[PATCH rcu 18/32] rcu-tasks: Track blocked RCU Tasks Trace readers

Paul E. McKenney
2022-06-20T22:55:47Z

This commit places any task that has ever blocked within its current
RCU Tasks Trace read-side critical section on a per-CPU list within the
rcu_tasks_percpu structure. Tasks are removed from this list when they
exit by the exit_tasks_rcu_finish_trace() function. The purpose of this
commit is to provide the information needed to eliminate the current
scan of the full task list.

This commit offsets the INT_MIN value for ->trc_reader_nesting with the
new nesting level in order to avoid queueing tasks that are exiting
their read-side critical sections.

[ paulmck: Apply kernel test robot feedback. ]
[ paulmck: Apply feedback from syzbot+9bb26e7c5e8e4fa7e641@syzkaller.appspotmail.com ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: syzbot <syzbot+9bb26e7c5e8e4fa7e641@syzkaller.appspotmail.com>
Tested-by: "Zhang, Qiang1" <qiang1.zhang@intel.com>
Cc: Peter Zijlstra <peterz@infradead.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>
---
include/linux/rcupdate.h | 11 +++++++++--
include/linux/rcupdate_trace.h | 2 +-
kernel/rcu/tasks.h | 22 +++++++++++++++++++++-
3 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 1e728d544fc1e..ebdfeead44e51 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -174,12 +174,19 @@ void synchronize_rcu_tasks(void);
#define TRC_NEED_QS_CHECKED 0x2 // Task has been checked for needing quiescent state.

u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new);
+void rcu_tasks_trace_qs_blkd(struct task_struct *t);

# define rcu_tasks_trace_qs(t) \
do { \
+ int ___rttq_nesting = READ_ONCE((t)->trc_reader_nesting); \
+ \
if (likely(!READ_ONCE((t)->trc_reader_special.b.need_qs)) && \
- likely(!READ_ONCE((t)->trc_reader_nesting))) \
+ likely(!___rttq_nesting)) { \
rcu_trc_cmpxchg_need_qs((t), 0, TRC_NEED_QS_CHECKED); \
+ } else if (___rttq_nesting && ___rttq_nesting != INT_MIN && \
+ !READ_ONCE((t)->trc_reader_special.b.blocked)) { \
+ rcu_tasks_trace_qs_blkd(t); \
+ } \
} while (0)
# else
# define rcu_tasks_trace_qs(t) do { } while (0)
@@ -188,7 +195,7 @@ u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new);
#define rcu_tasks_qs(t, preempt) \
do { \
rcu_tasks_classic_qs((t), (preempt)); \
- rcu_tasks_trace_qs((t)); \
+ rcu_tasks_trace_qs(t); \
} while (0)

# ifdef CONFIG_TASKS_RUDE_RCU
diff --git a/include/linux/rcupdate_trace.h b/include/linux/rcupdate_trace.h
index 6f9c358173989..9bc8cbb33340b 100644
--- a/include/linux/rcupdate_trace.h
+++ b/include/linux/rcupdate_trace.h
@@ -75,7 +75,7 @@ static inline void rcu_read_unlock_trace(void)
nesting = READ_ONCE(t->trc_reader_nesting) - 1;
barrier(); // Critical section before disabling.
// Disable IPI-based setting of .need_qs.
- WRITE_ONCE(t->trc_reader_nesting, INT_MIN);
+ WRITE_ONCE(t->trc_reader_nesting, INT_MIN + nesting);
if (likely(!READ_ONCE(t->trc_reader_special.s)) || nesting) {
WRITE_ONCE(t->trc_reader_nesting, nesting);
return; // We assume shallow reader nesting.
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index fd4508af055e6..bab75ec26bdbb 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1261,6 +1261,24 @@ void rcu_read_unlock_trace_special(struct task_struct *t)
}
EXPORT_SYMBOL_GPL(rcu_read_unlock_trace_special);

+/* Add a newly blocked reader task to its CPU's list. */
+void rcu_tasks_trace_qs_blkd(struct task_struct *t)
+{
+ unsigned long flags;
+ struct rcu_tasks_percpu *rtpcp;
+
+ local_irq_save(flags);
+ rtpcp = this_cpu_ptr(rcu_tasks_trace.rtpcpu);
+ raw_spin_lock_rcu_node(rtpcp); // irqs already disabled
+ t->trc_blkd_cpu = smp_processor_id();
+ if (!rtpcp->rtp_blkd_tasks.next)
+ INIT_LIST_HEAD(&rtpcp->rtp_blkd_tasks);
+ list_add(&t->trc_blkd_node, &rtpcp->rtp_blkd_tasks);
+ t->trc_reader_special.b.blocked = true;
+ raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+}
+EXPORT_SYMBOL_GPL(rcu_tasks_trace_qs_blkd);
+
/* Add a task to the holdout list, if it is not already on the list. */
static void trc_add_holdout(struct task_struct *t, struct list_head *bhp)
{
@@ -1586,9 +1604,11 @@ static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
/* Report any needed quiescent state for this exiting task. */
static void exit_tasks_rcu_finish_trace(struct task_struct *t)
{
+ union rcu_special trs = READ_ONCE(t->trc_reader_special);
+
rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
- if (WARN_ON_ONCE(rcu_ld_need_qs(t) & TRC_NEED_QS))
+ if (WARN_ON_ONCE(rcu_ld_need_qs(t) & TRC_NEED_QS) || trs.b.blocked)
rcu_read_unlock_trace_special(t);
else
WRITE_ONCE(t->trc_reader_nesting, 0);
--
2.31.1.189.g2e36527f23