Re: usage count in device driver and concurrency

[Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>]

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
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