usage count in device driver and concurrency

usage count in device driver and concurrency

[Adeshara Tushar]

Hi,
I am wondering how to handle device usage count in
open and release call of device driver if hardware
need to be initialized on first open and shutdown on
last close. I have seen som code like

int open()
{
        /*some code*/
        device->usage++;
        if(device->usage==1)
                init_hardware();
        /*rest of code*/
}
void release ()
{
        /*some code*/
        if(device->usage==1)
                shutdown_hardware();
        device->usage--;
        /*rest of code*/
}
                                                      
                                                      
               
However, it seems to me that this code can make
problem.
If device->usage=0, and two process A,B execute line
        device->usage++;
concurretly, device->usage will become 2 when they 
come to next line. This will result in hardware being
used without initialization. Same things can happen in
release call also, which will result in no shutdown of
hardware.
               I have seen this type of code in
        /linux-2.6.8/drivers/ide/ide-disk.c and
        /linux-2.6.8/drivers/ide/ide-floppy.c

    Please let me know if its bug or not before I
start working on patches.


Regards,
Tushar

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Re: usage count in device driver and concurrency

[Bartlomiej Zolnierkiewicz]

On 8/30/05, Adeshara Tushar <adesharatushar@yahoo.com> wrote:
> Hi,
> I am wondering how to handle device usage count in
> open and release call of device driver if hardware
> need to be initialized on first open and shutdown on
> last close. I have seen som code like
>
> int open()
> {
>         /*some code*/
>         device->usage++;
>         if(device->usage==1)
>                 init_hardware();
>         /*rest of code*/
> }
> void release ()
> {
>         /*some code*/
>         if(device->usage==1)
>                 shutdown_hardware();
>         device->usage--;
>         /*rest of code*/
> }
>
>
>
> However, it seems to me that this code can make
> problem.
> If device->usage=0, and two process A,B execute line
>         device->usage++;
> concurretly, device->usage will become 2 when they
> come to next line. This will result in hardware being
> used without initialization. Same things can happen in
> release call also, which will result in no shutdown of
> hardware.
>                I have seen this type of code in
>         /linux-2.6.8/drivers/ide/ide-disk.c and
>         /linux-2.6.8/drivers/ide/ide-floppy.c
>
>     Please let me know if its bug or not before I
> start working on patches.

Not a bug: ->open() and ->release() for block devices
are never called concurrently (because of bdev->bd_sem).

Bartlomiej