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Subject[v4.9-rt PATCH] ARM: mm: remove tasklist locking from update_sections_early()
Date
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
to simplify code analize in the future.

Cc: Kees Cook <keescook@chromium.org>
Cc: Laura Abbott <labbott@redhat.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
---
arch/arm/mm/init.c | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index 370581a..a77953a 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -693,30 +693,28 @@ 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
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