Date:	Fri, 9 Nov 2012 16:46:56 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v2 1/1] percpu_rw_semaphore: reimplement to not
	block the readers unnecessarily

On 11/08, Andrew Morton wrote:
>
> On Thu, 8 Nov 2012 14:48:49 +0100
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> >
> >  include/linux/percpu-rwsem.h |   83 +++++------------------------
> >  lib/Makefile                 |    2 +-
> >  lib/percpu-rwsem.c           |  123 ++++++++++++++++++++++++++++++++++++++++++
>
> The patch also uninlines everything.
>
> And it didn't export the resulting symbols to modules, so it isn't an
> equivalent.  We can export thing later if needed I guess.

Yes, currently it is only used by block_dev.c

> It adds percpu-rwsem.o to lib-y, so the CONFIG_BLOCK=n kernel will
> avoid including the code altogether, methinks?

I am going to add another user (uprobes), this was my motivation for
this patch. And perhaps it will have more users.

But I agree, CONFIG_PERCPU_RWSEM makes sense at least now, I'll send
the patch.

> > +#include <linux/percpu-rwsem.h>
> > +#include <linux/rcupdate.h>
> > +#include <linux/sched.h>
>
> This list is nowhere near sufficient to support this file's
> requirements.  atomic.h, percpu.h, rwsem.h, wait.h, errno.h and plenty
> more.  IOW, if it compiles, it was sheer luck.

OK, thanks, I'll send
send percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessarily.fix

> > +/*
> > + * A writer takes ->writer_mutex to exclude other writers and to force the
> > + * readers to switch to the slow mode, note the mutex_is_locked() check in
> > + * update_fast_ctr().
> > + *
> > + * After that the readers can only inc/dec the slow ->slow_read_ctr counter,
> > + * ->fast_read_ctr is stable. Once the writer moves its sum into the slow
> > + * counter it represents the number of active readers.
> > + *
> > + * Finally the writer takes ->rw_sem for writing and blocks the new readers,
> > + * then waits until the slow counter becomes zero.
> > + */
>
> Some overview of how fast/slow_read_ctr are supposed to work would be
> useful.  This comment seems to assume that the reader already knew
> that.

I hate to say this, but I'll try to update this comment too ;)

> > +void percpu_down_write(struct percpu_rw_semaphore *brw)
> > +{
> > +	/* also blocks update_fast_ctr() which checks mutex_is_locked() */
> > +	mutex_lock(&brw->writer_mutex);
> > +
> > +	/*
> > +	 * 1. Ensures mutex_is_locked() is visible to any down_read/up_read
> > +	 *    so that update_fast_ctr() can't succeed.
> > +	 *
> > +	 * 2. Ensures we see the result of every previous this_cpu_add() in
> > +	 *    update_fast_ctr().
> > +	 *
> > +	 * 3. Ensures that if any reader has exited its critical section via
> > +	 *    fast-path, it executes a full memory barrier before we return.
> > +	 */
> > +	synchronize_sched();
>
> Here's where I get horridly confused.  Your patch completely deRCUifies
> this code, yes?  Yet here we're using an RCU primitive.  And we seem to
> be using it not as an RCU primitive but as a handy thing which happens
> to have desirable side-effects.  But the implementation of
> synchronize_sched() differs considerably according to which rcu
> flavor-of-the-minute you're using.

It is documented that synchronize_sched() should play well with
preempt_disable/enable. From the comment:

	Note that preempt_disable(),
	local_irq_disable(), and so on may be used in place of
	rcu_read_lock_sched().

But I guess this needs more discussion, I see other emails in this
thread...

> And part 3 talks about the reader's critical section.  The only
> critical sections I can see on the reader side are already covered by
> mutex_lock() and preempt_diable().

Yes, but we need to ensure that if we take the lock for writing, we
should see all memory modifications done under down_read/up_read().

IOW. Suppose that the reader does

	percpu_down_read();
	STORE;
	percpu_up_read();	// no barriers in the fast path

The writer should see the result of that STORE under percpu_down_write().

Part 3 tries to say that at this point we should already see the result,
so we should not worry about acquire/release semantics.

> If this code isn't as brain damaged as it
> initially appears then please,

I hope ;)

> go easy on us simpletons in the next
> version?

Well, I'll try to update the comments... but the code is simple, I do
not think I can simplify it more. The nontrivial part is the barriers,
but this is always nontrivial.

Contrary, I am going to try to add some complications later, so that
it can have more users. In particular, I think it can replace
get_online_cpus/cpu_hotplug_begin, just we need
percpu_down_write_but_dont_deadlock_with_recursive_readers().

Oleg.

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