linux-kernel - Re: [PATCH] base: make module_create_drivers

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Date:	Fri, 10 Jun 2016 09:25:30 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	linux-kernel@...r.kernel.org,
	"v2 . 6 . 21+" <stable@...r.kernel.org>
Subject: Re: [PATCH] base: make module_create_drivers_dir race-free

On Fri, Jun 10, 2016 at 10:54:32AM +0200, Jiri Slaby wrote:
> Modules which register drivers via standard path (driver_register) in
> parallel can cause a warning:
> WARNING: CPU: 2 PID: 3492 at ../fs/sysfs/dir.c:31 sysfs_warn_dup+0x62/0x80
> sysfs: cannot create duplicate filename '/module/saa7146/drivers'
> Modules linked in: hexium_gemini(+) mxb(+) ...
> ...
> Call Trace:
> ...
>  [<ffffffff812e63a2>] sysfs_warn_dup+0x62/0x80
>  [<ffffffff812e6487>] sysfs_create_dir_ns+0x77/0x90
>  [<ffffffff8140f2c4>] kobject_add_internal+0xb4/0x340
>  [<ffffffff8140f5b8>] kobject_add+0x68/0xb0
>  [<ffffffff8140f631>] kobject_create_and_add+0x31/0x70
>  [<ffffffff8157a703>] module_add_driver+0xc3/0xd0
>  [<ffffffff8155e5d4>] bus_add_driver+0x154/0x280
>  [<ffffffff815604c0>] driver_register+0x60/0xe0
>  [<ffffffff8145bed0>] __pci_register_driver+0x60/0x70
>  [<ffffffffa0273e14>] saa7146_register_extension+0x64/0x90 [saa7146]
>  [<ffffffffa0033011>] hexium_init_module+0x11/0x1000 [hexium_gemini]
> ...
> 
> As can be (mostly) seen, driver_register causes this call sequence:
>   -> bus_add_driver
>     -> module_add_driver
>       -> module_create_drivers_dir
> The last one creates "drivers" directory in /sys/module/<...>. When
> this is done in parallel, the directory is attempted to be created
> twice at the same time.
> 
> This can be easily reproduced by loading mxb and hexium_gemini in
> parallel:
> while :; do
>   modprobe mxb &
>   modprobe hexium_gemini
>   wait
>   rmmod mxb hexium_gemini saa7146_vv saa7146
> done
> 
> saa7146 calls pci_register_driver for both mxb and hexium_gemini,
> which means /sys/module/saa7146/drivers is to be created for both of
> them.

Wow, good catch, I would never have expected that.

I'll go queue this up soon, thanks.

greg k-h