[v4.9-rt PATCH v2] ARM: mm: remove tasklist locking from update_sections_early()

[Grygorii Strashko <grygorii.strashko@ti.com>]

The below backtrace can be observed on -rt kernel with CONFIG_DEBUG_RODATA
option enabled:

 BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:993
 in_atomic(): 1, irqs_disabled(): 128, pid: 14, name: migration/0
 1 lock held by migration/0/14:
  #0:  (tasklist_lock){+.+...}, at: [<c01183e8>] update_sections_early+0x24/0xdc
 irq event stamp: 38
 hardirqs last  enabled at (37): [<c08f6f7c>] _raw_spin_unlock_irq+0x24/0x68
 hardirqs last disabled at (38): [<c01fdfe8>] multi_cpu_stop+0xd8/0x138
 softirqs last  enabled at (0): [<c01303ec>] copy_process.part.5+0x238/0x1b64
 softirqs last disabled at (0): [<  (null)>]   (null)
 Preemption disabled at: [<c01fe244>] cpu_stopper_thread+0x80/0x10c
 CPU: 0 PID: 14 Comm: migration/0 Not tainted 4.9.21-rt16-02220-g49e319c #15
 Hardware name: Generic DRA74X (Flattened Device Tree)
 [<c0112014>] (unwind_backtrace) from [<c010d370>] (show_stack+0x10/0x14)
 [<c010d370>] (show_stack) from [<c049beb8>] (dump_stack+0xa8/0xd4)
 [<c049beb8>] (dump_stack) from [<c01631a0>] (___might_sleep+0x1bc/0x2ac)
 [<c01631a0>] (___might_sleep) from [<c08f7244>] (__rt_spin_lock+0x1c/0x30)
 [<c08f7244>] (__rt_spin_lock) from [<c08f77a4>] (rt_read_lock+0x54/0x68)
 [<c08f77a4>] (rt_read_lock) from [<c01183e8>] (update_sections_early+0x24/0xdc)
 [<c01183e8>] (update_sections_early) from [<c01184b0>] (__fix_kernmem_perms+0x10/0x1c)
 [<c01184b0>] (__fix_kernmem_perms) from [<c01fe010>] (multi_cpu_stop+0x100/0x138)
 [<c01fe010>] (multi_cpu_stop) from [<c01fe24c>] (cpu_stopper_thread+0x88/0x10c)
 [<c01fe24c>] (cpu_stopper_thread) from [<c015edc4>] (smpboot_thread_fn+0x174/0x31c)
 [<c015edc4>] (smpboot_thread_fn) from [<c015a988>] (kthread+0xf0/0x108)
 [<c015a988>] (kthread) from [<c0108818>] (ret_from_fork+0x14/0x3c)
 Freeing unused kernel memory: 1024K (c0d00000 - c0e00000)

The stop_machine() is called with cpus = NULL from fix_kernmem_perms() and
mark_rodata_ro() which means only one CPU will execute
update_sections_early() while all other CPUs will spin and wait. Hence,
it's safe to remove tasklist locking from update_sections_early(). As part
of this change also mark functions which are local to this module as
static.

Cc: Kees Cook <keescook@chromium.org>
Cc: Laura Abbott <labbott@redhat.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
---
As I've checked it also can be applied to LKML as is.

Changes in v2:
 - added comment to update_sections_early()

v1: https://patchwork.kernel.org/patch/9686289/

 arch/arm/mm/init.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index 370581a..838f6b35 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -689,34 +689,37 @@ void set_section_perms(struct section_perm *perms, int n, bool set,
 
 }
 
+/**
+ * update_sections_early intended to be called only through stop_machine
+ * framework and executed by only one CPU while all other CPUs will spin and
+ * wait, so no locking is required in this function.
+ */
 static void update_sections_early(struct section_perm perms[], int n)
 {
 	struct task_struct *t, *s;
 
-	read_lock(&tasklist_lock);
 	for_each_process(t) {
 		if (t->flags & PF_KTHREAD)
 			continue;
 		for_each_thread(t, s)
 			set_section_perms(perms, n, true, s->mm);
 	}
-	read_unlock(&tasklist_lock);
 	set_section_perms(perms, n, true, current->active_mm);
 	set_section_perms(perms, n, true, &init_mm);
 }
 
-int __fix_kernmem_perms(void *unused)
+static int __fix_kernmem_perms(void *unused)
 {
 	update_sections_early(nx_perms, ARRAY_SIZE(nx_perms));
 	return 0;
 }
 
-void fix_kernmem_perms(void)
+static void fix_kernmem_perms(void)
 {
 	stop_machine(__fix_kernmem_perms, NULL, NULL);
 }
 
-int __mark_rodata_ro(void *unused)
+static int __mark_rodata_ro(void *unused)
 {
 	update_sections_early(ro_perms, ARRAY_SIZE(ro_perms));
 	return 0;
-- 
2.10.1