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 4.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

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-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From 6f542ebeaee0ee552a902ce3892220fc22c7ec8e Mon Sep 17 00:00:00 2001

From: Matija Glavinic Pecotic matija.glavinic-pecotic.ext@nokia.com Date: Thu, 3 Aug 2017 08:20:22 +0200 Subject: MIPS: Fix race on setting and getting cpu_online_mask

From: Matija Glavinic Pecotic matija.glavinic-pecotic.ext@nokia.com

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... 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.

Signed-off-by: Matija Glavinic Pecotic matija.glavinic-pecotic.ext@nokia.com Cc: Alexander Sverdlin alexander.sverdlin@nokia.com Cc: linux-mips@linux-mips.org Patchwork: https://patchwork.linux-mips.org/patch/16925/ Signed-off-by: Ralf Baechle ralf@linux-mips.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

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

--- a/arch/mips/kernel/smp.c +++ b/arch/mips/kernel/smp.c @@ -176,9 +176,6 @@ 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); @@ -186,6 +183,9 @@ 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 matija.glavinic-pecotic.ext@nokia.com are

queue-4.4/mips-fix-race-on-setting-and-getting-cpu_online_mask.patch queue-4.4/mips-smp-fix-deadlock-online-race.patch