[PATCH rcu 23/32] rcu-tasks: Scan running tasks for RCU Tasks Trace readers

Paul E. McKenney
2022-06-20T22:56:21Z

A running task might be within an RCU Tasks Trace read-side critical
section for any length of time, but will not be placed on any of the
per-CPU rcu_tasks_percpu structure's ->rtp_blkd_tasks lists. Therefore
any RCU Tasks Trace grace-period processing that does not scan the full
task list must interact with the running tasks.

This commit therefore causes the rcu_tasks_trace_pregp_step() function
to IPI each CPU in order to place the corresponding task on the holdouts
list and to record whether or not it was in an RCU Tasks Trace read-side
critical section. Yes, it is possible to avoid adding it to that list
if it is not a reader, but that would prevent the system from remembering
that this task was in a quiescent state. Which is why the running tasks
are unconditionally added to the holdout list.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: KP Singh <kpsingh@kernel.org>
---
kernel/rcu/tasks.h | 51 ++++++++++++++++++++++++++++++++++++----------
1 file changed, 40 insertions(+), 11 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 1aa6a24a9bc2b..a8f95864c921a 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -14,7 +14,7 @@

struct rcu_tasks;
typedef void (*rcu_tasks_gp_func_t)(struct rcu_tasks *rtp);
-typedef void (*pregp_func_t)(void);
+typedef void (*pregp_func_t)(struct list_head *hop);
typedef void (*pertask_func_t)(struct task_struct *t, struct list_head *hop);
typedef void (*postscan_func_t)(struct list_head *hop);
typedef void (*holdouts_func_t)(struct list_head *hop, bool ndrpt, bool *frptp);
@@ -661,7 +661,7 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
struct task_struct *t;

set_tasks_gp_state(rtp, RTGS_PRE_WAIT_GP);
- rtp->pregp_func();
+ rtp->pregp_func(&holdouts);

/*
* There were callbacks, so we need to wait for an RCU-tasks
@@ -791,7 +791,7 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
// disabling.

/* Pre-grace-period preparation. */
-static void rcu_tasks_pregp_step(void)
+static void rcu_tasks_pregp_step(struct list_head *hop)
{
/*
* Wait for all pre-existing t->on_rq and t->nvcsw transitions
@@ -1449,24 +1449,48 @@ static void trc_wait_for_one_reader(struct task_struct *t,
}
}

-/* Do first-round processing for the specified task. */
-static void rcu_tasks_trace_pertask(struct task_struct *t,
- struct list_head *hop)
+/*
+ * Initialize for first-round processing for the specified task.
+ * Return false if task is NULL or already taken care of, true otherwise.
+ */
+static bool rcu_tasks_trace_pertask_prep(struct task_struct *t, bool notself)
{
// During early boot when there is only the one boot CPU, there
// is no idle task for the other CPUs. Also, the grace-period
// kthread is always in a quiescent state. In addition, just return
// if this task is already on the list.
- if (unlikely(t == NULL) || t == current || !list_empty(&t->trc_holdout_list))
- return;
+ if (unlikely(t == NULL) || (t == current && notself) || !list_empty(&t->trc_holdout_list))
+ return false;

rcu_st_need_qs(t, 0);
t->trc_ipi_to_cpu = -1;
- trc_wait_for_one_reader(t, hop);
+ return true;
+}
+
+/* Do first-round processing for the specified task. */
+static void rcu_tasks_trace_pertask(struct task_struct *t, struct list_head *hop)
+{
+ if (rcu_tasks_trace_pertask_prep(t, true))
+ trc_wait_for_one_reader(t, hop);
+}
+
+/*
+ * Get the current CPU's current task on the holdout list.
+ * Calls to this function must be serialized.
+ */
+static void rcu_tasks_trace_pertask_handler(void *hop_in)
+{
+ struct list_head *hop = hop_in;
+ struct task_struct *t = current;
+
+ // Pull in the currently running task, but only if it is currently
+ // in an RCU tasks trace read-side critical section.
+ if (rcu_tasks_trace_pertask_prep(t, false))
+ trc_add_holdout(t, hop);
}

/* Initialize for a new RCU-tasks-trace grace period. */
-static void rcu_tasks_trace_pregp_step(void)
+static void rcu_tasks_trace_pregp_step(struct list_head *hop)
{
int cpu;

@@ -1474,9 +1498,14 @@ static void rcu_tasks_trace_pregp_step(void)
for_each_possible_cpu(cpu)
WARN_ON_ONCE(per_cpu(trc_ipi_to_cpu, cpu));

- // Disable CPU hotplug across the tasklist scan.
+ // Disable CPU hotplug across the CPU scan.
// This also waits for all readers in CPU-hotplug code paths.
cpus_read_lock();
+
+ // These smp_call_function_single() calls are serialized to
+ // allow safe access to the hop list.
+ for_each_online_cpu(cpu)
+ smp_call_function_single(cpu, rcu_tasks_trace_pertask_handler, hop, 1);
}

/*
--
2.31.1.189.g2e36527f23