4.9-stable review patch. If anyone has any objections, please let me know. ------------------ From: Matija Glavinic Pecotic <matija.glavinic-pecotic.ext@xxxxxxxxx> 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@xxxxxxxxx> Cc: Alexander Sverdlin <alexander.sverdlin@xxxxxxxxx> Cc: linux-mips@xxxxxxxxxxxxxx Patchwork: https://patchwork.linux-mips.org/patch/16925/ Signed-off-by: Ralf Baechle <ralf@xxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- 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.