[PATCH v4 tip/core/rcu 24/38] rcu-tasks: Split ->trc_reader_need_end

paulmck
2020-04-15T18:22:47Z

This commit splits ->trc_reader_need_end by using the rcu_special union.
This change permits readers to check to see if a memory barrier is
required without any added overhead in the common case where no such
barrier is required. This commit also adds the read-side checking.
Later commits will add the machinery to properly set the new
->trc_reader_special.b.need_mb field.

This commit also makes rcu_read_unlock_trace_special() tolerate nested
read-side critical sections within interrupt and NMI handlers.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
include/linux/rcupdate_trace.h | 11 +++++++----
include/linux/sched.h | 4 ++--
init/init_task.c | 1 +
kernel/fork.c | 1 +
kernel/rcu/tasks.h | 33 ++++++++++++++++++++-------------
5 files changed, 31 insertions(+), 19 deletions(-)

diff --git a/include/linux/rcupdate_trace.h b/include/linux/rcupdate_trace.h
index ed97e10..c42b365c 100644
--- a/include/linux/rcupdate_trace.h
+++ b/include/linux/rcupdate_trace.h
@@ -31,7 +31,7 @@ static inline int rcu_read_lock_trace_held(void)

#ifdef CONFIG_TASKS_TRACE_RCU

-void rcu_read_unlock_trace_special(struct task_struct *t);
+void rcu_read_unlock_trace_special(struct task_struct *t, int nesting);

/**
* rcu_read_lock_trace - mark beginning of RCU-trace read-side critical section
@@ -50,6 +50,8 @@ static inline void rcu_read_lock_trace(void)
struct task_struct *t = current;

WRITE_ONCE(t->trc_reader_nesting, READ_ONCE(t->trc_reader_nesting) + 1);
+ if (t->trc_reader_special.b.need_mb)
+ smp_mb(); // Pairs with update-side barriers
rcu_lock_acquire(&rcu_trace_lock_map);
}

@@ -69,10 +71,11 @@ static inline void rcu_read_unlock_trace(void)

rcu_lock_release(&rcu_trace_lock_map);
nesting = READ_ONCE(t->trc_reader_nesting) - 1;
- WRITE_ONCE(t->trc_reader_nesting, nesting);
- if (likely(!READ_ONCE(t->trc_reader_need_end)) || nesting)
+ if (likely(!READ_ONCE(t->trc_reader_special.s)) || nesting) {
+ WRITE_ONCE(t->trc_reader_nesting, nesting);
return; // We assume shallow reader nesting.
- rcu_read_unlock_trace_special(t);
+ }
+ rcu_read_unlock_trace_special(t, nesting);
}

void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 864f60e..9437b53 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -613,7 +613,7 @@ union rcu_special {
u8 blocked;
u8 need_qs;
u8 exp_hint; /* Hint for performance. */
- u8 pad; /* No garbage from compiler! */
+ u8 need_mb; /* Readers need smp_mb(). */
} b; /* Bits. */
u32 s; /* Set of bits. */
};
@@ -727,7 +727,7 @@ struct task_struct {
#ifdef CONFIG_TASKS_TRACE_RCU
int trc_reader_nesting;
int trc_ipi_to_cpu;
- bool trc_reader_need_end;
+ union rcu_special trc_reader_special;
bool trc_reader_checked;
struct list_head trc_holdout_list;
#endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
diff --git a/init/init_task.c b/init/init_task.c
index e8b3740..825972d 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -143,6 +143,7 @@ struct task_struct init_task
#endif
#ifdef CONFIG_TASKS_TRACE_RCU
.trc_reader_nesting = 0,
+ .trc_reader_special.s = 0,
.trc_holdout_list = LIST_HEAD_INIT(init_task.trc_holdout_list),
#endif
#ifdef CONFIG_CPUSETS
diff --git a/kernel/fork.c b/kernel/fork.c
index 1eff609..559cdb1 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1685,6 +1685,7 @@ static inline void rcu_copy_process(struct task_struct *p)
#endif /* #ifdef CONFIG_TASKS_RCU */
#ifdef CONFIG_TASKS_TRACE_RCU
p->trc_reader_nesting = 0;
+ p->trc_reader_special.s = 0;
INIT_LIST_HEAD(&p->trc_holdout_list);
#endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
}
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 6f4e8b1..4a730f1 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -723,10 +723,17 @@ DEFINE_RCU_TASKS(rcu_tasks_trace, rcu_tasks_wait_gp, call_rcu_tasks_trace,
"RCU Tasks Trace");

/* If we are the last reader, wake up the grace-period kthread. */
-void rcu_read_unlock_trace_special(struct task_struct *t)
+void rcu_read_unlock_trace_special(struct task_struct *t, int nesting)
{
- WRITE_ONCE(t->trc_reader_need_end, false);
- if (atomic_dec_and_test(&trc_n_readers_need_end))
+ int nq = t->trc_reader_special.b.need_qs;
+
+ if (t->trc_reader_special.b.need_mb)
+ smp_mb(); // Pairs with update-side barriers.
+ // Update .need_qs before ->trc_reader_nesting for irq/NMI handlers.
+ if (nq)
+ WRITE_ONCE(t->trc_reader_special.b.need_qs, false);
+ WRITE_ONCE(t->trc_reader_nesting, nesting);
+ if (nq && atomic_dec_and_test(&trc_n_readers_need_end))
wake_up(&trc_wait);
}
EXPORT_SYMBOL_GPL(rcu_read_unlock_trace_special);
@@ -777,8 +784,8 @@ static void trc_read_check_handler(void *t_in)
// Get here if the task is in a read-side critical section. Set
// its state so that it will awaken the grace-period kthread upon
// exit from that critical section.
- WARN_ON_ONCE(t->trc_reader_need_end);
- WRITE_ONCE(t->trc_reader_need_end, true);
+ WARN_ON_ONCE(t->trc_reader_special.b.need_qs);
+ WRITE_ONCE(t->trc_reader_special.b.need_qs, true);

reset_ipi:
// Allow future IPIs to be sent on CPU and for task.
@@ -804,8 +811,8 @@ static bool trc_inspect_reader(struct task_struct *t, void *arg)
// exit from that critical section.
if (unlikely(t->trc_reader_nesting)) {
atomic_inc(&trc_n_readers_need_end); // One more to wait on.
- WARN_ON_ONCE(t->trc_reader_need_end);
- WRITE_ONCE(t->trc_reader_need_end, true);
+ WARN_ON_ONCE(t->trc_reader_special.b.need_qs);
+ WRITE_ONCE(t->trc_reader_special.b.need_qs, true);
}
return true;
}
@@ -880,7 +887,7 @@ static void rcu_tasks_trace_pregp_step(void)
static void rcu_tasks_trace_pertask(struct task_struct *t,
struct list_head *hop)
{
- WRITE_ONCE(t->trc_reader_need_end, false);
+ WRITE_ONCE(t->trc_reader_special.b.need_qs, false);
WRITE_ONCE(t->trc_reader_checked, false);
t->trc_ipi_to_cpu = -1;
trc_wait_for_one_reader(t, hop);
@@ -912,7 +919,7 @@ static void show_stalled_task_trace(struct task_struct *t, bool *firstreport)
".i"[is_idle_task(t)],
".N"[cpu > 0 && tick_nohz_full_cpu(cpu)],
t->trc_reader_nesting,
- " N"[!!t->trc_reader_need_end],
+ " N"[!!t->trc_reader_special.b.need_qs],
cpu);
sched_show_task(t);
}
@@ -976,11 +983,11 @@ static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
break; // Count reached zero.
// Stall warning time, so make a list of the offenders.
for_each_process_thread(g, t)
- if (READ_ONCE(t->trc_reader_need_end))
+ if (READ_ONCE(t->trc_reader_special.b.need_qs))
trc_add_holdout(t, &holdouts);
firstreport = true;
list_for_each_entry_safe(t, g, &holdouts, trc_holdout_list)
- if (READ_ONCE(t->trc_reader_need_end)) {
+ if (READ_ONCE(t->trc_reader_special.b.need_qs)) {
show_stalled_task_trace(t, &firstreport);
trc_del_holdout(t);
}
@@ -999,8 +1006,8 @@ void exit_tasks_rcu_finish_trace(struct task_struct *t)
WRITE_ONCE(t->trc_reader_checked, true);
WARN_ON_ONCE(t->trc_reader_nesting);
WRITE_ONCE(t->trc_reader_nesting, 0);
- if (WARN_ON_ONCE(READ_ONCE(t->trc_reader_need_end)))
- rcu_read_unlock_trace_special(t);
+ if (WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs)))
+ rcu_read_unlock_trace_special(t, 0);
}

/**
--
2.9.5