On Tue, Nov 15, 2016 at 09:44:45AM +0800, Boqun Feng wrote:
> 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 <[email protected]>
> > Signed-off-by: Lance Roy <[email protected]>
> > Signed-off-by: Paul E. McKenney <[email protected]>
> > [ 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?
Quite possibly...
Thanx, Paul
> 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)
> >
