| 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/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. |
| |
| 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 |
| @@ -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. |