Re: [PATCH 5/8] kernel: convert process/thread iterators to for_each_*_rcu

[Michal Hocko]

On Fri 04-09-26 16:29:57, Ye Liu wrote:
> From: Ye Liu <liuye@xxxxxxxxxx>
> 
> Replace the manual rcu_read_lock()/rcu_read_unlock() and guard(rcu)
> pairs combined with for_each_process(), for_each_thread() and
> for_each_process_thread() loops across kernel/ with the
> for_each_*_rcu() macros.
> 
> No functional change.
> 
> Signed-off-by: Ye Liu <liuye@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  kernel/cpu.c             | 4 +---
>  kernel/freezer.c         | 4 +---
>  kernel/hung_task.c       | 7 ++-----
>  kernel/locking/lockdep.c | 4 +---
>  kernel/rcu/update.c      | 4 +---
>  kernel/sched/core.c      | 3 +--
>  kernel/sched/debug.c     | 4 +---
>  kernel/trace/fgraph.c    | 8 ++------
>  kernel/unwind/deferred.c | 3 +--
>  9 files changed, 11 insertions(+), 30 deletions(-)
> 
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index b3c8553d7bd6..bc70fd21561a 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1254,8 +1254,7 @@ void clear_tasks_mm_cpumask(int cpu)
>  	 * full-fledged tasklist_lock.
>  	 */
>  	WARN_ON(cpu_online(cpu));
> -	rcu_read_lock();
> -	for_each_process(p) {
> +	for_each_process_rcu(p) {
>  		struct task_struct *t;
>  
>  		/*
> @@ -1268,7 +1267,6 @@ void clear_tasks_mm_cpumask(int cpu)
>  		arch_clear_mm_cpumask_cpu(cpu, t->mm);
>  		task_unlock(t);
>  	}
> -	rcu_read_unlock();
>  }
>  
>  /* Take this CPU down. */
> diff --git a/kernel/freezer.c b/kernel/freezer.c
> index a76bf957fb32..a27b2382fda9 100644
> --- a/kernel/freezer.c
> +++ b/kernel/freezer.c
> @@ -217,11 +217,9 @@ void thaw_process(struct task_struct *p)
>  {
>  	struct task_struct *t;
>  
> -	rcu_read_lock();
> -	for_each_thread(p, t) {
> +	for_each_thread_rcu(p, t) {
>  		__thaw_task(t);
>  	}
> -	rcu_read_unlock();
>  }
>  
>  /**
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index 6fcc94ce4ca9..76871f78497f 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -315,8 +315,7 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
>  		return;
>  
>  	this_round_count = 0;
> -	rcu_read_lock();
> -	for_each_process_thread(g, t) {
> +	for_each_process_thread_rcu(g, t) {
>  		if (!max_count--)
>  			goto unlock;
>  		if (time_after(jiffies, last_break + HUNG_TASK_LOCK_BREAK)) {
> @@ -337,9 +336,7 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
>  			hung_task_info(t, timeout, this_round_count);
>  		}
>  	}
> - unlock:
> -	rcu_read_unlock();
> -
> +unlock:
>  	if (!this_round_count)
>  		return;
>  
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index c56a7f91d72e..9c3dce0f0c4b 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -6834,15 +6834,13 @@ void debug_show_all_locks(void)
>  	}
>  	pr_warn("\nShowing all locks held in the system:\n");
>  
> -	rcu_read_lock();
> -	for_each_process_thread(g, p) {
> +	for_each_process_thread_rcu(g, p) {
>  		if (!p->lockdep_depth)
>  			continue;
>  		lockdep_print_held_locks(p);
>  		touch_nmi_watchdog();
>  		touch_all_softlockup_watchdogs();
>  	}
> -	rcu_read_unlock();
>  
>  	pr_warn("\n");
>  	pr_warn("=============================================\n\n");
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 2a778b8ab4ad..c48a84a0da82 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -548,15 +548,13 @@ void synchronize_rcu_trivial_preempt(void)
>  	struct task_struct *t;
>  
>  	smp_mb(); // Order prior accesses before grace-period start.
> -	rcu_read_lock(); // Protect task list.
> -	for_each_process_thread(g, t) {
> +	for_each_process_thread_rcu(g, t) {
>  		if (t == current)
>  			continue;  // Don't deadlock on ourselves!
>  		// Order later rcu_read_lock() on other tasks after QS.
>  		while (smp_load_acquire(&t->rcu_trivial_preempt_nesting))
>  			continue;
>  	}
> -	rcu_read_unlock();
>  }
>  EXPORT_SYMBOL_GPL(synchronize_rcu_trivial_preempt);
>  #endif // #if IS_ENABLED(CONFIG_TRIVIAL_PREEMPT_RCU)
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 0697ed0f1c3d..a41cdd2ff02f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2005,8 +2005,7 @@ static void uclamp_sync_util_min_rt_default(void)
>  	smp_mb__after_spinlock();
>  	read_unlock(&tasklist_lock);
>  
> -	guard(rcu)();
> -	for_each_process_thread(g, p)
> +	for_each_process_thread_rcu(g, p)
>  		uclamp_update_util_min_rt_default(p);
>  }
>  
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 72236db67983..8f37b1a7adfe 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -1029,14 +1029,12 @@ static void print_rq(struct seq_file *m, struct rq *rq, int rq_cpu)
>  #endif
>  		   "\n");
>  
> -	rcu_read_lock();
> -	for_each_process_thread(g, p) {
> +	for_each_process_thread_rcu(g, p) {
>  		if (task_cpu(p) != rq_cpu)
>  			continue;
>  
>  		print_task(m, rq, p);
>  	}
> -	rcu_read_unlock();
>  }
>  
>  void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> index 40d373d65f9b..e522433ec8a5 100644
> --- a/kernel/trace/fgraph.c
> +++ b/kernel/trace/fgraph.c
> @@ -1057,11 +1057,10 @@ static int alloc_retstack_tasklist(unsigned long **ret_stack_list)
>  		}
>  	}
>  
> -	rcu_read_lock();
> -	for_each_process_thread(g, t) {
> +	for_each_process_thread_rcu(g, t) {
>  		if (start == end) {
>  			ret = -EAGAIN;
> -			goto unlock;
> +			goto free;
>  		}
>  
>  		if (t->ret_stack == NULL) {
> @@ -1074,9 +1073,6 @@ static int alloc_retstack_tasklist(unsigned long **ret_stack_list)
>  			t->ret_stack = ret_stack_list[start++];
>  		}
>  	}
> -
> -unlock:
> -	rcu_read_unlock();
>  free:
>  	for (i = start; i < end; i++)
>  		kmem_cache_free(fgraph_stack_cachep, ret_stack_list[i]);
> diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
> index 5bea47314254..61fe2d4727ce 100644
> --- a/kernel/unwind/deferred.c
> +++ b/kernel/unwind/deferred.c
> @@ -319,9 +319,8 @@ void unwind_deferred_cancel(struct unwind_work *work)
>  
>  	synchronize_srcu(&unwind_srcu);
>  
> -	guard(rcu)();
>  	/* Clear this bit from all threads */
> -	for_each_process_thread(g, t) {
> +	for_each_process_thread_rcu(g, t) {
>  		atomic_long_andnot(BIT(bit),
>  				   &t->unwind_info.unwind_mask);
>  		if (t->unwind_info.cache)
> -- 
> 2.25.1
> 

-- 
Michal Hocko
SUSE Labs