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Subject[PATCH] leds: trigger: fix potential deadlock with libata
We have the following potential deadlock condition:

========================================================
WARNING: possible irq lock inversion dependency detected
5.10.0-rc2+ #25 Not tainted
--------------------------------------------------------
swapper/3/0 just changed the state of lock:
ffff8880063bd618 (&host->lock){-...}-{2:2}, at: ata_bmdma_interrupt+0x27/0x200
but this lock took another, HARDIRQ-READ-unsafe lock in the past:
(&trig->leddev_list_lock){.+.?}-{2:2}

and interrupts could create inverse lock ordering between them.

other info that might help us debug this:
Possible interrupt unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&trig->leddev_list_lock);
local_irq_disable();
lock(&host->lock);
lock(&trig->leddev_list_lock);
<Interrupt>
lock(&host->lock);

*** DEADLOCK ***

no locks held by swapper/3/0.

the shortest dependencies between 2nd lock and 1st lock:
-> (&trig->leddev_list_lock){.+.?}-{2:2} ops: 46 {
HARDIRQ-ON-R at:
lock_acquire+0x15f/0x420
_raw_read_lock+0x42/0x90
led_trigger_event+0x2b/0x70
rfkill_global_led_trigger_worker+0x94/0xb0
process_one_work+0x240/0x560
worker_thread+0x58/0x3d0
kthread+0x151/0x170
ret_from_fork+0x1f/0x30
IN-SOFTIRQ-R at:
lock_acquire+0x15f/0x420
_raw_read_lock+0x42/0x90
led_trigger_event+0x2b/0x70
kbd_bh+0x9e/0xc0
tasklet_action_common.constprop.0+0xe9/0x100
tasklet_action+0x22/0x30
__do_softirq+0xcc/0x46d
run_ksoftirqd+0x3f/0x70
smpboot_thread_fn+0x116/0x1f0
kthread+0x151/0x170
ret_from_fork+0x1f/0x30
SOFTIRQ-ON-R at:
lock_acquire+0x15f/0x420
_raw_read_lock+0x42/0x90
led_trigger_event+0x2b/0x70
rfkill_global_led_trigger_worker+0x94/0xb0
process_one_work+0x240/0x560
worker_thread+0x58/0x3d0
kthread+0x151/0x170
ret_from_fork+0x1f/0x30
INITIAL READ USE at:
lock_acquire+0x15f/0x420
_raw_read_lock+0x42/0x90
led_trigger_event+0x2b/0x70
rfkill_global_led_trigger_worker+0x94/0xb0
process_one_work+0x240/0x560
worker_thread+0x58/0x3d0
kthread+0x151/0x170
ret_from_fork+0x1f/0x30
}
... key at: [<ffffffff83da4c00>] __key.0+0x0/0x10
... acquired at:
_raw_read_lock+0x42/0x90
led_trigger_blink_oneshot+0x3b/0x90
ledtrig_disk_activity+0x3c/0xa0
ata_qc_complete+0x26/0x450
ata_do_link_abort+0xa3/0xe0
ata_port_freeze+0x2e/0x40
ata_hsm_qc_complete+0x94/0xa0
ata_sff_hsm_move+0x177/0x7a0
ata_sff_pio_task+0xc7/0x1b0
process_one_work+0x240/0x560
worker_thread+0x58/0x3d0
kthread+0x151/0x170
ret_from_fork+0x1f/0x30

-> (&host->lock){-...}-{2:2} ops: 69 {
IN-HARDIRQ-W at:
lock_acquire+0x15f/0x420
_raw_spin_lock_irqsave+0x52/0xa0
ata_bmdma_interrupt+0x27/0x200
__handle_irq_event_percpu+0xd5/0x2b0
handle_irq_event+0x57/0xb0
handle_edge_irq+0x8c/0x230
asm_call_irq_on_stack+0xf/0x20
common_interrupt+0x100/0x1c0
asm_common_interrupt+0x1e/0x40
native_safe_halt+0xe/0x10
arch_cpu_idle+0x15/0x20
default_idle_call+0x59/0x1c0
do_idle+0x22c/0x2c0
cpu_startup_entry+0x20/0x30
start_secondary+0x11d/0x150
secondary_startup_64_no_verify+0xa6/0xab
INITIAL USE at:
lock_acquire+0x15f/0x420
_raw_spin_lock_irqsave+0x52/0xa0
ata_dev_init+0x54/0xe0
ata_link_init+0x8b/0xd0
ata_port_alloc+0x1f1/0x210
ata_host_alloc+0xf1/0x130
ata_host_alloc_pinfo+0x14/0xb0
ata_pci_sff_prepare_host+0x41/0xa0
ata_pci_bmdma_prepare_host+0x14/0x30
piix_init_one+0x21f/0x600
local_pci_probe+0x48/0x80
pci_device_probe+0x105/0x1c0
really_probe+0x221/0x490
driver_probe_device+0xe9/0x160
device_driver_attach+0xb2/0xc0
__driver_attach+0x91/0x150
bus_for_each_dev+0x81/0xc0
driver_attach+0x1e/0x20
bus_add_driver+0x138/0x1f0
driver_register+0x91/0xf0
__pci_register_driver+0x73/0x80
piix_init+0x1e/0x2e
do_one_initcall+0x5f/0x2d0
kernel_init_freeable+0x26f/0x2cf
kernel_init+0xe/0x113
ret_from_fork+0x1f/0x30
}
... key at: [<ffffffff83d9fdc0>] __key.6+0x0/0x10
... acquired at:
__lock_acquire+0x9da/0x2370
lock_acquire+0x15f/0x420
_raw_spin_lock_irqsave+0x52/0xa0
ata_bmdma_interrupt+0x27/0x200
__handle_irq_event_percpu+0xd5/0x2b0
handle_irq_event+0x57/0xb0
handle_edge_irq+0x8c/0x230
asm_call_irq_on_stack+0xf/0x20
common_interrupt+0x100/0x1c0
asm_common_interrupt+0x1e/0x40
native_safe_halt+0xe/0x10
arch_cpu_idle+0x15/0x20
default_idle_call+0x59/0x1c0
do_idle+0x22c/0x2c0
cpu_startup_entry+0x20/0x30
start_secondary+0x11d/0x150
secondary_startup_64_no_verify+0xa6/0xab

This lockdep splat is reported after:
commit e918188611f0 ("locking: More accurate annotations for read_lock()")

To clarify:
- read-locks are recursive only in interrupt context (when
in_interrupt() returns true)
- after acquiring host->lock in CPU1, another cpu (i.e. CPU2) may call
write_lock(&trig->leddev_list_lock) that would be blocked by CPU0
that holds trig->leddev_list_lock in read-mode
- when CPU1 (ata_ac_complete()) tries to read-lock
trig->leddev_list_lock, it would be blocked by the write-lock waiter
on CPU2 (because we are not in interrupt context, so the read-lock is
not recursive)
- at this point if an interrupt happens on CPU0 and
ata_bmdma_interrupt() is executed it will try to acquire host->lock,
that is held by CPU1, that is currently blocked by CPU2, so:

* CPU0 blocked by CPU1
* CPU1 blocked by CPU2
* CPU2 blocked by CPU0

*** DEADLOCK ***

The deadlock scenario is better represented by the following schema
(thanks to Boqun Feng <boqun.feng@gmail.com> for the schema and the
detailed explanation of the deadlock condition):

CPU 0: CPU 1: CPU 2:
----- ----- -----
led_trigger_event():
read_lock(&trig->leddev_list_lock);
<workqueue>
ata_hsm_qc_complete():
spin_lock_irqsave(&host->lock);
write_lock(&trig->leddev_list_lock);
ata_port_freeze():
ata_do_link_abort():
ata_qc_complete():
ledtrig_disk_activity():
led_trigger_blink_oneshot():
read_lock(&trig->leddev_list_lock);
// ^ not in in_interrupt() context, so could get blocked by CPU 2
<interrupt>
ata_bmdma_interrupt():
spin_lock_irqsave(&host->lock);

Fix by using read_lock_irqsave/irqrestore() in led_trigger_event(), so
that no interrupt can happen in between, preventing the deadlock
condition.

Link: https://lore.kernel.org/lkml/20201101092614.GB3989@xps-13-7390/
Fixes: eb25cb9956cc ("leds: convert IDE trigger to common disk trigger")
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
---
drivers/leds/led-triggers.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/leds/led-triggers.c b/drivers/leds/led-triggers.c
index 91da90cfb11d..16d1a93a10a8 100644
--- a/drivers/leds/led-triggers.c
+++ b/drivers/leds/led-triggers.c
@@ -378,14 +378,15 @@ void led_trigger_event(struct led_trigger *trig,
enum led_brightness brightness)
{
struct led_classdev *led_cdev;
+ unsigned long flags;

if (!trig)
return;

- read_lock(&trig->leddev_list_lock);
+ read_lock_irqsave(&trig->leddev_list_lock, flags);
list_for_each_entry(led_cdev, &trig->led_cdevs, trig_list)
led_set_brightness(led_cdev, brightness);
- read_unlock(&trig->leddev_list_lock);
+ read_unlock_irqrestore(&trig->leddev_list_lock, flags);
}
EXPORT_SYMBOL_GPL(led_trigger_event);

--
2.27.0
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