Re: [PATCH 1/2] srcu: Remove needless rcu_seq_done() check while holding read lock

[Pingfan Liu <kernelfans@gmail.com>]

On Sun, Nov 20, 2022 at 05:00:43AM +0000, Zhang, Qiang1 wrote:
> >
> >As the code changes, now, srcu_gp_start_if_needed() holds the srcu read
> >lock, gets the gp_seq snap and call srcu_funnel_gp_start() passing that
> >snap value.
> >
> >As the rcu_seq_snap() promises "a full grace period has elapsed since
> >the current time".  In srcu_funnel_gp_start(), the statement
> >  rcu_seq_done(&ssp->srcu_gp_seq, s)
> >always return false.
> 
> Hi Pingfan
> 
> Please correct me if I understand your commit message incorrectly. because the srcu_gp_start_if_needed() is
> protected  by srcu read lock,  and the  rcu_seq_snap()  seq is a  at the end of the current or next srcu grace period, 
> so as long as we are still in the srcu_gp_start_if_needed(), it also means that we are in SRCU critical section.
> the current or next srcu grace period will not end, it also means that we can not invoke rcu_seq_end() to update 
> 'ssp->secu_gp_seq' , so the rcu_seq_done(&ssp->srcu_gp_seq, s) is always return false.
> 

Yes, you totally got it.

Thanks

	Pingfan

> Thanks
> Zqiang
> 
> >
> >The same condition stands for srcu_funnel_exp_start(). Hence removing
> >all the checks of rcu_seq_done().
> >
> >Test info:
> >  Running "tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 10h --configs 18*SRCU-P"
> >without any failure.
> >
> >Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> >Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> To: rcu@vger.kernel.org
> ---
>  kernel/rcu/srcutree.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 1c304fec89c0..7df59fc8073e 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -851,8 +851,7 @@ static void srcu_funnel_exp_start(struct srcu_struct *ssp, struct srcu_node *snp
>  	if (snp)
>  		for (; snp != NULL; snp = snp->srcu_parent) {
>  			sgsne = READ_ONCE(snp->srcu_gp_seq_needed_exp);
> -			if (rcu_seq_done(&ssp->srcu_gp_seq, s) ||
> -			    (!srcu_invl_snp_seq(sgsne) && ULONG_CMP_GE(sgsne, s)))
> +			if (!srcu_invl_snp_seq(sgsne) && ULONG_CMP_GE(sgsne, s))
>  				return;
>  			spin_lock_irqsave_rcu_node(snp, flags);
>  			sgsne = snp->srcu_gp_seq_needed_exp;
> @@ -878,6 +877,9 @@ static void srcu_funnel_exp_start(struct srcu_struct *ssp, struct srcu_node *snp
>   *
>   * Note that this function also does the work of srcu_funnel_exp_start(),
>   * in some cases by directly invoking it.
> + *
> + * The srcu read lock should be hold around this function. And s is a seq snap
> + * after holding that lock.
>   */
>  static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
>  				 unsigned long s, bool do_norm)
> @@ -898,8 +900,6 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
>  	if (snp_leaf)
>  		/* Each pass through the loop does one level of the srcu_node tree. */
>  		for (snp = snp_leaf; snp != NULL; snp = snp->srcu_parent) {
> -			if (rcu_seq_done(&ssp->srcu_gp_seq, s) && snp != snp_leaf)
> -				return; /* GP already done and CBs recorded. */
>  			spin_lock_irqsave_rcu_node(snp, flags);
>  			snp_seq = snp->srcu_have_cbs[idx];
>  			if (!srcu_invl_snp_seq(snp_seq) && ULONG_CMP_GE(snp_seq, s)) {
> @@ -935,9 +935,8 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
>  	if (!do_norm && ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, s))
>  		WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, s);
>  
> -	/* If grace period not already done and none in progress, start it. */
> -	if (!rcu_seq_done(&ssp->srcu_gp_seq, s) &&
> -	    rcu_seq_state(ssp->srcu_gp_seq) == SRCU_STATE_IDLE) {
> +	/* If grace period not already in progress, start it. */
> +	if (rcu_seq_state(ssp->srcu_gp_seq) == SRCU_STATE_IDLE) {
>  		WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed));
>  		srcu_gp_start(ssp);
>  
> -- 
> 2.31.1
>