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

From: Boqun Feng
Date: Mon Nov 14 2016 - 20:44:58 EST


On Mon, Nov 14, 2016 at 10:36:36AM -0800, Paul E. McKenney 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.
>
> 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;

__srcu_read_lock() used to be called with preemption disabled. I guess
the reason was because we have two percpu variables to increase. So with
only one percpu right, could we remove the preempt_{dis,en}able() in
srcu_read_lock() and use this_cpu_inc() here?

Regards,
Boqun

> }
> 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)
>

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