On 10.03.20 17:29, Jan Beulich wrote:
On 10.03.2020 08:28, Juergen Gross wrote:
@@ -143,51 +143,75 @@ 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:
+ * cpu_count holds the number of cpu required to finish barrier handling.
+ * Cpus are synchronized via softirq mechanism. rcu_barrier() is regarded to
+ * be active if cpu_count is not zero. In case rcu_barrier() is called on
+ * multiple cpus it is enough to check for cpu_count being not zero on entry
+ * and to call process_pending_softirqs() in a loop until cpu_count drops to
+ * zero, as syncing has been requested already and we don't need to sync
+ * multiple times.
+ * In order to avoid hangs when rcu_barrier() is called mutiple times on the
+ * same cpu in fast sequence and a slave cpu couldn't drop out of the
+ * barrier handling fast enough a second counter done_count is needed.
+ */
+static atomic_t cpu_count = ATOMIC_INIT(0);
+static atomic_t done_count = ATOMIC_INIT(0);
From its use below this looks more like "todo_count" or
"pending_count".
+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.
Juergen
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