Date:	Mon, 27 Feb 2012 10:30:35 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, patches@...aro.org
Subject: Re: [PATCH 2/2 RFC] srcu: implement Peter's checking algorithm

On Mon, Feb 27, 2012 at 04:01:04PM +0800, Lai Jiangshan wrote:
> >From 40724998e2d121c2b5a5bd75114625cfd9d4f9a9 Mon Sep 17 00:00:00 2001
> From: Lai Jiangshan <laijs@...fujitsu.com>
> Date: Mon, 27 Feb 2012 14:22:47 +0800
> Subject: [PATCH 2/2] srcu: implement Peter's checking algorithm
> 
> This patch implement the algorithm as Peter's:
> https://lkml.org/lkml/2012/2/1/119
> 
> o	Make the checking lock-free and we can perform parallel checking,
> 	Although almost parallel checking makes no sense, but we need it
> 	when 1) the original checking task is preempted for long, 2)
> 	sychronize_srcu_expedited(), 3) avoid lock(see next)
> 
> o	Since it is lock-free, we save a mutex in state machine for
> 	call_srcu().
> 
> o	Remove the SRCU_REF_MASK and remove the coupling with the flipping.
> 	(so we can remove the preempt_disable() in future, but use
> 	 __this_cpu_inc() instead.)
> 
> o	reduce a smp_mb(), simplify the comments and make the smp_mb() pairs
> 	more intuitive.

Hello, Lai,

Interesting approach!

What happens given the following sequence of events?

o	CPU 0 in srcu_readers_active_idx_check() invokes
	srcu_readers_seq_idx(), getting some number back.

o	CPU 0 invokes srcu_readers_active_idx(), summing the
	->c[] array up through CPU 3.

o	CPU 1 invokes __srcu_read_lock(), and increments its counter
	but not yet its ->seq[] element.

o	CPU 0 completes its summing of the ->c[] array, incorrectly
	obtaining zero.

o	CPU 0 invokes srcu_readers_seq_idx(), getting the same
	number back that it got last time.

o	In parallel with the previous step, CPU 1 executes out of order
	(as permitted by the lack of a second memory barrier in
	__srcu_read_lock()), starting up the critical section before
	incrementing its ->seq[] element.

o	Because CPU 0 is not aware that CPU 1 is an SRCU reader, it
	completes the SRCU grace period before CPU 1 completes its
	SRCU read-side critical section.

This actually might be safe, but I need to think more about it.  In the
meantime, I figured I should ask your thoughts.

							Thanx, Paul

> Inspired-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> ---
>  include/linux/srcu.h |    7 +--
>  kernel/srcu.c        |  137 ++++++++++++++++++++-----------------------------
>  2 files changed, 57 insertions(+), 87 deletions(-)
> 
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index 5b49d41..15354db 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -32,18 +32,13 @@
> 
>  struct srcu_struct_array {
>  	unsigned long c[2];
> +	unsigned long seq[2];
>  };
> 
> -/* Bit definitions for field ->c above and ->snap below. */
> -#define SRCU_USAGE_BITS		1
> -#define SRCU_REF_MASK		(ULONG_MAX >> SRCU_USAGE_BITS)
> -#define SRCU_USAGE_COUNT	(SRCU_REF_MASK + 1)
> -
>  struct srcu_struct {
>  	unsigned completed;
>  	struct srcu_struct_array __percpu *per_cpu_ref;
>  	struct mutex mutex;
> -	unsigned long snap[NR_CPUS];
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  	struct lockdep_map dep_map;
>  #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> diff --git a/kernel/srcu.c b/kernel/srcu.c
> index 47ee35d..376b583 100644
> --- a/kernel/srcu.c
> +++ b/kernel/srcu.c
> @@ -73,10 +73,25 @@ EXPORT_SYMBOL_GPL(init_srcu_struct);
>  #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> 
>  /*
> + * Returns approximate total sequence of readers on the specified rank
> + * of per-CPU counters.
> + */
> +static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
> +{
> +	int cpu;
> +	unsigned long sum = 0;
> +	unsigned long t;
> +
> +	for_each_possible_cpu(cpu) {
> +		t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
> +		sum += t;
> +	}
> +	return sum;
> +}
> +
> +/*
>   * Returns approximate number of readers active on the specified rank
> - * of per-CPU counters.  Also snapshots each counter's value in the
> - * corresponding element of sp->snap[] for later use validating
> - * the sum.
> + * of per-CPU counters.
>   */
>  static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
>  {
> @@ -87,26 +102,36 @@ static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
>  	for_each_possible_cpu(cpu) {
>  		t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
>  		sum += t;
> -		sp->snap[cpu] = t;
>  	}
> -	return sum & SRCU_REF_MASK;
> +	return sum;
>  }
> 
> -/*
> - * To be called from the update side after an index flip.  Returns true
> - * if the modulo sum of the counters is stably zero, false if there is
> - * some possibility of non-zero.
> - */
>  static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
>  {
>  	int cpu;
> +	unsigned long seq;
> +
> +	seq = srcu_readers_seq_idx(sp, idx);
> +
> +	/*
> +	 * smp_mb() A pairs with smp_mb() B for critical section.
> +	 * It ensures that the SRCU read-side critical section whose
> +	 * read-lock is not seen by the following srcu_readers_active_idx()
> +	 * will see any updates that before the current task performed before.
> +	 * (So we don't need to care these readers this time)
> +	 *
> +	 * Also, if we see the increment of the seq, we must see the
> +	 * increment of the active counter in the following
> +	 * srcu_readers_active_idx().
> +	 */
> +	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 modulo sum of the counters
> +	 * 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
> @@ -122,53 +147,26 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
>  		return false;
> 
>  	/*
> -	 * Since the caller recently flipped ->completed, we can see at
> -	 * most one increment of each CPU's counter from this point
> -	 * forward.  The reason for this is that the reader CPU must have
> -	 * fetched the index before srcu_readers_active_idx checked
> -	 * that CPU's counter, but not yet incremented its counter.
> -	 * Its eventual counter increment will follow the read in
> -	 * srcu_readers_active_idx(), and that increment is immediately
> -	 * followed by smp_mb() B.  Because smp_mb() D is between
> -	 * the ->completed flip and srcu_readers_active_idx()'s read,
> -	 * that CPU's subsequent load of ->completed must see the new
> -	 * value, and therefore increment the counter in the other rank.
> -	 */
> -	smp_mb(); /* A */
> -
> -	/*
> -	 * Now, we check the ->snap array that srcu_readers_active_idx()
> -	 * filled in from the per-CPU counter values. Since
> -	 * __srcu_read_lock() increments the upper bits of the per-CPU
> -	 * counter, an increment/decrement pair will change the value
> -	 * of the counter.  Since there is only one possible increment,
> -	 * the only way to wrap the counter is to have a huge number of
> -	 * counter decrements, which requires a huge number of tasks and
> -	 * huge SRCU read-side critical-section nesting levels, even on
> -	 * 32-bit systems.
> -	 *
> -	 * All of the ways of confusing the readings require that the scan
> -	 * in srcu_readers_active_idx() see the read-side task's decrement,
> -	 * but not its increment.  However, between that decrement and
> -	 * increment are smb_mb() B and C.  Either or both of these pair
> -	 * with smp_mb() A above to ensure that the scan below will see
> -	 * the read-side tasks's increment, thus noting a difference in
> -	 * the counter values between the two passes.
> +	 * Validation step, smp_mb() D pairs with smp_mb() C. If the above
> +	 * srcu_readers_active_idx() see a decrement of the active counter
> +	 * in srcu_read_unlock(), it should see one of these for corresponding
> +	 * srcu_read_lock():
> +	 * 	See the increment of the active counter,
> +	 * 	Failed to see the increment of the active counter.
> +	 * The second one can cause srcu_readers_active_idx() incorrectly
> +	 * return zero, but it means the above srcu_readers_seq_idx() does not
> +	 * see the increment of the seq(ref: comments of smp_mb() A),
> +	 * and the following srcu_readers_seq_idx() sees the increment of
> +	 * the seq. The seq is changed.
>  	 *
> -	 * Therefore, if srcu_readers_active_idx() returned zero, and
> -	 * none of the counters changed, we know that the zero was the
> -	 * correct sum.
> -	 *
> -	 * Of course, it is possible that a task might be delayed
> -	 * for a very long time in __srcu_read_lock() after fetching
> -	 * the index but before incrementing its counter.  This
> -	 * possibility will be dealt with in __synchronize_srcu().
> +	 * This smp_mb() D pairs with smp_mb() C for critical section.
> +	 * then any of the current task's subsequent code will happen after
> +	 * that SRCU read-side critical section whose read-unlock is seen in
> +	 * srcu_readers_active_idx().
>  	 */
> -	for_each_possible_cpu(cpu)
> -		if (sp->snap[cpu] !=
> -		    ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]))
> -			return false;  /* False zero reading! */
> -	return true;
> +	smp_mb(); /* D */
> +
> +	return srcu_readers_seq_idx(sp, idx) == seq;
>  }
> 
>  /**
> @@ -216,9 +214,9 @@ int __srcu_read_lock(struct srcu_struct *sp)
>  	preempt_disable();
>  	idx = rcu_dereference_index_check(sp->completed,
>  					  rcu_read_lock_sched_held()) & 0x1;
> -	ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) +=
> -		SRCU_USAGE_COUNT + 1;
> +	ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;
>  	smp_mb(); /* B */  /* Avoid leaking the critical section. */
> +	ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;
>  	preempt_enable();
>  	return idx;
>  }
> @@ -258,17 +256,6 @@ static void wait_idx(struct srcu_struct *sp, int idx, bool expedited)
>  	int trycount = 0;
> 
>  	/*
> -	 * If a reader fetches the index before the ->completed increment,
> -	 * but increments its counter after srcu_readers_active_idx_check()
> -	 * sums it, then smp_mb() D will pair with __srcu_read_lock()'s
> -	 * smp_mb() B to ensure that the SRCU read-side critical section
> -	 * will see any updates that the current task performed before its
> -	 * call to synchronize_srcu(), or to synchronize_srcu_expedited(),
> -	 * as the case may be.
> -	 */
> -	smp_mb(); /* D */
> -
> -	/*
>  	 * SRCU read-side critical sections are normally short, so wait
>  	 * a small amount of time before possibly blocking.
>  	 */
> @@ -281,18 +268,6 @@ static void wait_idx(struct srcu_struct *sp, int idx, bool expedited)
>  				schedule_timeout_interruptible(1);
>  		}
>  	}
> -
> -	/*
> -	 * The following smp_mb() E pairs with srcu_read_unlock()'s
> -	 * smp_mb C to ensure that if srcu_readers_active_idx_check()
> -	 * sees srcu_read_unlock()'s counter decrement, then any
> -	 * of the current task's subsequent code will happen after
> -	 * that SRCU read-side critical section.
> -	 *
> -	 * It also ensures the order between the above waiting and
> -	 * the next flipping.
> -	 */
> -	smp_mb(); /* E */
>  }
> 
>  static void srcu_flip(struct srcu_struct *sp)
> -- 
> 1.7.4.4
> 

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