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Date:   Wed, 19 Dec 2018 08:01:10 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Daniel Vetter <daniel.vetter@ffwll.ch>
Cc:     DRI Development <dri-devel@lists.freedesktop.org>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Ramalingam C <ramalingam.c@intel.com>,
        Arend van Spriel <aspriel@gmail.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Bartosz Golaszewski <brgl@bgdev.pl>,
        Heikki Krogerus <heikki.krogerus@linux.intel.com>,
        Vivek Gautam <vivek.gautam@codeaurora.org>,
        Joe Perches <joe@perches.com>,
        Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [PATCH] sysfs: Disable lockdep for driver bind/unbind files
Message-ID: <20181219070110.GB21256@kroah.com>
References: <20181218201443.4950-1-daniel.vetter@ffwll.ch>
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On Tue, Dec 18, 2018 at 09:14:43PM +0100, Daniel Vetter wrote:
> This is the much more correct fix for my earlier attempt at:
> 
> https://lkml.org/lkml/2018/12/10/118
> 
> Short recap:
> 
> - There's not actually a locking issue, it's just lockdep being a bit
>   too eager to complain about a possible deadlock.
> 
> - Contrary to what I claimed the real problem is recursion on
>   kn->count. Greg pointed me at sysfs_break_active_protection(), used
>   by the scsi subsystem to allow a sysfs file to unbind itself. That
>   would be a real deadlock, which isn't what's happening here. Also,
>   breaking the active protection means we'd need to manually handle
>   all the lifetime fun.
> 
> - With Rafael we discussed the task_work approach, which kinda works,
>   but has two downsides: It's a functional change for a lockdep
>   annotation issue, and it won't work for the bind file (which needs
>   to get the errno from the driver load function back to userspace).
> 
> - Greg also asked why this never showed up: To hit this you need to
>   unregister a 2nd driver from the unload code of your first driver. I
>   guess only gpus do that. The bug has always been there, but only
>   with a recent patch series did we add more locks so that lockdep
>   built a chain from unbinding the snd-hda driver to the
>   acpi_video_unregister call.
> 
> Full lockdep splat:
> 
> [12301.898799] ============================================
> [12301.898805] WARNING: possible recursive locking detected
> [12301.898811] 4.20.0-rc7+ #84 Not tainted
> [12301.898815] --------------------------------------------
> [12301.898821] bash/5297 is trying to acquire lock:
> [12301.898826] 00000000f61c6093 (kn->count#39){++++}, at: kernfs_remove_by_name_ns+0x3b/0x80
> [12301.898841] but task is already holding lock:
> [12301.898847] 000000005f634021 (kn->count#39){++++}, at: kernfs_fop_write+0xdc/0x190
> [12301.898856] other info that might help us debug this:
> [12301.898862]  Possible unsafe locking scenario:
> [12301.898867]        CPU0
> [12301.898870]        ----
> [12301.898874]   lock(kn->count#39);
> [12301.898879]   lock(kn->count#39);
> [12301.898883] *** DEADLOCK ***
> [12301.898891]  May be due to missing lock nesting notation
> [12301.898899] 5 locks held by bash/5297:
> [12301.898903]  #0: 00000000cd800e54 (sb_writers#4){.+.+}, at: vfs_write+0x17f/0x1b0
> [12301.898915]  #1: 000000000465e7c2 (&of->mutex){+.+.}, at: kernfs_fop_write+0xd3/0x190
> [12301.898925]  #2: 000000005f634021 (kn->count#39){++++}, at: kernfs_fop_write+0xdc/0x190
> [12301.898936]  #3: 00000000414ef7ac (&dev->mutex){....}, at: device_release_driver_internal+0x34/0x240
> [12301.898950]  #4: 000000003218fbdf (register_count_mutex){+.+.}, at: acpi_video_unregister+0xe/0x40
> [12301.898960] stack backtrace:
> [12301.898968] CPU: 1 PID: 5297 Comm: bash Not tainted 4.20.0-rc7+ #84
> [12301.898974] Hardware name: Hewlett-Packard HP EliteBook 8460p/161C, BIOS 68SCF Ver. F.01 03/11/2011
> [12301.898982] Call Trace:
> [12301.898989]  dump_stack+0x67/0x9b
> [12301.898997]  __lock_acquire+0x6ad/0x1410
> [12301.899003]  ? kernfs_remove_by_name_ns+0x3b/0x80
> [12301.899010]  ? find_held_lock+0x2d/0x90
> [12301.899017]  ? mutex_spin_on_owner+0xe4/0x150
> [12301.899023]  ? find_held_lock+0x2d/0x90
> [12301.899030]  ? lock_acquire+0x90/0x180
> [12301.899036]  lock_acquire+0x90/0x180
> [12301.899042]  ? kernfs_remove_by_name_ns+0x3b/0x80
> [12301.899049]  __kernfs_remove+0x296/0x310
> [12301.899055]  ? kernfs_remove_by_name_ns+0x3b/0x80
> [12301.899060]  ? kernfs_name_hash+0xd/0x80
> [12301.899066]  ? kernfs_find_ns+0x6c/0x100
> [12301.899073]  kernfs_remove_by_name_ns+0x3b/0x80
> [12301.899080]  bus_remove_driver+0x92/0xa0
> [12301.899085]  acpi_video_unregister+0x24/0x40
> [12301.899127]  i915_driver_unload+0x42/0x130 [i915]
> [12301.899160]  i915_pci_remove+0x19/0x30 [i915]
> [12301.899169]  pci_device_remove+0x36/0xb0
> [12301.899176]  device_release_driver_internal+0x185/0x240
> [12301.899183]  unbind_store+0xaf/0x180
> [12301.899189]  kernfs_fop_write+0x104/0x190
> [12301.899195]  __vfs_write+0x31/0x180
> [12301.899203]  ? rcu_read_lock_sched_held+0x6f/0x80
> [12301.899209]  ? rcu_sync_lockdep_assert+0x29/0x50
> [12301.899216]  ? __sb_start_write+0x13c/0x1a0
> [12301.899221]  ? vfs_write+0x17f/0x1b0
> [12301.899227]  vfs_write+0xb9/0x1b0
> [12301.899233]  ksys_write+0x50/0xc0
> [12301.899239]  do_syscall_64+0x4b/0x180
> [12301.899247]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> [12301.899253] RIP: 0033:0x7f452ac7f7a4
> [12301.899259] Code: 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 80 00 00 00 00 8b 05 aa f0 2c 00 48 63 ff 85 c0 75 13 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 54 f3 c3 66 90 55 53 48 89 d5 48 89 f3 48 83
> [12301.899273] RSP: 002b:00007ffceafa6918 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> [12301.899282] RAX: ffffffffffffffda RBX: 000000000000000d RCX: 00007f452ac7f7a4
> [12301.899288] RDX: 000000000000000d RSI: 00005612a1abf7c0 RDI: 0000000000000001
> [12301.899295] RBP: 00005612a1abf7c0 R08: 000000000000000a R09: 00005612a1c46730
> [12301.899301] R10: 000000000000000a R11: 0000000000000246 R12: 000000000000000d
> [12301.899308] R13: 0000000000000001 R14: 00007f452af4a740 R15: 000000000000000d
> 
> Locking around I've noticed that usb and i2c already handle similar
> recursion problems, where a sysfs file can unbind the same type of
> sysfs somewhere else in the hierarchy. Relevant commits are:
> 
> commit 356c05d58af05d582e634b54b40050c73609617b
> Author: Alan Stern <stern@rowland.harvard.edu>
> Date:   Mon May 14 13:30:03 2012 -0400
> 
>     sysfs: get rid of some lockdep false positives
> 
> commit e9b526fe704812364bca07edd15eadeba163ebfb
> Author: Alexander Sverdlin <alexander.sverdlin@nsn.com>
> Date:   Fri May 17 14:56:35 2013 +0200
> 
>     i2c: suppress lockdep warning on delete_device
> 
> Implement the same trick for driver bind/unbind.
> 
> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Ramalingam C <ramalingam.c@intel.com>
> Cc: Arend van Spriel <aspriel@gmail.com>
> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Cc: Geert Uytterhoeven <geert+renesas@glider.be>
> Cc: Bartosz Golaszewski <brgl@bgdev.pl>
> Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> Cc: Vivek Gautam <vivek.gautam@codeaurora.org>
> Cc: Joe Perches <joe@perches.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> ---
>  drivers/base/bus.c     | 4 ++--
>  include/linux/device.h | 4 ++++
>  2 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/base/bus.c b/drivers/base/bus.c
> index 8bfd27ec73d6..5d2411b848cd 100644
> --- a/drivers/base/bus.c
> +++ b/drivers/base/bus.c
> @@ -195,7 +195,7 @@ static ssize_t unbind_store(struct device_driver *drv, const char *buf,
>  	bus_put(bus);
>  	return err;
>  }
> -static DRIVER_ATTR_WO(unbind);
> +static DRIVER_ATTR_IGNORE_LOCKDEP(unbind, S_IWUSR, NULL, unbind_store);
>  
>  /*
>   * Manually attach a device to a driver.
> @@ -231,7 +231,7 @@ static ssize_t bind_store(struct device_driver *drv, const char *buf,
>  	bus_put(bus);
>  	return err;
>  }
> -static DRIVER_ATTR_WO(bind);
> +static DRIVER_ATTR_IGNORE_LOCKDEP(bind, S_IWUSR, NULL, bind_store);
>  
>  static ssize_t show_drivers_autoprobe(struct bus_type *bus, char *buf)
>  {
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 1b25c7a43f4c..d2cb1a6c5d95 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -326,6 +326,10 @@ struct driver_attribute {
>  	struct driver_attribute driver_attr_##_name = __ATTR_RO(_name)
>  #define DRIVER_ATTR_WO(_name) \
>  	struct driver_attribute driver_attr_##_name = __ATTR_WO(_name)
> +#define DRIVER_ATTR_IGNORE_LOCKDEP(_name, _mode, _show, _store) \
> +	struct driver_attribute driver_attr_##_name =		\
> +		__ATTR_IGNORE_LOCKDEP(_name, _mode, _show, _store)
> +

I don't want to give driver writers the ability to shoot themselves in
the foot, can you just put this in the bus.c file itself so that no one
else will abuse this and "open code" the unbind/bind attributes instead
of using a #define for it?

Also I've been trying to get rid of the "specify the mode" macros, so
that everyone uses the RO, WO, and RW versions, so adding a generic one
here is not going to help with that effort :)

thanks,

greg k-h