Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

On Fri, Oct 6, 2017 at 12:02 AM, Andrea Parri [off-list ref] wrote:

On Thu, Oct 05, 2017 at 04:02:06PM +0000, Mathieu Desnoyers wrote:

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----- On Oct 5, 2017, at 8:12 AM, Peter Zijlstra peterz@infradead.org wrote:

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On Wed, Oct 04, 2017 at 02:37:53PM -0700, Paul E. McKenney wrote:

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diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c
new file mode 100644
index 000000000000..b0d79a5f5981
--- /dev/null
+++ b/arch/powerpc/kernel/membarrier.c

@@ -0,0 +1,45 @@

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+void membarrier_arch_register_private_expedited(struct task_struct *p)
+{
+  struct task_struct *t;
+
+  if (get_nr_threads(p) == 1) {
+          set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
+          return;
+  }
+  /*
+   * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
+   * fork is protected by siglock.
+   */
+  spin_lock(&p->sighand->siglock);
+  for_each_thread(p, t)
+          set_ti_thread_flag(task_thread_info(t),
+                          TIF_MEMBARRIER_PRIVATE_EXPEDITED);

I'm not sure this works correctly vs CLONE_VM without CLONE_THREAD.

The intent here is to hold the sighand siglock to provide mutual
exclusion against invocation of membarrier_fork(p, clone_flags)
by copy_process().

copy_process() grabs spin_lock(&current->sighand->siglock) for both
CLONE_THREAD and not CLONE_THREAD flags.

What am I missing here ?

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+  spin_unlock(&p->sighand->siglock);
+  /*
+   * Ensure all future scheduler executions will observe the new
+   * thread flag state for this process.
+   */
+  synchronize_sched();

This relies on the flag being read inside rq->lock, right?

Yes. The flag is read by membarrier_arch_switch_mm(), invoked
within switch_mm_irqs_off(), called by context_switch() before
finish_task_switch() releases the rq lock.

I fail to graps the relation between this synchronize_sched() and rq->lock.

s/graps/grasp

  Andrea

(Besides, we made no reference to rq->lock while discussing:

  https://github.com/paulmckrcu/litmus/commit/47039df324b60ace0cf7b2403299580be730119b
  replace membarrier_arch_sched_in with switch_mm_irqs_off )

Could you elaborate?

  Andrea

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Is the comment clear enough, or do you have suggestions for
improvements ?

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Thanks,

Mathieu

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+}

--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com