Re: read / write / close sync for device files

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On Mon, Jul 4, 2011 at 9:40 PM, Greg KH <greg@xxxxxxxxx> wrote:
> On Mon, Jul 04, 2011 at 06:13:49PM +0530, Amit Nagal wrote:
>> Hi ,
>> if  a application is  doing read /write I/O with a  usb device ,  in 2
>> different threads in userspace ,
> What type of USB device?

Thanx for reply .

i am using USB hid interface and using USBHID  driver with hid-raw
device interface .

Actually i have 2 threads in userspace app  : one for read and another
one for write .

my problem is like this :
In case of usb disconnect  , if read() returns first with error and
application calls close() in read thread itself in userspace   , while
write() is still in progress() in usb driver in a separate thread   ,
is this ok and safe  ?

or shall the application be designed such that both until read and
write return with error , it should not call close in userspace ?

Also to test this  , i applied a 10 seconds delay at the end of
hidraw_read() function in hidraw driver like :

static ssize_t hidraw_read(struct file *file, char __user *buffer,
size_t count, loff_t *ppos)
                 /********************* test block starts */

                printk(KERN_EMERG "[%s] - sleep start\n" , __func__);
                mdelay(10000);  /* 10s delay */
	printk(KERN_EMERG "[%s] - sleep end\n" , __func__);

                /*****************  test block ends */

	return ret;

now while hidraw_read is executing 10s delay  , i disconnects  usb
device  , calls close(fd) .
i observed  , while close(fd) returns with 0 successfully immediately
, usb driver release() function corresponding to close() was called
only when hidraw_read returns after completing delay which is approx
9s later calling to close() in userspace  .

so do the usb-core layers or VFS layers maintains some locks that take
care that if read / write is in progress
and application calls close() , corresponding release() method in usb
driver will be called only upon completion
of all read / writes ?

Amit Nagal

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