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Subject[PATCH 4.4 42/56] MIPS: Fix race on setting and getting cpu_online_mask
Date
4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

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
@@ -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.

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