On Wed, Sep 28, 2011 at 11:11 AM, Michael Blizek <
michi1@michaelblizek.twilightparadox.com> wrote:

Hi!

On 21:41 Tue 27 Sep     , Venkatram Tummala wrote:

quoted

On Tue, Sep 27, 2011 at 9:19 PM, rohan puri [off-list ref]

wrote:
...

quoted

quoted

in device_open() ->

if(var)
      return -EBUSY
var++

&

in device_release() ->

var--


I think this should do the job.

This will prevent other processes to open the file until a process

releases

quoted

it. This is not what i need. Only the threads in a process shouldn't be

able

quoted

to open the file if it is already opened in the process. Other processes
should be able to open it.

You could create something like this:

DEFINE_MUTEX(pidlist_lock);
LIST_HEAD(pidlist);

struct pidlist_node{
       struct list_head lh;
       pid_t pid;
}

static struct pidlist_node *get_pin(void)
{
       struct list_head *curr = pidlist.next;
       while (curr != pidlist) {
               struct struct pidlist_node *pin = container_of(curr, struct
pidlist_node, lh);
               if (pin->pid == current->pid) {

Instead of pid check, AFAIK here tgid comparison should be done. Threads in
a single process may have different pid but same tgid and we want to
restrict access to only one thread in a single process. Right?

                       return pin;
               }
       }
       return 0;
}

int open(void)
{
       struct pidlist_node *pin;

       mutex_lock(&pidlist_lock);

       pin = get_pin();
       if (pin != 0) {
               mutex_unlock(&pidlist_lock);
               return -EBUSY;
       }

       pin = kmalloc(sizeof(struct pidlist_node), GFP_KERNEL);
       if (pin == 0) {
               mutex_unlock(&pidlist_lock);
               return -ENOMEM;
       }

       pin->pid = current->pid;
       list_add(&(pin->lh), &pidlist);

       mutex_unlock(&pidlist_lock);
}

int close(void)
{
       struct pidlist_node *pin;

       mutex_lock(&pidlist_lock);

       pin = get_pin();
       if (pin != 0) {
               list_del(&(pin->lh));
               kfree(pin);
       }

       mutex_unlock(&pidlist_lock);
}

       -Michi
--
programing a layer 3+4 network protocol for mesh networks
see http://michaelblizek.twilightparadox.com

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