[PATCH 6.1 14/16] rcu: Export rcu_request_urgent_qs_task()

[Alexey Nepomnyashih]

From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>

commit 43a89baecfe200cb4530f42b9fcf904925d6d14a upstream.

If a CPU is executing a long series of non-sleeping system calls,
RCU grace periods can be delayed for on the order of a couple hundred
milliseconds.  This is normally not a problem, but if each system call
does a call_rcu(), those callbacks can stack up.  RCU will eventually
notice this callback storm, but use of rcu_request_urgent_qs_task()
allows the code invoking call_rcu() to give RCU a heads up.

This function is not for general use, not yet, anyway.

Reported-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/bpf/20230706033447.54696-11-alexei.starovoitov@xxxxxxxxx
Signed-off-by: Alexey Nepomnyashih <sdl@xxxxxxxx>
---
 include/linux/rcutiny.h | 2 ++
 include/linux/rcutree.h | 1 +
 kernel/rcu/rcu.h        | 4 ++--
 3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 768196a5f39d..68ebe147e45d 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -138,6 +138,8 @@ static inline int rcu_needs_cpu(void)
 	return 0;
 }
 
+static inline void rcu_request_urgent_qs_task(struct task_struct *t) { }
+
 /*
  * Take advantage of the fact that there is only one CPU, which
  * allows us to ignore virtualization-based context switches.
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 5efb51486e8a..8d0cecced199 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -21,6 +21,7 @@ void rcu_softirq_qs(void);
 void rcu_note_context_switch(bool preempt);
 int rcu_needs_cpu(void);
 void rcu_cpu_stall_reset(void);
+void rcu_request_urgent_qs_task(struct task_struct *t);
 
 /*
  * Note a virtualization-based context switch.  This is simply a
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index af6a06b86298..edff841a1a69 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -452,7 +452,8 @@ static inline bool rcu_gp_is_normal(void) { return true; }
 static inline bool rcu_gp_is_expedited(void) { return false; }
 static inline void rcu_expedite_gp(void) { }
 static inline void rcu_unexpedite_gp(void) { }
-static inline void rcu_request_urgent_qs_task(struct task_struct *t) { }
+static inline void rcu_async_hurry(void) { }
+static inline void rcu_async_relax(void) { }
 #else /* #ifdef CONFIG_TINY_RCU */
 bool rcu_gp_is_normal(void);     /* Internal RCU use. */
 bool rcu_gp_is_expedited(void);  /* Internal RCU use. */
@@ -464,7 +465,6 @@ void show_rcu_tasks_gp_kthreads(void);
 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
 static inline void show_rcu_tasks_gp_kthreads(void) {}
 #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
-void rcu_request_urgent_qs_task(struct task_struct *t);
 #endif /* #else #ifdef CONFIG_TINY_RCU */
 
 #define RCU_SCHEDULER_INACTIVE	0
-- 
2.43.0