of kfree_rcu(). And we might need to come up with some sort of lockdep
> > check for "safe to use normal spinlock in -rt".
>
> Oh. We do have this already, it is called CONFIG_PROVE_RAW_LOCK_NESTING.
> This one will scream if you do
> raw_spin_lock();
> spin_lock();
>
> Sadly, as of today, there is code triggering this which needs to be
> addressed first (but it is one list of things to do).
>
> Given the thread so far, is it okay if I repost the series with
> migrate_disable() instead of accepting a possible migration before
> grabbing the lock? I would prefer to avoid the extra RT case (avoiding
> memory allocations in a possible atomic context) until we get there.
I prefer something like the following to make it possible to invoke
kfree_rcu() from atomic context considering call_rcu() is already callable
from such contexts. Thoughts?
(Only build tested)
---8<-----------------------
From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>
Subject: [PATCH] rcu/tree: Avoid allocating in non-preemptible context for
PREEMPT_RT kernels
Per recent discussions, kfree_rcu() is a low-level facility which should be
callable in atomic context (raw spinlock sections, IRQ disable sections etc).
However, it depends on page allocation which acquires sleeping locks on
PREEMPT_RT.
In order to support all usecases, avoid the allocation of pages for
PREEMPT_RT. The page allocation is just an optimization which does not
break functionality. Further, in future patches the pages will be
pre-allocated reducing the likelihood that page allocations will be
needed.
We also convert the spinlock_t to raw_spinlock_t so that does not sleep
in PREEMPT_RT's raw atomic critical sections.
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
kernel/rcu/tree.c | 42 +++++++++++++++++++++++++-----------------
1 file changed, 25 insertions(+), 17 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f288477ee1c26..ba831712fb307 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2905,7 +2905,7 @@ 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;
@@ -2939,12 +2939,12 @@ 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) {
@@ -3047,14 +3047,14 @@ 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);
}
/*
@@ -3067,16 +3067,16 @@ 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
kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
- struct rcu_head *head, rcu_callback_t func)
+ struct rcu_head *head, rcu_callback_t func, bool alloc)
{
struct kfree_rcu_bulk_data *bnode;
@@ -3092,6 +3092,10 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
if (!bnode) {
WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE);
+ /* If allocation is not allowed, don't do it. */
+ if (!alloc)
+ return false;
+
bnode = (struct kfree_rcu_bulk_data *)
__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
}
@@ -3138,11 +3142,15 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
{
unsigned long flags;
struct kfree_rcu_cpu *krcp;
+ bool alloc = true;
+
+ if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
+ alloc = false;
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)) {
@@ -3156,7 +3164,7 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
* Under high memory pressure GFP_NOWAIT can fail,
* in that case the emergency path is maintained.
*/
- if (unlikely(!kfree_call_rcu_add_ptr_to_bulk(krcp, head, func))) {
+ if (unlikely(!kfree_call_rcu_add_ptr_to_bulk(krcp, head, func, alloc))) {
head->func = func;
head->next = krcp->head;
krcp->head = head;
@@ -3173,7 +3181,7 @@ 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);
@@ -3205,11 +3213,11 @@ 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;
@@ -3236,15 +3244,15 @@ 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);
}
}
@@ -4140,7 +4148,7 @@ 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;
--
2.26.1.301.g55bc3eb7cb9-goog
