Patch "MIPS: Fix race on setting and getting cpu_online_mask" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

MIPS: Fix race on setting and getting cpu_online_mask

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mips-fix-race-on-setting-and-getting-cpu_online_mask.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Matija Glavinic Pecotic <[email protected]>
Date: Thu, 3 Aug 2017 08:20:22 +0200
Subject: MIPS: Fix race on setting and getting cpu_online_mask

From: Matija Glavinic Pecotic <[email protected]>

commit 6f542ebeaee0ee552a902ce3892220fc22c7ec8e upstream.

While testing cpu hoptlug (cpu down and up in loops) on kernel 4.4, it was observed that occasionally check for cpu online will fail in kernel/cpu.c, _cpu_up:

https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/tree/kernel/cpu.c?h=v4.4.79#n485
518 /* Arch-specific enabling code. */
519 ret = __cpu_up(cpu, idle);
520
521 if (ret != 0)
522 goto out_notify;
523 BUG_ON(!cpu_online(cpu));

Reason is race between start_secondary and _cpu_up. cpu_callin_map is set before cpu_online_mask. In __cpu_up, cpu_callin_map is waited for, but cpu online mask is not, resulting in race in which secondary processor started and set cpu_callin_map, but not yet set the online mask,resulting in above BUG being hit.

Upstream differs in the area. cpu_online check is in bringup_wait_for_ap, which is after cpu reached AP_ONLINE_IDLE,where secondary passed its start function. Nonetheless, fix makes start_secondary safe and not depending on other locks throughout the code. It protects as well against cpu_online checks put in between sometimes in the future.

Fix this by moving completion after all flags are set.

Patchwork: https://patchwork.linux-mips.org/patch/16925/

---
 arch/mips/kernel/smp.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

@@ -371,9 +371,6 @@ asmlinkage void start_secondary(void)
 	cpumask_set_cpu(cpu, &cpu_coherent_mask);
 	notify_cpu_starting(cpu);
 
-	complete(&cpu_running);
-	synchronise_count_slave(cpu);
-
 	set_cpu_online(cpu, true);
 
 	set_cpu_sibling_map(cpu);

@@ -381,6 +378,9 @@ asmlinkage void start_secondary(void)
 
 	calculate_cpu_foreign_map();
 
+	complete(&cpu_running);
+	synchronise_count_slave(cpu);
+
 	/*
 	 * irq will be enabled in ->smp_finish(), enabling it too early
 	 * is dangerous.

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/mips-fix-race-on-setting-and-getting-cpu_online_mask.patch queue-4.9/mips-smp-fix-deadlock-online-race.patch