Re: [PATCH v5 04/45] percpu_rwlock: Implement the core design ofPer-CPU Reader-Writer Locks

From: Oleg Nesterov
Date: Sun Feb 10 2013 - 13:09:47 EST

On 02/08, Paul E. McKenney wrote:
> On Tue, Jan 22, 2013 at 01:03:53PM +0530, Srivatsa S. Bhat wrote:
> >
> > void percpu_read_unlock(struct percpu_rwlock *pcpu_rwlock)
> > {
> > - read_unlock(&pcpu_rwlock->global_rwlock);
> We need an smp_mb() here to keep the critical section ordered before the
> this_cpu_dec() below. Otherwise, if a writer shows up just after we
> exit the fastpath, that writer is not guaranteed to see the effects of
> our critical section. Equivalently, the prior read-side critical section
> just might see some of the writer's updates, which could be a bit of
> a surprise to the reader.

Agreed, we should not assume that a "reader" doesn't write. And we should
ensure that this "read" section actually completes before this_cpu_dec().

> > + /*
> > + * We never allow heterogeneous nesting of readers. So it is trivial
> > + * to find out the kind of reader we are, and undo the operation
> > + * done by our corresponding percpu_read_lock().
> > + */
> > + if (__this_cpu_read(*pcpu_rwlock->reader_refcnt)) {
> > + this_cpu_dec(*pcpu_rwlock->reader_refcnt);
> > + smp_wmb(); /* Paired with smp_rmb() in sync_reader() */
> Given an smp_mb() above, I don't understand the need for this smp_wmb().
> Isn't the idea that if the writer sees ->reader_refcnt decremented to
> zero, it also needs to see the effects of the corresponding reader's
> critical section?

I am equally confused ;)

OTOH, we can probably aboid any barrier if reader_nested_percpu() == T.

> > +static void announce_writer_inactive(struct percpu_rwlock *pcpu_rwlock)
> > +{
> > + unsigned int cpu;
> > +
> > + drop_writer_signal(pcpu_rwlock, smp_processor_id());
> Why do we drop ourselves twice? More to the point, why is it important to
> drop ourselves first?

And don't we need mb() _before_ we clear ->writer_signal ?

> > +static inline void sync_reader(struct percpu_rwlock *pcpu_rwlock,
> > + unsigned int cpu)
> > +{
> > + smp_rmb(); /* Paired with smp_[w]mb() in percpu_read_[un]lock() */
> As I understand it, the purpose of this memory barrier is to ensure
> that the stores in drop_writer_signal() happen before the reads from
> ->reader_refcnt in reader_uses_percpu_refcnt(), thus preventing the
> race between a new reader attempting to use the fastpath and this writer
> acquiring the lock. Unless I am confused, this must be smp_mb() rather
> than smp_rmb().

And note that before sync_reader() we call announce_writer_active() which
already adds mb() before sync_all_readers/sync_reader, so this rmb() looks

But, at the same time, could you confirm that we do not need another mb()
after sync_all_readers() in percpu_write_lock() ? I mean, without mb(),
can't this reader_uses_percpu_refcnt() LOAD leak into the critical section
protected by ->global_rwlock? Then this LOAD can be re-ordered with other
memory operations done by the writer.

Srivatsa, I think that the code would be more understandable if you kill
the helpers like sync_reader/raise_writer_signal. Perhaps even all "write"
helpers, I am not sure. At least, it seems to me that all barriers should
be moved to percpu_write_lock/unlock. But I won't insist of course, up to

And cosmetic nit... How about

struct xxx {
unsigned long reader_refcnt;
bool writer_signal;

struct percpu_rwlock {
struct xxx __percpu *xxx;
rwlock_t global_rwlock;


This saves one alloc_percpu() and ensures that reader_refcnt/writer_signal
are always in the same cache-line.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at