ARM: CPU hotplug: Delegate complete() to surviving CPU
The ARM implementation of arch_cpu_idle_dead() invokes complete(), but
does so after RCU has stopped watching the outgoing CPU, which results
in lockdep complaints because complete() invokes functions containing RCU
readers. In addition, if the outgoing CPU really were to consume several
seconds of its five-second allotted time, multiple RCU updates could
complete, possibly giving the outgoing CPU an inconsistent view of the
scheduler data structures on which complete() relies.
This (untested, probably does not build) commit avoids this problem by
polling the outgoing CPU. The polling strategy in this prototype patch
is quite naive, with one jiffy between each poll and without any sort
of adaptive spin phase. The key point is that the polling CPU uses
atomic_dec_and_test(), which evicts the flag from the outgoing CPU's
cache. The outgoing CPU simply does an atomic_set() of the value 1 which
causes the next atomic_dec_and_test() to return true, and which also
minimizes opportunities for other data to get pulled into the outgoing
CPU's cache. This pulling of values from the outgoing CPU's cache is
important because the outgoing CPU might be unceremoniously powered off
before it has time to execute any code after the atomic_set().
Underflow is avoided because there can be at most 5,000 invocations of
atomic_dec_and_test() for a given offline operation, and the counter is
set back to zero each time.
Reported-by: Peng Fan <van.freenix@gmail.com>
Reported-by: Russell King - ARM Linux <linux@armlinux.org.uk>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: <linux-arm-kernel@lists.infradead.org>
diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index b4fbf00..f9a4990 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -241,7 +241,7 @@ int __cpu_disable(void)
return 0;
}
-static DECLARE_COMPLETION(cpu_died);
+static atomic_t cpu_died;
/*
* called on the thread which is asking for a CPU to be shutdown -
@@ -249,7 +249,17 @@ static DECLARE_COMPLETION(cpu_died);
*/
void __cpu_die(unsigned int cpu)
{
- if (!wait_for_completion_timeout(&cpu_died, msecs_to_jiffies(5000))) {
+ unsigned long deadline = jiffies + msecs_to_jiffies(5000);
+ char ret = 0;
+
+ while (time_before(jiffies, deadline)) {
+ ret = atomic_dec_and_test(&cpu_died);
+ if (ret)
+ break;
+ schedule_timeout_interruptible(1);
+ }
+ atomic_set(&cpu_died, 0);
+ if (!ret) {
pr_err("CPU%u: cpu didn't die\n", cpu);
return;
}
@@ -295,7 +305,7 @@ void arch_cpu_idle_dead(void)
* this returns, power and/or clocks can be removed at any point
* from this CPU and its cache by platform_cpu_kill().
*/
- complete(&cpu_died);
+ atomic_set(&cpu_died, 1);
/*
* Ensure that the cache lines associated with that completion are
