linux-kernel - Re: [PATCH 08/10] srcu: Add __srcu_clone_read

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Message-ID: <Zo4gtyzNvinXBOHU@Boquns-Mac-mini.home>
Date: Tue, 9 Jul 2024 22:48:39 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...nel.org, andrii@...nel.org, linux-kernel@...r.kernel.org,
	rostedt@...dmis.org, mhiramat@...nel.org, oleg@...hat.com,
	jolsa@...nel.org, clm@...a.com, paulmck@...nel.org
Subject: Re: [PATCH 08/10] srcu: Add __srcu_clone_read_lock()

On Mon, Jul 08, 2024 at 11:12:49AM +0200, Peter Zijlstra wrote:
> In order to support carrying an srcu_read_lock() section across fork,
> where both the parent and child process will do: srcu_read_unlock(),
> it is needed to account for the extra decrement with an extra
> increment at fork time.
> 

We also need to dup the per-task lock held stack in order to maintain
consistent data for lockdep, right?

Regards,
Boqun

> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  include/linux/srcu.h     |    1 +
>  include/linux/srcutiny.h |   10 ++++++++++
>  kernel/rcu/srcutree.c    |    5 +++++
>  3 files changed, 16 insertions(+)
> 
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -55,6 +55,7 @@ void call_srcu(struct srcu_struct *ssp,
>  		void (*func)(struct rcu_head *head));
>  void cleanup_srcu_struct(struct srcu_struct *ssp);
>  int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
> +void __srcu_clone_read_lock(struct srcu_struct *ssp, int idx);
>  void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
>  void synchronize_srcu(struct srcu_struct *ssp);
>  unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
> --- a/include/linux/srcutiny.h
> +++ b/include/linux/srcutiny.h
> @@ -71,6 +71,16 @@ static inline int __srcu_read_lock(struc
>  	return idx;
>  }
>  
> +static inline void __srcu_clone_read_lock(struct srcu_struct *ssp, int idx)
> +{
> +	int newval;
> +
> +	preempt_disable();  // Needed for PREEMPT_AUTO
> +	newval = READ_ONCE(ssp->srcu_lock_nesting[idx]) + 1;
> +	WRITE_ONCE(ssp->srcu_lock_nesting[idx], newval);
> +	preempt_enable();
> +}
> +
>  static inline void synchronize_srcu_expedited(struct srcu_struct *ssp)
>  {
>  	synchronize_srcu(ssp);
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -720,6 +720,11 @@ int __srcu_read_lock(struct srcu_struct
>  }
>  EXPORT_SYMBOL_GPL(__srcu_read_lock);
>  
> +void __srcu_clone_read_lock(struct srcu_struct *ssp, int idx)
> +{
> +	this_cpu_inc(ssp->sda->srcu_lock_count[idx].counter);
> +}
> +
>  /*
>   * Removes the count for the old reader from the appropriate per-CPU
>   * element of the srcu_struct.  Note that this may well be a different
> 
>