Commit 7c405fb3 authored by Boqun Feng's avatar Boqun Feng
Browse files

rcu: Use an intermediate irq_work to start process_srcu() 



Since commit c27cea44 ("rcu: Re-implement RCU Tasks Trace in terms
of SRCU-fast") we switched to SRCU in BPF. However as BPF instrument can
happen basically everywhere (including where a scheduler lock is held),
call_srcu() now needs to avoid acquiring scheduler lock because
otherwise it could cause deadlock [1]. Fix this by following what the
previous RCU Tasks Trace did: using an irq_work to delay the queuing of
the work to start process_srcu().

[boqun: Apply Joel's feedback]
[boqun: Apply Andrea's test feedback]

Reported-by: default avatarAndrea Righi <arighi@nvidia.com>
Closes: https://lore.kernel.org/all/abjzvz_tL_siV17s@gpd4/
Fixes: commit c27cea44 ("rcu: Re-implement RCU Tasks Trace in terms of SRCU-fast")
Link: https://lore.kernel.org/rcu/3c4c5a29-24ea-492d-aeee-e0d9605b4183@nvidia.com/

 [1]
Suggested-by: default avatarZqiang <qiang.zhang@linux.dev>
Tested-by: default avatarAndrea Righi <arighi@nvidia.com>
Tested-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Tested-by: default avatarJoel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: default avatarBoqun Feng <boqun@kernel.org>
parent 61bbcfb5

include/linux/srcutree.h

+1 −0
Original line number Diff line number Diff line
@@ -95,6 +95,7 @@ struct srcu_usage { 
	unsigned long reschedule_jiffies;

	unsigned long reschedule_count;

	struct delayed_work work;

	struct irq_work irq_work;

	struct srcu_struct *srcu_ssp;

};

kernel/rcu/srcutree.c

+28 −2
Original line number Diff line number Diff line
@@ -19,6 +19,7 @@ 
#include <linux/mutex.h>

#include <linux/percpu.h>

#include <linux/preempt.h>

#include <linux/irq_work.h>

#include <linux/rcupdate_wait.h>

#include <linux/sched.h>

#include <linux/smp.h>
@@ -75,6 +76,7 @@ static bool __read_mostly srcu_init_done; 
static void srcu_invoke_callbacks(struct work_struct *work);

static void srcu_reschedule(struct srcu_struct *ssp, unsigned long delay);

static void process_srcu(struct work_struct *work);

static void srcu_irq_work(struct irq_work *work);

static void srcu_delay_timer(struct timer_list *t);



/*
@@ -216,6 +218,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static) 
	mutex_init(&ssp->srcu_sup->srcu_barrier_mutex);

	atomic_set(&ssp->srcu_sup->srcu_barrier_cpu_cnt, 0);

	INIT_DELAYED_WORK(&ssp->srcu_sup->work, process_srcu);

	init_irq_work(&ssp->srcu_sup->irq_work, srcu_irq_work);

	ssp->srcu_sup->sda_is_static = is_static;

	if (!is_static) {

		ssp->sda = alloc_percpu(struct srcu_data);
@@ -716,6 +719,8 @@ void cleanup_srcu_struct(struct srcu_struct *ssp) 
		return; /* Just leak it! */

	if (WARN_ON(srcu_readers_active(ssp)))

		return; /* Just leak it! */

	/* Wait for irq_work to finish first as it may queue a new work. */

	irq_work_sync(&sup->irq_work);

	flush_delayed_work(&sup->work);

	for_each_possible_cpu(cpu) {

		struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
@@ -1121,9 +1126,13 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp, 
		// it isn't.  And it does not have to be.  After all, it

		// can only be executed during early boot when there is only

		// the one boot CPU running with interrupts still disabled.

		//

		// Use an irq_work here to avoid acquiring runqueue lock with

		// srcu rcu_node::lock held. BPF instrument could introduce the

		// opposite dependency, hence we need to break the possible

		// locking dependency here.

		if (likely(srcu_init_done))

			queue_delayed_work(rcu_gp_wq, &sup->work,

					   !!srcu_get_delay(ssp));

			irq_work_queue(&sup->irq_work);

		else if (list_empty(&sup->work.work.entry))

			list_add(&sup->work.work.entry, &srcu_boot_list);

	}
@@ -1982,6 +1991,23 @@ static void process_srcu(struct work_struct *work) 
	srcu_reschedule(ssp, curdelay);

}



static void srcu_irq_work(struct irq_work *work)

{

	struct srcu_struct *ssp;

	struct srcu_usage *sup;

	unsigned long delay;

	unsigned long flags;



	sup = container_of(work, struct srcu_usage, irq_work);

	ssp = sup->srcu_ssp;



	raw_spin_lock_irqsave_rcu_node(ssp->srcu_sup, flags);

	delay = srcu_get_delay(ssp);

	raw_spin_unlock_irqrestore_rcu_node(ssp->srcu_sup, flags);



	queue_delayed_work(rcu_gp_wq, &sup->work, !!delay);

}



void srcutorture_get_gp_data(struct srcu_struct *ssp, int *flags,

			     unsigned long *gp_seq)

{