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Re: [Xen-devel] [PATCH v4 2/6] xen/rcu: don't use stop_machine_run() for rcu_barrier()



On 10.03.20 17:37, Jan Beulich wrote:
On 10.03.2020 17:34, Jürgen Groß wrote:
On 10.03.20 17:29, Jan Beulich wrote:
On 10.03.2020 08:28, Juergen Gross wrote:
+void rcu_barrier(void)
   {
-    atomic_t cpu_count = ATOMIC_INIT(0);
-    return stop_machine_run(rcu_barrier_action, &cpu_count, NR_CPUS);
+    unsigned int n_cpus;
+
+    while ( !get_cpu_maps() )
+    {
+        process_pending_softirqs();
+        if ( !atomic_read(&cpu_count) )
+            return;
+
+        cpu_relax();
+    }
+
+    n_cpus = num_online_cpus();
+
+    if ( atomic_cmpxchg(&cpu_count, 0, n_cpus) == 0 )
+    {
+        atomic_add(n_cpus, &done_count);
+        cpumask_raise_softirq(&cpu_online_map, RCU_SOFTIRQ);
+    }
+
+    while ( atomic_read(&done_count) )

Don't you leave a window for races here, in that done_count
gets set to non-zero only after setting cpu_count? A CPU
losing the cmpxchg attempt above may observe done_count
still being zero, and hence exit without waiting for the
count to actually _drop_ to zero.

This can only be a cpu not having joined the barrier handling, so it
will do that later.

I'm afraid I don't understand - if two CPUs independently call
rcu_barrier(), neither should fall through here without waiting
at all, I would think?

Oh, good catch!

I have thought more about this problem and I think using counters only
for doing rendezvous accounting is rather risky. I'll have a try using
a cpumask instead.


Juergen

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