Re: [PATCH rcu 06/12] srcu: Add srcu_read_lock_lite() and srcu_read_unlock_lite()

[Paul E. McKenney]

On Mon, Nov 11, 2024 at 06:24:58PM +0530, Neeraj Upadhyay wrote:
> 
> >  
> >  /*
> > - * Returns approximate total of the readers' ->srcu_lock_count[] values
> > - * for the rank of per-CPU counters specified by idx.
> > + * Computes approximate total of the readers' ->srcu_lock_count[] values
> > + * for the rank of per-CPU counters specified by idx, and returns true if
> > + * the caller did the proper barrier (gp), and if the count of the locks
> > + * matches that of the unlocks passed in.
> >   */
> > -static unsigned long srcu_readers_lock_idx(struct srcu_struct *ssp, int idx)
> > +static bool srcu_readers_lock_idx(struct srcu_struct *ssp, int idx, bool gp, unsigned long unlocks)
> >  {
> >  	int cpu;
> > +	unsigned long mask = 0;
> >  	unsigned long sum = 0;
> >  
> >  	for_each_possible_cpu(cpu) {
> >  		struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
> >  
> >  		sum += atomic_long_read(&sdp->srcu_lock_count[idx]);
> > +		if (IS_ENABLED(CONFIG_PROVE_RCU))
> > +			mask = mask | READ_ONCE(sdp->srcu_reader_flavor);
> >  	}
> > -	return sum;
> > +	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)),
> > +		  "Mixed reader flavors for srcu_struct at %ps.\n", ssp);
> 
> I am trying to understand the (unlikely) case where synchronize_srcu() is done before any
> srcu reader lock/unlock lite call is done. Can new SRCU readers fail to observe the
> updates?

If a SRCU reader fail to observe the index flip, then isn't it the case
that the synchronize_rcu() invoked from srcu_readers_active_idx_check()
must wait on it?

> > +	if (mask & SRCU_READ_FLAVOR_LITE && !gp)
> > +		return false;
> 
> So, srcu_readers_active_idx_check() can potentially return false for very long
> time, until the CPU executing srcu_readers_active_idx_check() does
> at least one read lock/unlock lite call?

That is correct.  The theory is that until after an srcu_read_lock_lite()
has executed, there is no need to wait on it.  Does the practice match the
theory in this case, or is there some sequence of events that I missed?

> > +	return sum == unlocks;
> >  }
> >  
> >  /*
> > @@ -473,6 +482,7 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
> >   */
> >  static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
> >  {
> > +	bool did_gp = !!(raw_cpu_read(ssp->sda->srcu_reader_flavor) & SRCU_READ_FLAVOR_LITE);
> 
> sda->srcu_reader_flavor is only set when CONFIG_PROVE_RCU is enabled. But we
> need the reader flavor information for srcu lite variant to work. So, lite
> variant does not work when CONFIG_PROVE_RCU is disabled. Am I missing something
> obvious here?

At first glance, it appears that I am the one who missed something obvious.
Including in testing, which failed to uncover this issue.

Thank you for the careful reviews!

							Thanx, Paul

> - Neeraj
> 
> >  	unsigned long unlocks;
> >  
> >  	unlocks = srcu_readers_unlock_idx(ssp, idx);
> > @@ -482,13 +492,16 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
> >  	 * unlock is counted. Needs to be a smp_mb() as the read side may
> >  	 * contain a read from a variable that is written to before the
> >  	 * synchronize_srcu() in the write side. In this case smp_mb()s
> > -	 * A and B act like the store buffering pattern.
> > +	 * A and B (or X and Y) act like the store buffering pattern.
> >  	 *
> > -	 * This smp_mb() also pairs with smp_mb() C to prevent accesses
> > -	 * after the synchronize_srcu() from being executed before the
> > -	 * grace period ends.
> > +	 * This smp_mb() also pairs with smp_mb() C (or, in the case of X,
> > +	 * Z) to prevent accesses after the synchronize_srcu() from being
> > +	 * executed before the grace period ends.
> >  	 */
> > -	smp_mb(); /* A */
> > +	if (!did_gp)
> > +		smp_mb(); /* A */
> > +	else
> > +		synchronize_rcu(); /* X */
> >  
> >  	/*
> >  	 * If the locks are the same as the unlocks, then there must have
> > @@ -546,7 +559,7 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
> >  	 * which are unlikely to be configured with an address space fully
> >  	 * populated with memory, at least not anytime soon.
> >  	 */
> > -	return srcu_readers_lock_idx(ssp, idx) == unlocks;
> > +	return srcu_readers_lock_idx(ssp, idx, did_gp, unlocks);
> >  }
> >  
>