From 46c498c2cdee5efe44f617bcd4f388179be36115 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Tue, 26 Feb 2013 18:44:33 +0100 Subject: stop_machine: Mark per cpu stopper enabled early commit 14e568e78 (stop_machine: Use smpboot threads) introduced the following regression: Before this commit the stopper enabled bit was set in the online notifier. CPU0 CPU1 cpu_up cpu online hotplug_notifier(ONLINE) stopper(CPU1)->enabled = true; ... stop_machine() The conversion to smpboot threads moved the enablement to the wakeup path of the parked thread. The majority of users seem to have the following working order: CPU0 CPU1 cpu_up cpu online unpark_threads() wakeup(stopper[CPU1]) .... stopper thread runs stopper(CPU1)->enabled = true; stop_machine() But Konrad and Sander have observed: CPU0 CPU1 cpu_up cpu online unpark_threads() wakeup(stopper[CPU1]) .... stop_machine() stopper thread runs stopper(CPU1)->enabled = true; Now the stop machinery kicks CPU0 into the stop loop, where it gets stuck forever because the queue code saw stopper(CPU1)->enabled == false, so CPU0 waits for CPU1 to enter stomp_machine, but the CPU1 stopper work got discarded due to enabled == false. Add a pre_unpark function to the smpboot thread descriptor and call it before waking the thread. This fixes the problem at hand, but the stop_machine code should be more robust. The stopper->enabled flag smells fishy at best. Thanks to Konrad for going through a loop of debug patches and providing the information to decode this issue. Reported-and-tested-by: Konrad Rzeszutek Wilk Reported-and-tested-by: Sander Eikelenboom Cc: Srivatsa S. Bhat Cc: Rusty Russell Link: http://lkml.kernel.org/r/alpine.LFD.2.02.1302261843240.22263@ionos Signed-off-by: Thomas Gleixner --- kernel/smpboot.c | 2 ++ 1 file changed, 2 insertions(+) (limited to 'kernel/smpboot.c') diff --git a/kernel/smpboot.c b/kernel/smpboot.c index d4abac26177..8eaed9aa9cf 100644 --- a/kernel/smpboot.c +++ b/kernel/smpboot.c @@ -209,6 +209,8 @@ static void smpboot_unpark_thread(struct smp_hotplug_thread *ht, unsigned int cp { struct task_struct *tsk = *per_cpu_ptr(ht->store, cpu); + if (ht->pre_unpark) + ht->pre_unpark(cpu); kthread_unpark(tsk); } -- cgit v1.2.3-70-g09d2