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rcu-tasks: Split ->trc_reader_need_end

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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>
This commit is contained in:
Paul E. McKenney 2020-03-17 16:02:06 -07:00
parent b0afa0f056
commit 276c410448
5 changed files with 31 additions and 19 deletions
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11
include/linux/rcupdate_trace.h
View File

@ -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);

4
include/linux/sched.h
View File

@ -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 */

1
init/init_task.c
View File

@ -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

1
kernel/fork.c
View File

@ -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 */
}

33
kernel/rcu/tasks.h
View File

@ -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;
}
@ -884,7 +891,7 @@ 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);
@ -916,7 +923,7 @@ 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);
}
@ -980,11 +987,11 @@ 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);
}
@ -1003,8 +1010,8 @@ 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);
}
/**