Re: [RFC PATCH] SRCU: More efficient reader counts.

[Paul E. McKenney]

On Thu, Nov 17, 2016 at 02:03:35PM -0800, Lance Roy 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().
> 
> Possible bug: There is no guarantee that the lock counter won't overflow
> during srcu_readers_active_idx_check(), as there are no memory barriers
> around srcu_flip() (see comment in srcu_readers_active_idx_check() for
> details). However, this problem was already present before this patch.

This patch differs from the previous one in a few (good) code-style
changes, comment changes, and of course the commit log.  Good.  Once
discussion converges, I will apply the agreed-upon commit, which might
well be this one.

However, let's first take a look at the overflow issue.

If a given program could have ULONG_MAX or more readers at any given
time, there would of course be overflow.  However, each read must have
an srcu_read_lock() outstanding, and the resulting four-byte return
value must be stored somewhere.  Because the full address space is at
most ULONG_MAX, the maximum number of outstanding readers is at most
ULONG_MAX/4, even in the degenerate case where a single CPU/task invokes
srcu_read_lock() in a tight loop.  And even this assumes that the entire
address space can somehow be devoted to srcu_read_lock() return values.
ULONG_MAX/4 is therefore a hard upper bound on the number of outstanding
SRCU readers.

Now srcu_readers_active_idx_check() checks for strict equality between
the number of locks and unlocks, so we can in theory tolerate ULONG_MAX-1
readers.  So, the question is whether ULONG_MAX/4 readers can result
in the updater seeing ULONG_MAX reads, due to memory misordering and
other issues.

Because there are no memory barriers surrounding srcu_flip(), the updater
could miss an extremely large number of srcu_read_unlock()s.  However,
each missed srcu_read_unlock() must have a corresponding srcu_read_lock(),
and there is a full memory barrier between between the srcu_flip() and
the read of the lock count.  There is also a full barrier between any
srcu_read_lock()'s increment of the lock count and that CPU's/task's next
srcu_read_lock()'s fetch of the index.  Therefore, if the updater misses
counting a given srcu_read_lock(), that CPU's/task's next srcu_read_lock()
must see the new value of the index.  Because srcu_read_lock() disables
preemption across the index fetch and the lock increment, there can be at
most NR_CPUS-1 srcu_read_lock() calls that missed the recent srcu_flip()'s
update to the index.  (I said NR_CPUS earlier, but Mathieu is correct
in pointing out that srcu_flip() has to run somewhere.)

The maximum number of locks that the updater can see is therefore:

o	ULONG_MAX/4 for a full set of missed srcu_read_unlock()s.

o	ULONG_MAX/4 for a full set of srcu_read_lock()s.

o	NR_CPUS-1 for a full set of subsequent srcu_read_lock()s that
	missed the flip.

This totals to ULONG_MAX/2+NR_CPUS-1.  So as long as there are no more
than ULONG_MAX/2 CPUs, we should be good.  And given that the biggest
system I have hard evidence of is 4K CPUs, we have ample headrooom
compared to the ~2G value of ULONG_MAX/2, even on 32-bit systems.

So, what am I missing here?

							Thanx, Paul

> Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Signed-off-by: Lance Roy <ldr709@xxxxxxxxx>
> ---
>  include/linux/srcu.h    |   4 +-
>  kernel/rcu/rcutorture.c |  20 ++++++++-
>  kernel/rcu/srcu.c       | 116 ++++++++++++++++++------------------------------
>  3 files changed, 63 insertions(+), 77 deletions(-)
> 
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index dc8eb63..0caea34 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 bf08fee..2450c61 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -555,10 +555,26 @@ 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 9b9cdd5..38e9aae 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 +180,42 @@ 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.
> +	 * Possible bug: There is no guarantee that there haven't been ULONG_MAX
> +	 * increments of ->lock_count[] since the unlocks were counted, meaning
> +	 * that this could return true even if there are still active readers.
> +	 * Since there are no memory barriers around srcu_flip(), the CPU is not
> +	 * required to increment ->completed before running
> +	 * srcu_readers_unlock_idx(), which means that there could be an
> +	 * arbitrarily large number of critical sections that execute after
> +	 * srcu_readers_unlock_idx() but use the old value of ->completed.
>  	 */
> -	smp_mb(); /* D */
> -
> -	return srcu_readers_seq_idx(sp, idx) == seq;
> +	return srcu_readers_lock_idx(sp, idx) == unlocks;
>  }
> 
>  /**
> @@ -266,8 +233,12 @@ 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 +269,8 @@ 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 +284,7 @@ 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 +319,7 @@ 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)
> -- 
> 2.9.0
>