Re: [PATCH RFC tip/core/rcu] SRCU rewrite

[Mathieu Desnoyers]

----- On Nov 14, 2016, at 1:36 PM, Paul E. McKenney paulmck@xxxxxxxxxxxxxxxxxx wrote:

> SRCU uses two per-cpu counters: a nesting counter to count the number of
> active critical sections, and a sequence counter to ensure that the nesting
> counters don't change while they are being added together in
> srcu_readers_active_idx_check().
> 
> This patch instead uses per-cpu lock and unlock counters. Because the both
> counters only increase and srcu_readers_active_idx_check() reads the unlock
> counter before the lock counter, this achieves the same end without having
> to increment two different counters in srcu_read_lock(). This also saves a
> smp_mb() in srcu_readers_active_idx_check().
> 
> A possible problem with this patch is that it can only handle
> ULONG_MAX - NR_CPUS simultaneous readers, whereas the old version could
> handle up to ULONG_MAX.

I think for the above we ended up agreeing that the old version did have
similar limitations as the new one ? I would have expected the sentence
above to be removed from the changelog.

Thanks,

Mathieu

> 
> Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Signed-off-by: Lance Roy <ldr709@xxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> [ paulmck: Queued for 4.12, that is, merge window after this coming one. ]
> 
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index dc8eb63c6568..0caea34d8c5f 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -34,8 +34,8 @@
> #include <linux/workqueue.h>
> 
> struct srcu_struct_array {
> -	unsigned long c[2];
> -	unsigned long seq[2];
> +	unsigned long lock_count[2];
> +	unsigned long unlock_count[2];
> };
> 
> struct rcu_batch {
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index 87c51225ceec..6e4fd7680c70 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -564,10 +564,24 @@ static void srcu_torture_stats(void)
> 	pr_alert("%s%s per-CPU(idx=%d):",
> 		 torture_type, TORTURE_FLAG, idx);
> 	for_each_possible_cpu(cpu) {
> +		unsigned long l0, l1;
> +		unsigned long u0, u1;
> 		long c0, c1;
> +		struct srcu_struct_array* counts =
> +			per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu);
> 
> -		c0 = (long)per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu)->c[!idx];
> -		c1 = (long)per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu)->c[idx];
> +		u0 = counts->unlock_count[!idx];
> +		u1 = counts->unlock_count[idx];
> +
> +		/* Make sure that a lock is always counted if the corresponding
> +		   unlock is counted. */
> +		smp_rmb();
> +
> +		l0 = counts->lock_count[!idx];
> +		l1 = counts->lock_count[idx];
> +
> +		c0 = (long)(l0 - u0);
> +		c1 = (long)(l1 - u1);
> 		pr_cont(" %d(%ld,%ld)", cpu, c0, c1);
> 	}
> 	pr_cont("\n");
> diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
> index 9b9cdd549caa..edfdfadec821 100644
> --- a/kernel/rcu/srcu.c
> +++ b/kernel/rcu/srcu.c
> @@ -141,34 +141,38 @@ EXPORT_SYMBOL_GPL(init_srcu_struct);
> #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> 
> /*
> - * Returns approximate total of the readers' ->seq[] values for the
> + * Returns approximate total of the readers' ->lock_count[] values for the
>  * rank of per-CPU counters specified by idx.
>  */
> -static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
> +static unsigned long srcu_readers_lock_idx(struct srcu_struct *sp, int idx)
> {
> 	int cpu;
> 	unsigned long sum = 0;
> 	unsigned long t;
> 
> 	for_each_possible_cpu(cpu) {
> -		t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
> +		struct srcu_struct_array* cpu_counts =
> +			per_cpu_ptr(sp->per_cpu_ref, cpu);
> +		t = READ_ONCE(cpu_counts->lock_count[idx]);
> 		sum += t;
> 	}
> 	return sum;
> }
> 
> /*
> - * Returns approximate number of readers active on the specified rank
> - * of the per-CPU ->c[] counters.
> + * Returns approximate total of the readers' ->unlock_count[] values for the
> + * rank of per-CPU counters specified by idx.
>  */
> -static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
> +static unsigned long srcu_readers_unlock_idx(struct srcu_struct *sp, int idx)
> {
> 	int cpu;
> 	unsigned long sum = 0;
> 	unsigned long t;
> 
> 	for_each_possible_cpu(cpu) {
> -		t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
> +		struct srcu_struct_array* cpu_counts =
> +			per_cpu_ptr(sp->per_cpu_ref, cpu);
> +		t = READ_ONCE(cpu_counts->unlock_count[idx]);
> 		sum += t;
> 	}
> 	return sum;
> @@ -176,79 +180,43 @@ static unsigned long srcu_readers_active_idx(struct
> srcu_struct *sp, int idx)
> 
> /*
>  * Return true if the number of pre-existing readers is determined to
> - * be stably zero.  An example unstable zero can occur if the call
> - * to srcu_readers_active_idx() misses an __srcu_read_lock() increment,
> - * but due to task migration, sees the corresponding __srcu_read_unlock()
> - * decrement.  This can happen because srcu_readers_active_idx() takes
> - * time to sum the array, and might in fact be interrupted or preempted
> - * partway through the summation.
> + * be zero.
>  */
> static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
> {
> -	unsigned long seq;
> +	unsigned long unlocks;
> 
> -	seq = srcu_readers_seq_idx(sp, idx);
> +	unlocks = srcu_readers_unlock_idx(sp, idx);
> 
> 	/*
> -	 * The following smp_mb() A pairs with the smp_mb() B located in
> -	 * __srcu_read_lock().  This pairing ensures that if an
> -	 * __srcu_read_lock() increments its counter after the summation
> -	 * in srcu_readers_active_idx(), then the corresponding SRCU read-side
> -	 * critical section will see any changes made prior to the start
> -	 * of the current SRCU grace period.
> +	 * Make sure that a lock is always counted if the corresponding 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.
> 	 *
> -	 * Also, if the above call to srcu_readers_seq_idx() saw the
> -	 * increment of ->seq[], then the call to srcu_readers_active_idx()
> -	 * must see the increment of ->c[].
> +	 * This smp_mb() also pairs with smp_mb() C to prevent writes after the
> +	 * synchronize_srcu() from being executed before the grace period ends.
> 	 */
> 	smp_mb(); /* A */
> 
> 	/*
> -	 * Note that srcu_readers_active_idx() can incorrectly return
> -	 * zero even though there is a pre-existing reader throughout.
> -	 * To see this, suppose that task A is in a very long SRCU
> -	 * read-side critical section that started on CPU 0, and that
> -	 * no other reader exists, so that the sum of the counters
> -	 * is equal to one.  Then suppose that task B starts executing
> -	 * srcu_readers_active_idx(), summing up to CPU 1, and then that
> -	 * task C starts reading on CPU 0, so that its increment is not
> -	 * summed, but finishes reading on CPU 2, so that its decrement
> -	 * -is- summed.  Then when task B completes its sum, it will
> -	 * incorrectly get zero, despite the fact that task A has been
> -	 * in its SRCU read-side critical section the whole time.
> -	 *
> -	 * We therefore do a validation step should srcu_readers_active_idx()
> -	 * return zero.
> -	 */
> -	if (srcu_readers_active_idx(sp, idx) != 0)
> -		return false;
> -
> -	/*
> -	 * The remainder of this function is the validation step.
> -	 * The following smp_mb() D pairs with the smp_mb() C in
> -	 * __srcu_read_unlock().  If the __srcu_read_unlock() was seen
> -	 * by srcu_readers_active_idx() above, then any destructive
> -	 * operation performed after the grace period will happen after
> -	 * the corresponding SRCU read-side critical section.
> +	 * If the locks are the same as the unlocks, then there must of have
> +	 * been no readers on this index at some time in between. This does not
> +	 * mean that there are no more readers, as one could have read the
> +	 * current index but have incremented the lock counter yet.
> 	 *
> -	 * Note that there can be at most NR_CPUS worth of readers using
> -	 * the old index, which is not enough to overflow even a 32-bit
> -	 * integer.  (Yes, this does mean that systems having more than
> -	 * a billion or so CPUs need to be 64-bit systems.)  Therefore,
> -	 * the sum of the ->seq[] counters cannot possibly overflow.
> -	 * Therefore, the only way that the return values of the two
> -	 * calls to srcu_readers_seq_idx() can be equal is if there were
> -	 * no increments of the corresponding rank of ->seq[] counts
> -	 * in the interim.  But the missed-increment scenario laid out
> -	 * above includes an increment of the ->seq[] counter by
> -	 * the corresponding __srcu_read_lock().  Therefore, if this
> -	 * scenario occurs, the return values from the two calls to
> -	 * srcu_readers_seq_idx() will differ, and thus the validation
> -	 * step below suffices.
> +	 * Note that there can be at most NR_CPUS worth of readers using the old
> +	 * index that haven't incremented ->lock_count[] yet.  Therefore, the
> +	 * sum of the ->lock_count[]s cannot increment enough times to overflow
> +	 * and end up equal the sum of the ->unlock_count[]s, as long as there
> +	 * are at most ULONG_MAX - NR_CPUS readers at a time.  (Yes, this does
> +	 * mean that systems having more than a billion or so CPUs need to be
> +	 * 64-bit systems.)  Therefore, the only way that the return values of
> +	 * the two calls to srcu_readers_(un)lock_idx() can be equal is if there
> +	 * are no active readers using this index.
> 	 */
> -	smp_mb(); /* D */
> -
> -	return srcu_readers_seq_idx(sp, idx) == seq;
> +	return srcu_readers_lock_idx(sp, idx) == unlocks;
> }
> 
> /**
> @@ -266,8 +234,12 @@ static bool srcu_readers_active(struct srcu_struct *sp)
> 	unsigned long sum = 0;
> 
> 	for_each_possible_cpu(cpu) {
> -		sum += READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[0]);
> -		sum += READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[1]);
> +		struct srcu_struct_array* cpu_counts =
> +			per_cpu_ptr(sp->per_cpu_ref, cpu);
> +		sum += READ_ONCE(cpu_counts->lock_count[0]);
> +		sum += READ_ONCE(cpu_counts->lock_count[1]);
> +		sum -= READ_ONCE(cpu_counts->unlock_count[0]);
> +		sum -= READ_ONCE(cpu_counts->unlock_count[1]);
> 	}
> 	return sum;
> }
> @@ -298,9 +270,8 @@ int __srcu_read_lock(struct srcu_struct *sp)
> 	int idx;
> 
> 	idx = READ_ONCE(sp->completed) & 0x1;
> -	__this_cpu_inc(sp->per_cpu_ref->c[idx]);
> +	__this_cpu_inc(sp->per_cpu_ref->lock_count[idx]);
> 	smp_mb(); /* B */  /* Avoid leaking the critical section. */
> -	__this_cpu_inc(sp->per_cpu_ref->seq[idx]);
> 	return idx;
> }
> EXPORT_SYMBOL_GPL(__srcu_read_lock);
> @@ -314,7 +285,7 @@ EXPORT_SYMBOL_GPL(__srcu_read_lock);
> void __srcu_read_unlock(struct srcu_struct *sp, int idx)
> {
> 	smp_mb(); /* C */  /* Avoid leaking the critical section. */
> -	this_cpu_dec(sp->per_cpu_ref->c[idx]);
> +	this_cpu_inc(sp->per_cpu_ref->unlock_count[idx]);
> }
> EXPORT_SYMBOL_GPL(__srcu_read_unlock);
> 
> @@ -349,7 +320,7 @@ static bool try_check_zero(struct srcu_struct *sp, int idx,
> int trycount)
> 
> /*
>  * Increment the ->completed counter so that future SRCU readers will
> - * use the other rank of the ->c[] and ->seq[] arrays.  This allows
> + * use the other rank of the ->(un)lock_count[] arrays.  This allows
>  * us to wait for pre-existing readers in a starvation-free manner.
>  */
>  static void srcu_flip(struct srcu_struct *sp)

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com