How can I wake up one process from the wait queue?

How can I wake up one process from the wait queue?

[Rock Lee]

Hi, everyoneI am implementing a simple driver to experiment with wait queue.Two or more read processes block until a write process changes a flag and call wake_up_interruptible().I expect that a write process will only wake up one read process.However, once a write process calls  wake_up_interruptible() , all the read processes are awaken. How can I wake up one process from the wait queue? Here is the snippet of the simple dirver(just for experimenting, kernel 2.6.18):
static ssize_t rlwait_read(struct file *filp, char __user *userp, size_t size, loff_t *off){    struct rlwait_t *rock = (struct rlwait_t *)filp->private_data;    DECLARE_WAITQUEUE(wait, current);        add_wait_queue(&rock->r_wait_head, &wait);        while (rock->r_flag == 0) {        set_current_state(TASK_INTERRUPTIBLE);        schedule();    }   
    remove_wait_queue(&rock->r_wait_head, &wait);    set_current_state(TASK_RUNNING);    return 0;}
static ssize_t rlwait_write(struct file *filp, const char __user *userp, size_t size, loff_t *off){    struct rlwait_t *rock = (struct rlwait_t *)filp->private_data;            rock->r_flag = 1;    wake_up_interruptible(&rock->r_wait_head);            return size; }
Thans in advice.
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How can I wake up one process from the wait queue?

[Raghavendra]

On Thursday 30 October 2014 02:02 PM, Rock Lee wrote:
>
> Hi, everyone
>
> I am implementing a simple driver to experiment with wait queue.Two or 
> more read processes block until a write process changes a flag and 
> call wake_up_interruptible().I expect that a write process will only 
> wake up one read process.However, once a write process calls 
> wake_up_interruptible() , all the read processes are awaken. How can I 
> wake up one process from the wait queue?
>
You can use exclusive wait queues, which wakes up only one process from 
the queue.

Although the general idea that is followed is: all the process are woken 
up once the resource is made available. And then the access to that 
resource is guarded by a lock, which in turn allows only one process to 
access the resource. Rest of the processes are pushed backed to sleep(or 
busy wait).

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How can I wake up one process from the wait queue?

[Pranay Srivastava]

Hi

You got to use exclusive wait queues here. Needs a bit more lines but
you can specify exactly how many tasks you want woken up.

You should be able to look it up easily on how to use it.

On Thu, Oct 30, 2014 at 2:02 PM, Rock Lee <rocklee_104@sina.com> wrote:
> Hi, everyone
>
> I am implementing a simple driver to experiment with wait queue.Two or more
> read processes block until a write process changes a flag and call
> wake_up_interruptible().I expect that a write process will only wake up one
> read process.However, once a write process calls  wake_up_interruptible() ,
> all the read processes are awaken. How can I wake up one process from the
> wait queue? Here is the snippet of the simple dirver(just for experimenting,
> kernel 2.6.18):
>
>
> static ssize_t rlwait_read(struct file *filp, char __user *userp, size_t
> size, loff_t *off)
>
> {
>
>     struct rlwait_t *rock = (struct rlwait_t *)filp->private_data;
>
>     DECLARE_WAITQUEUE(wait, current);
>
>
>
>     add_wait_queue(&rock->r_wait_head, &wait);
>
>
>
>     while (rock->r_flag == 0) {
>
>         set_current_state(TASK_INTERRUPTIBLE);
>
>         schedule();
>
>     }
>
>
>     remove_wait_queue(&rock->r_wait_head, &wait);
>
>     set_current_state(TASK_RUNNING);
>
>     return 0;
>
> }
>
>
> static ssize_t rlwait_write(struct file *filp, const char __user *userp,
> size_t size, loff_t *off)
>
> {
>
>     struct rlwait_t *rock = (struct rlwait_t *)filp->private_data;
>
>
>
>     rock->r_flag = 1;
>
>     wake_up_interruptible(&rock->r_wait_head);
>
>
>
>     return size;
>
> }
>
>
> Thans in advice.
>
>
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>



-- 
        ---P.K.S