Re: waiting on a condition

[Neil Horman <nhorman@redhat.com>]

Martijn Sipkema wrote:
> L.S.
> 
> I'd like to do something similar as can be done using a POSIX condition
> variable in the kernel, i.e. wait for some condition to become true. The
> pthread_cond_wait() function allows atomically unlocking a mutex and
> waiting on a condition. I think I should do something like:
> (the condition is updated from an interrupt handler)
> 
> disable interrupts
> acquire spinlock
> if condition not satisfied
>     add task to wait queue
>     set task to sleep
> release spinlock
> restore interrupts
> schedule
> 
> Now, this will only work with preemption disabled within the critical
> section. How would something like this be done whith preemption
> enabled?
> 
you above algorithm seems rather prone to deadlock.  Everything else in 
the kernel does more or less this operation by using a wait queue and a 
call to schedule to make tasks sleep until an event is signaled with a 
call to one of the wake_up family of functions.  Then a spinlock is used 
  to protect any critical data regions in smp environments.  Search the 
kernel for calls to add_wait_queue and wake_up[_interruptible] for 
examples of how this is implemented.

HTH
Neil

> 
> --ms
> 
> 
> 
> 
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  *Neil Horman
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  *Red Hat, Inc.
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