Re: [PATCH 2/5] srcu: Initialize SRCU after timers

[Paul E. McKenney]

On Fri, Apr 09, 2021 at 12:38:59AM +0200, Frederic Weisbecker wrote:
> Once srcu_init() is called, the SRCU core is free to queue delayed
> workqueues, which rely on timers. However init_timers() is called
> several steps after rcu_init(). Any call_srcu() in-between would finish
> its course inside a dangerously uninitialized timer core.
> 
> Make sure we stay in early SRCU mode until everything is well settled.
> 
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> Cc: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>

Good catches!  Apparently not so many people are using SRCU that early
in boot, but good to get it working.  I queued this patch and the
preceding one (1/5).

								
							Thanx, Paul

> ---
>  include/linux/srcu.h  | 6 ++++++
>  init/main.c           | 2 ++
>  kernel/rcu/rcu.h      | 6 ------
>  kernel/rcu/srcutree.c | 5 +++++
>  kernel/rcu/tiny.c     | 1 -
>  kernel/rcu/tree.c     | 1 -
>  6 files changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index a0895bbf71ce..e6011a9975af 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -64,6 +64,12 @@ unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
>  unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
>  bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
>  
> +#ifdef CONFIG_SRCU
> +void srcu_init(void);
> +#else /* #ifdef CONFIG_SRCU */
> +static inline void srcu_init(void) { }
> +#endif /* #else #ifdef CONFIG_SRCU */
> +
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  
>  /**
> diff --git a/init/main.c b/init/main.c
> index 53b278845b88..1bc5cc9e52ef 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -42,6 +42,7 @@
>  #include <linux/profile.h>
>  #include <linux/kfence.h>
>  #include <linux/rcupdate.h>
> +#include <linux/srcu.h>
>  #include <linux/moduleparam.h>
>  #include <linux/kallsyms.h>
>  #include <linux/writeback.h>
> @@ -956,6 +957,7 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
>  	tick_init();
>  	rcu_init_nohz();
>  	init_timers();
> +	srcu_init();
>  	hrtimers_init();
>  	softirq_init();
>  	timekeeping_init();
> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> index d64b842f4078..b3af34068051 100644
> --- a/kernel/rcu/rcu.h
> +++ b/kernel/rcu/rcu.h
> @@ -422,12 +422,6 @@ do {									\
>  
>  #endif /* #if defined(CONFIG_SRCU) || !defined(CONFIG_TINY_RCU) */
>  
> -#ifdef CONFIG_SRCU
> -void srcu_init(void);
> -#else /* #ifdef CONFIG_SRCU */
> -static inline void srcu_init(void) { }
> -#endif /* #else #ifdef CONFIG_SRCU */
> -
>  #ifdef CONFIG_TINY_RCU
>  /* Tiny RCU doesn't expedite, as its purpose in life is instead to be tiny. */
>  static inline bool rcu_gp_is_normal(void) { return true; }
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 10e681ea7051..108f9ca06047 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -1384,6 +1384,11 @@ void __init srcu_init(void)
>  {
>  	struct srcu_struct *ssp;
>  
> +	/*
> +	 * Once that is set, call_srcu() can follow the normal path and
> +	 * queue delayed work. This must follow RCU workqueues creation
> +	 * and timers initialization.
> +	 */
>  	srcu_init_done = true;
>  	while (!list_empty(&srcu_boot_list)) {
>  		ssp = list_first_entry(&srcu_boot_list, struct srcu_struct,
> diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> index c8a029fbb114..340b3f8b090d 100644
> --- a/kernel/rcu/tiny.c
> +++ b/kernel/rcu/tiny.c
> @@ -221,5 +221,4 @@ void __init rcu_init(void)
>  {
>  	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
>  	rcu_early_boot_tests();
> -	srcu_init();
>  }
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 5c214705c33f..740f5cd34459 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4714,7 +4714,6 @@ void __init rcu_init(void)
>  	WARN_ON(!rcu_gp_wq);
>  	rcu_par_gp_wq = alloc_workqueue("rcu_par_gp", WQ_MEM_RECLAIM, 0);
>  	WARN_ON(!rcu_par_gp_wq);
> -	srcu_init();
>  
>  	/* Fill in default value for rcutree.qovld boot parameter. */
>  	/* -After- the rcu_node ->lock fields are initialized! */
> -- 
> 2.25.1
>