Hi,
Greg KH wrote:
Ok, but if we unplug a device, then disconnect will be called even if we
opened a sysfs file.
Yes but the device structure will still be in memory, so you will be ok.
disconnect method isn't supposed to clear the device structure in memory ?
It all depends on the bus you are talking about :)
usb, but should be for all bus that accept a hot remove of the device...
Couldn't be a race between the moment we read our private data and check
it is valid and the moment we use it :
Process A (read/write sysfs file) Process B (disconnect)
recover our private data from struct device
check it is valid
free our private data
do operation on private data
No, you should not be freeing your private data on your own. You should
do that in the device release function.
But that's what I do !
What type of driver?
a driver for an usb modem.
I free it in device release function = usb_disconnect in my case =
Process B. Process A is call by sysfs and don't seem serialized with
Process B.
Is there another place where I could free my private data later : I don't
think so.
For usb, no, you are correct. I was referring to the driver core when
you were mentioning the sysfs stuff, sorry.
Hm, in thinking about it, it might make more sense to rework the usb
core to handle this better. Possibly add a release() callback to the
driver when the device is actually being freed. Wouldn't be that hard
to do so, and might cut down on some of the common locking errors.
So a diconnect when the device is removed and a released when the
resource counter reach 0 ?
I believe some drivers expected that sysfs read/write callback are always
called when the device is plugged so they don't check if
to_usb_interface/usb_get_intfdata return valid pointer.
Then they should be fixed. Any specific examples?
I am a little lasy to list all, but some drivers in driver/usb should
have this problem : the first driver I look : ./misc/phidgetkit.c do
[1]. So sysfs read don't check if to_usb_interface or usb_get_intfdata
return NULL pointer...
And it is a bit your fault, as many developper should have read your
great tutorial [2] ;)
Also I always see driver free their privatre data in device disconnect,
so if read/write from sysfs aren't serialized with device disconnect
there are still a possible race like I show in my example.
Yes, you are correct. Again, any specific drivers you see with this
problem?
I believe near all drivers that use sysfs via device_create_file, as I
never see them use mutex in read/write in order to check there aren't in
the same time in their disconnect that could free there private data
when they do operation on it...
Couldn't be possible the make device_remove_file blocking until all the
open file are closed ?
thanks,
Matthieu
[1]
#define show_input(value) \
static ssize_t show_input##value(struct device *dev, char *buf) \
{ \
struct usb_interface *intf = to_usb_interface(dev); \
struct phidget_interfacekit *kit = usb_get_intfdata(intf); \
\
return sprintf(buf, "%d\n", kit->inputs[value - 1]); \
} \
static DEVICE_ATTR(input##value, S_IRUGO, show_input##value, NULL);
[2] http://www.linuxjournal.com/article/7353
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