Re: Kernel 2.2: tq_scheduler functions scheduling and waiting

[Andrew Morton <andrewm@uow.edu.au>]

Arthur Naseef wrote:
> 
> Andrew:
> 
> Excellent.  I will look at the 2.4 sources.
> 
> In addition to the TASK_ZOMBIE issue you mention, I believe there
> is an issue of false termination of wait queues.  Consider this:
> 
>         - Task places itself on a wait queue
>         - Calls schedule()
>         - tq_scheduler function does the same
> 
> Now, there are two events which could place the task in TASK_RUNNING
> and no clear way to differentiate.  And, since most of the kernel
> code does not check that the wait condition was actually met, this
> could lead to all types of problems, right?
> 

Yes.  The situation where one task is on two waitqueues
is rare, but does happen.  And yes, there is code out there
which does a bare schedule() and *assumes* that once the
schedule has returned, the thing it was waiting for has
indeed occurred.

Generally this is poor practice - it's safer to loop
over the schedule() call until the condition you're
sleeping on has been tested.

You really shouldn't be sleeping in this way on tq_scheduler
if there's any way in which the sleep can take an extended
period of time.  You may end up putting important kernel
tasks to sleep.

Best to use schedule_task(), or an independent kernel thread.

-