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    SubjectRe: [PATCH RFC tip/core/rcu] SRCU rewrite
    ----- On Nov 14, 2016, at 1:36 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com 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@efficios.com>
    > Signed-off-by: Lance Roy <ldr709@gmail.com>
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > [ 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

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