Merge tag 'rcu.2024.07.12a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull RCU updates from Paul McKenney:

 - Update Tasks RCU and Tasks Rude RCU description in Requirements.rst
   and clarify rcu_assign_pointer() and rcu_dereference() ordering
   properties

 - Add lockdep assertions for RCU readers, limit inline wakeups for
   callback-bypass synchronize_rcu(), add an
   rcutree.nohz_full_patience_delay to reduce nohz_full OS jitter, add
   Uladzislau Rezki as RCU maintainer, and fix a subtle
   callback-migration memory-ordering issue

 - Remove a number of redundant memory barriers

 - Remove unnecessary bypass-list lock-contention mitigation, use
   parking API instead of open-coded ad-hoc equivalent, and upgrade
   obsolete comments

 - Revert avoidance of a deadlock that can no longer occur and properly
   synchronize Tasks Trace RCU checking of runqueues

 - Add tests for handling of double-call_rcu() bug, add missing
   MODULE_DESCRIPTION, and add a script that histograms the number of
   calls to RCU updaters

 - Fill out SRCU polled-grace-period API

* tag 'rcu.2024.07.12a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu: (29 commits)
  rcu: Fix rcu_barrier() VS post CPUHP_TEARDOWN_CPU invocation
  rcu: Eliminate lockless accesses to rcu_sync->gp_count
  MAINTAINERS: Add Uladzislau Rezki as RCU maintainer
  rcu: Add rcutree.nohz_full_patience_delay to reduce nohz_full OS jitter
  rcu/exp: Remove redundant full memory barrier at the end of GP
  rcu: Remove full memory barrier on RCU stall printout
  rcu: Remove full memory barrier on boot time eqs sanity check
  rcu/exp: Remove superfluous full memory barrier upon first EQS snapshot
  rcu: Remove superfluous full memory barrier upon first EQS snapshot
  rcu: Remove full ordering on second EQS snapshot
  srcu: Fill out polled grace-period APIs
  srcu: Update cleanup_srcu_struct() comment
  srcu: Add NUM_ACTIVE_SRCU_POLL_OLDSTATE
  srcu: Disable interrupts directly in srcu_gp_end()
  rcu: Disable interrupts directly in rcu_gp_init()
  rcu/tree: Reduce wake up for synchronize_rcu() common case
  rcu/tasks: Fix stale task snaphot for Tasks Trace
  tools/rcu: Add rcu-updaters.sh script
  rcutorture: Add missing MODULE_DESCRIPTION() macros
  rcutorture: Fix rcu_torture_fwd_cb_cr() data race
  ...

1 files changed, 7 insertions, 7 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 59ce0841eb1f..35a35e36024b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4466,12 +4466,7 @@ int task_call_func(struct task_struct *p, task_call_f func, void *arg)
  * @cpu: The CPU on which to snapshot the task.
  *
  * Returns the task_struct pointer of the task "currently" running on
- * the specified CPU.  If the same task is running on that CPU throughout,
- * the return value will be a pointer to that task's task_struct structure.
- * If the CPU did any context switches even vaguely concurrently with the
- * execution of this function, the return value will be a pointer to the
- * task_struct structure of a randomly chosen task that was running on
- * that CPU somewhere around the time that this function was executing.
+ * the specified CPU.
  *
  * If the specified CPU was offline, the return value is whatever it
  * is, perhaps a pointer to the task_struct structure of that CPU's idle
@@ -4485,11 +4480,16 @@ int task_call_func(struct task_struct *p, task_call_f func, void *arg)
  */
 struct task_struct *cpu_curr_snapshot(int cpu)
 {
+	struct rq *rq = cpu_rq(cpu);
 	struct task_struct *t;
+	struct rq_flags rf;
 
-	smp_mb(); /* Pairing determined by caller's synchronization design. */
+	rq_lock_irqsave(rq, &rf);
+	smp_mb__after_spinlock(); /* Pairing determined by caller's synchronization design. */
 	t = rcu_dereference(cpu_curr(cpu));
+	rq_unlock_irqrestore(rq, &rf);
 	smp_mb(); /* Pairing determined by caller's synchronization design. */
+
 	return t;
 }