[patch V4 15/37] cpu/hotplug: Rework sparse_irq locking in bringup_cpu()

[Thomas Gleixner <tglx@xxxxxxxxxxxxx>]

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

There is no harm to hold sparse_irq lock until the upcoming CPU completes
in cpuhp_online_idle(). This allows to remove cpu_online() synchronization
from architecture code.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
---
V4: Amend comment about sparse irq lock - Peter Z.
---
 kernel/cpu.c |   34 ++++++++++++++++++++++++----------
 1 file changed, 24 insertions(+), 10 deletions(-)

--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -558,7 +558,7 @@ static int cpuhp_kick_ap(int cpu, struct
        return ret;
 }
 
-static int bringup_wait_for_ap(unsigned int cpu)
+static int bringup_wait_for_ap_online(unsigned int cpu)
 {
        struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
 
@@ -579,15 +579,12 @@ static int bringup_wait_for_ap(unsigned
         */
        if (!cpu_smt_allowed(cpu))
                return -ECANCELED;
-
-       if (st->target <= CPUHP_AP_ONLINE_IDLE)
-               return 0;
-
-       return cpuhp_kick_ap(cpu, st, st->target);
+       return 0;
 }
 
 static int bringup_cpu(unsigned int cpu)
 {
+       struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
        struct task_struct *idle = idle_thread_get(cpu);
        int ret;
 
@@ -600,16 +597,33 @@ static int bringup_cpu(unsigned int cpu)
        /*
         * Some architectures have to walk the irq descriptors to
         * setup the vector space for the cpu which comes online.
-        * Prevent irq alloc/free across the bringup.
+        *
+        * Prevent irq alloc/free across the bringup by acquiring the
+        * sparse irq lock. Hold it until the upcoming CPU completes the
+        * startup in cpuhp_online_idle() which allows to avoid
+        * intermediate synchronization points in the architecture code.
         */
        irq_lock_sparse();
 
        /* Arch-specific enabling code. */
        ret = __cpu_up(cpu, idle);
-       irq_unlock_sparse();
        if (ret)
-               return ret;
-       return bringup_wait_for_ap(cpu);
+               goto out_unlock;
+
+       ret = bringup_wait_for_ap_online(cpu);
+       if (ret)
+               goto out_unlock;
+
+       irq_unlock_sparse();
+
+       if (st->target <= CPUHP_AP_ONLINE_IDLE)
+               return 0;
+
+       return cpuhp_kick_ap(cpu, st, st->target);
+
+out_unlock:
+       irq_unlock_sparse();
+       return ret;
 }
 
 static int finish_cpu(unsigned int cpu)