WARNING: suspicious RCU usage

[Paul E. McKenney]

On Fri, Dec 15, 2017 at 06:36:49PM -0200, Fabio Estevam wrote:
> Hi Paul,
> 
> On Fri, Dec 15, 2017 at 4:23 PM, Paul E. McKenney
> <paulmck at linux.vnet.ibm.com> wrote:
> 
> > How about this one, also untested etc.?
> 
> This one gives a build warning:
> 
> arch/arm/kernel/smp.c: In function ‘__cpu_die’:
> arch/arm/kernel/smp.c:262:5: warning: ‘ret’ may be used uninitialized
> in this function [-Wmaybe-uninitialized]
>   if (!ret) {
>      ^

That would be me being stupid.  Please see below for an updated patch.

> but when I run suspend/resume I don't see the RCU warning with this
> patch applied.

So some progress, at least!  Thank you for your testing efforts!!!

							Thanx, Paul

------------------------------------------------------------------------

commit e1bb1ddc3402220eba1138322fedd520801ee510
Author: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
Date:   Mon Dec 11 09:40:58 2017 -0800

    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 at gmail.com>
    Reported-by: Russell King - ARM Linux <linux at armlinux.org.uk>
    Signed-off-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
    Cc: Russell King <linux at armlinux.org.uk>
    Cc: Ingo Molnar <mingo at kernel.org>
    Cc: "Peter Zijlstra (Intel)" <peterz at infradead.org>
    Cc: Michal Hocko <mhocko at suse.com>
    Cc: Thomas Gleixner <tglx at linutronix.de>
    Cc: Fabio Estevam <fabio.estevam at nxp.com>
    Cc: <linux-arm-kernel at lists.infradead.org>

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index b4fbf00ee4ad..f9a4990689f3 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