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

[Pingfan Liu <kernelfans@gmail.com>]

On Mon, Nov 21, 2022 at 05:13:45PM -0800, Paul E. McKenney wrote:
> On Sun, Nov 20, 2022 at 11:40:13AM +0800, Pingfan Liu 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.
> > 
> > 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.
> 
> Nice catch!!!  And I do appreciate the testing.
> 
> But if we are going to do this, let's please also place lockdep assertions
> in srcu_funnel_exp_start() and srcu_funnel_gp_start() to verify that these
> functions are in fact invoked within an SRCU read-side critical section.
> Also a WARN_ON_ONCE(rcu_seq_done(&ssp->srcu_gp_seq, s)), please.
> 

OK, I will do both.

> Things changed to make the rcu_seq_done() unnecessary, so they could just
> as easily change again.
> 

Yes. It is wise to prevent from the future variation.

Thanks,

	Pingfan

> 							Thanx, Paul
> 
> > 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
> >