Messages in this thread Patch in this message | | | Date | Thu, 18 Oct 2012 11:32:30 -0400 (EDT) | From | Mikulas Patocka <> | Subject | Re: [PATCH] percpu-rwsem: use barrier in unlock path |
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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
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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); }
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