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Re: [Xen-devel] [PATCH v7 2/5] xen/rcu: don't use stop_machine_run() for rcu_barrier()

On 26/03/2020 08:50, Jürgen Groß wrote:
On 26.03.20 09:49, Jan Beulich wrote:
On 26.03.2020 08:24, Jürgen Groß wrote:
On 26.03.20 07:58, Jan Beulich wrote:
On 25.03.2020 17:13, Julien Grall wrote:
On 25/03/2020 10:55, Juergen Gross wrote:
@@ -143,51 +143,90 @@ static int qhimark = 10000;
    static int qlowmark = 100;
    static int rsinterval = 1000;
    -struct rcu_barrier_data {
-    struct rcu_head head;
-    atomic_t *cpu_count;
+ * rcu_barrier() handling:
+ * Two counters are used to synchronize rcu_barrier() work:
+ * - cpu_count holds the number of cpus required to finish barrier handling. + *   It is decremented by each cpu when it has performed all pending rcu calls. + * - pending_count shows whether any rcu_barrier() activity is running and + *   it is used to synchronize leaving rcu_barrier() only after all cpus + *   have finished their processing. pending_count is initialized to nr_cpus + 1 + *   and it is decremented by each cpu when it has seen that cpu_count has + *   reached 0. The cpu where rcu_barrier() has been called will wait until + *   pending_count has been decremented to 1 (so all cpus have seen cpu_count + *   reaching 0) and will then set pending_count to 0 indicating there is no
+ *   rcu_barrier() running.
+ * Cpus are synchronized via softirq mechanism. rcu_barrier() is regarded to + * be active if pending_count is not zero. In case rcu_barrier() is called on + * multiple cpus it is enough to check for pending_count being not zero on entry + * and to call process_pending_softirqs() in a loop until pending_count drops to
+ * zero, before starting the new rcu_barrier() processing.
+ */
+static atomic_t cpu_count = ATOMIC_INIT(0);
+static atomic_t pending_count = ATOMIC_INIT(0);
      static void rcu_barrier_callback(struct rcu_head *head)
-    struct rcu_barrier_data *data = container_of(
-        head, struct rcu_barrier_data, head);
-    atomic_inc(data->cpu_count);
+    smp_mb__before_atomic();     /* Make all writes visible to other cpus. */

smp_mb__before_atomic() will order both read and write. However, the
comment suggest only the write are required to be ordered.

So either the barrier is too strong or the comment is incorrect. Can
you clarify it?

Neither is the case, I guess: There simply is no smp_wmb__before_atomic()
in Linux, and if we want to follow their model we shouldn't have one
either. I'd rather take the comment to indicate that if one appeared, it
could be used here.

Right. Currently we have the choice of either using
smp_mb__before_atomic() which is too strong for Arm, or smp_wmb() which
is too strong for x86.

For x86 smp_wmb() is actually only very slightly too strong - it expands
to just barrier(), after all. So overall perhaps that's the better
choice here (with a suitable comment)?

Fine with me.

I am happy with that.


Julien Grall



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