relayfs: Convert to hotplug state machine

Install the callbacks via the state machine. They are installed at run time but
relay_prepare_cpu() does not need to be invoked by the boot CPU because
relay_open() was not yet invoked and there are no pools that need to be created.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: rt@linutronix.de
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20160818125731.27256-3-bigeasy@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

4 files changed, 26 insertions, 45 deletions

diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index 9e6d10786e29..4c79f40fcebc 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -21,6 +21,7 @@ enum cpuhp_state {
         CPUHP_PROFILE_PREPARE,
         CPUHP_X2APIC_PREPARE,
         CPUHP_SMPCFD_PREPARE,
+        CPUHP_RELAY_PREPARE,
         CPUHP_RCUTREE_PREP,
         CPUHP_NOTIFY_PREPARE,
         CPUHP_TIMERS_DEAD,
diff --git a/include/linux/relay.h b/include/linux/relay.h
index eb295e373b90..ecbb34a382b8 100644
--- a/include/linux/relay.h
+++ b/include/linux/relay.h
@@ -288,5 +288,11 @@ static inline void subbuf_start_reserve(struct rchan_buf *buf,
  */
 extern const struct file_operations relay_file_operations;
 
+#ifdef CONFIG_RELAY
+int relay_prepare_cpu(unsigned int cpu);
+#else
+#define relay_prepare_cpu     NULL
+#endif
+
 #endif /* _LINUX_RELAY_H */
 
diff --git a/kernel/cpu.c b/kernel/cpu.c
index d14ae4438e8e..0c0d4b2ddd1c 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -23,6 +23,7 @@
 #include <linux/tick.h>
 #include <linux/irq.h>
 #include <linux/smpboot.h>
+#include <linux/relay.h>
 
 #include <trace/events/power.h>
 #define CREATE_TRACE_POINTS
@@ -1272,6 +1273,11 @@ static struct cpuhp_step cpuhp_bp_states[] = {
                 .startup.single         = smpcfd_prepare_cpu,
                 .teardown.single        = smpcfd_dead_cpu,
         },
+        [CPUHP_RELAY_PREPARE] = {
+                .name                   = "relay:prepare",
+                .startup.single         = relay_prepare_cpu,
+                .teardown.single        = NULL,
+        },
         [CPUHP_RCUTREE_PREP] = {
                 .name                   = "RCU/tree:prepare",
                 .startup.single         = rcutree_prepare_cpu,
diff --git a/kernel/relay.c b/kernel/relay.c
index ed157378f6cb..fc9b4a4af463 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -513,48 +513,25 @@ static void setup_callbacks(struct rchan *chan,
         chan->cb = cb;
 }
 
-/**
- *      relay_hotcpu_callback - CPU hotplug callback
- *      @nb: notifier block
- *      @action: hotplug action to take
- *      @hcpu: CPU number
- *
- *      Returns the success/failure of the operation. (%NOTIFY_OK, %NOTIFY_BAD)
- */
-static int relay_hotcpu_callback(struct notifier_block *nb,
-                                unsigned long action,
-                                void *hcpu)
+int relay_prepare_cpu(unsigned int cpu)
 {
-        unsigned int hotcpu = (unsigned long)hcpu;
         struct rchan *chan;
         struct rchan_buf *buf;
 
-        switch(action) {
-        case CPU_UP_PREPARE:
-        case CPU_UP_PREPARE_FROZEN:
-                mutex_lock(&relay_channels_mutex);
-                list_for_each_entry(chan, &relay_channels, list) {
-                        if ((buf = *per_cpu_ptr(chan->buf, hotcpu)))
-                                continue;
-                        buf = relay_open_buf(chan, hotcpu);
-                        if (!buf) {
-                                printk(KERN_ERR
-                                        "relay_hotcpu_callback: cpu %d buffer "
-                                        "creation failed\n", hotcpu);
-                                mutex_unlock(&relay_channels_mutex);
-                                return notifier_from_errno(-ENOMEM);
-                        }
-                        *per_cpu_ptr(chan->buf, hotcpu) = buf;
+        mutex_lock(&relay_channels_mutex);
+        list_for_each_entry(chan, &relay_channels, list) {
+                if ((buf = *per_cpu_ptr(chan->buf, cpu)))
+                        continue;
+                buf = relay_open_buf(chan, cpu);
+                if (!buf) {
+                        pr_err("relay: cpu %d buffer creation failed\n", cpu);
+                        mutex_unlock(&relay_channels_mutex);
+                        return -ENOMEM;
                 }
-                mutex_unlock(&relay_channels_mutex);
-                break;
-        case CPU_DEAD:
-        case CPU_DEAD_FROZEN:
-                /* No need to flush the cpu : will be flushed upon
-                 * final relay_flush() call. */
-                break;
+                *per_cpu_ptr(chan->buf, cpu) = buf;
         }
-        return NOTIFY_OK;
+        mutex_unlock(&relay_channels_mutex);
+        return 0;
 }
 
 /**
@@ -1387,12 +1364,3 @@ const struct file_operations relay_file_operations = {
         .splice_read    = relay_file_splice_read,
 };
 EXPORT_SYMBOL_GPL(relay_file_operations);
-
-static __init int relay_init(void)
-{
-
-        hotcpu_notifier(relay_hotcpu_callback, 0);
-        return 0;
-}
-
-early_initcall(relay_init);