On Tue, Mar 12, 2019 at 12:16:34PM +0900, Yoshihiro Shimoda wrote:
> From: Phong Hoang <phong.hoang.wz@xxxxxxxxxxx>
>
> This patch fixes deadlock warning if removing PWM device
> when CONFIG_PROVE_LOCKING is enabled.
>
> This issue can be reproceduced by the following steps on
> the R-Car H3 Salvator-X board if the backlight is disabled:
>
> # cd /sys/class/pwm/pwmchip0
> # echo 0 > export
> # ls
> device export npwm power pwm0 subsystem uevent unexport
> # cd device/driver
> # ls
> bind e6e31000.pwm uevent unbind
> # echo e6e31000.pwm > unbind
>
> [ 87.659974] ======================================================
> [ 87.666149] WARNING: possible circular locking dependency detected
> [ 87.672327] 5.0.0 #7 Not tainted
> [ 87.675549] ------------------------------------------------------
> [ 87.681723] bash/2986 is trying to acquire lock:
> [ 87.686337] 000000005ea0e178 (kn->count#58){++++}, at: kernfs_remove_by_name_ns+0x50/0xa0
> [ 87.694528]
> [ 87.694528] but task is already holding lock:
> [ 87.700353] 000000006313b17c (pwm_lock){+.+.}, at: pwmchip_remove+0x28/0x13c
> [ 87.707405]
> [ 87.707405] which lock already depends on the new lock.
> [ 87.707405]
> [ 87.715574]
> [ 87.715574] the existing dependency chain (in reverse order) is:
> [ 87.723048]
> [ 87.723048] -> #1 (pwm_lock){+.+.}:
> [ 87.728017] __mutex_lock+0x70/0x7e4
> [ 87.732108] mutex_lock_nested+0x1c/0x24
> [ 87.736547] pwm_request_from_chip.part.6+0x34/0x74
> [ 87.741940] pwm_request_from_chip+0x20/0x40
> [ 87.746725] export_store+0x6c/0x1f4
> [ 87.750820] dev_attr_store+0x18/0x28
> [ 87.754998] sysfs_kf_write+0x54/0x64
> [ 87.759175] kernfs_fop_write+0xe4/0x1e8
> [ 87.763615] __vfs_write+0x40/0x184
> [ 87.767619] vfs_write+0xa8/0x19c
> [ 87.771448] ksys_write+0x58/0xbc
> [ 87.775278] __arm64_sys_write+0x18/0x20
> [ 87.779721] el0_svc_common+0xd0/0x124
> [ 87.783986] el0_svc_compat_handler+0x1c/0x24
> [ 87.788858] el0_svc_compat+0x8/0x18
> [ 87.792947]
> [ 87.792947] -> #0 (kn->count#58){++++}:
> [ 87.798260] lock_acquire+0xc4/0x22c
> [ 87.802353] __kernfs_remove+0x258/0x2c4
> [ 87.806790] kernfs_remove_by_name_ns+0x50/0xa0
> [ 87.811836] remove_files.isra.1+0x38/0x78
> [ 87.816447] sysfs_remove_group+0x48/0x98
> [ 87.820971] sysfs_remove_groups+0x34/0x4c
> [ 87.825583] device_remove_attrs+0x6c/0x7c
> [ 87.830197] device_del+0x11c/0x33c
> [ 87.834201] device_unregister+0x14/0x2c
> [ 87.838638] pwmchip_sysfs_unexport+0x40/0x4c
> [ 87.843509] pwmchip_remove+0xf4/0x13c
> [ 87.847773] rcar_pwm_remove+0x28/0x34
> [ 87.852039] platform_drv_remove+0x24/0x64
> [ 87.856651] device_release_driver_internal+0x18c/0x21c
> [ 87.862391] device_release_driver+0x14/0x1c
> [ 87.867175] unbind_store+0xe0/0x124
> [ 87.871265] drv_attr_store+0x20/0x30
> [ 87.875442] sysfs_kf_write+0x54/0x64
> [ 87.879618] kernfs_fop_write+0xe4/0x1e8
> [ 87.884055] __vfs_write+0x40/0x184
> [ 87.888057] vfs_write+0xa8/0x19c
> [ 87.891887] ksys_write+0x58/0xbc
> [ 87.895716] __arm64_sys_write+0x18/0x20
> [ 87.900154] el0_svc_common+0xd0/0x124
> [ 87.904417] el0_svc_compat_handler+0x1c/0x24
> [ 87.909289] el0_svc_compat+0x8/0x18
I'm not sure I understand this correctly. The above looks like pwm_lock
is held as part of the syscall writing 0 to the export attribute and the
second callchain looks like it's originating from the write to the
unbind attribute for the driver. But pwm_request_from_chip() should have
already released the lock before it returned, so how can the above
situation even happen?
Thierry
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