Re: [PATCH] percpu-rwsem: use barrier in unlock path

[Mikulas Patocka <mpatocka@redhat.com>]

On Thu, 18 Oct 2012, Lai Jiangshan wrote:

> On 10/18/2012 04:28 AM, Steven Rostedt wrote:
> > On Wed, Oct 17, 2012 at 11:07:21AM -0400, Mikulas Patocka wrote:
> >>>
> >>> Even the previous patch is applied, percpu_down_read() still
> >>> needs mb() to pair with it.
> >>
> >> percpu_down_read uses rcu_read_lock which should guarantee that memory 
> >> accesses don't escape in front of a rcu-protected section.
> > 
> > You do realize that rcu_read_lock() does nothing more that a barrier(),
> > right?
> > 
> > Paul worked really hard to get rcu_read_locks() to not call HW barriers.
> > 
> >>
> >> If rcu_read_unlock has only an unlock barrier and not a full barrier, 
> >> memory accesses could be moved in front of rcu_read_unlock and reordered 
> >> with this_cpu_inc(*p->counters), but it doesn't matter because 
> >> percpu_down_write does synchronize_rcu(), so it never sees these accesses 
> >> halfway through.
> > 
> > Looking at the patch, you are correct. The read side doesn't need the
> > memory barrier as the worse thing that will happen is that it sees the
> > locked = false, and will just grab the mutex unnecessarily.
> 
> ---------------------
> A memory barrier can be added iff these two things are known:
> 	1) it disables the disordering between what and what.
> 	2) what is the corresponding mb() that it pairs with.
> 
> You tried to add a mb() in percpu_up_write(), OK, I know it disables the disordering
> between the writes to the protected data and the statement "p->locked = false",
> But I can't find out the corresponding mb() that it pairs with.

Or alternativelly, instead of barrier, we can do this.

Mikulas

---

percpu-rwsem: use barrier in unlock path

The lock is considered unlocked when p->locked is set to false.
Use barrier prevent reordering of operations around p->locked.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 include/linux/percpu-rwsem.h |    6 ++++++
 1 file changed, 6 insertions(+)

Index: linux-3.6.2-fast/include/linux/percpu-rwsem.h
===================================================================
--- linux-3.6.2-fast.orig/include/linux/percpu-rwsem.h	2012-10-17 20:48:40.000000000 +0200
+++ linux-3.6.2-fast/include/linux/percpu-rwsem.h	2012-10-18 17:19:24.000000000 +0200
@@ -66,6 +66,12 @@ static inline void percpu_down_write(str
 
 static inline void percpu_up_write(struct percpu_rw_semaphore *p)
 {
+	/*
+	 * Make sure that other processes that are in rcu section and that
+	 * may have seen partially modified state exit the rcu section and
+	 * try to grab the mutex.
+	 */
+	synchronize_rcu();
 	p->locked = false;
 	mutex_unlock(&p->mtx);
 }