On Mon, 2005-04-18 at 14:38 -0500, Linas Vepstas wrote:
>
> Hi,
>
> The patch below appears to fix a problem where a number of dead processes
> linger on the system. On a highly loaded system, dozens of processes
> were found stuck in do_exit(), calling thier very last schedule(), and
> then being lost forever.
>
> Processes that are PF_DEAD are cleaned up *after* the context switch,
> in a routine called finish_task_switch(task_t *prev). The "prev" gets
> the value returned by _switch() in entry.S, but this value comes from
>
> __switch_to (struct task_struct *prev,
> struct task_struct *new)
> {
> old_thread = ¤t->thread; ///XXX shouldn't this be prev, not current?
> last = _switch(old_thread, new_thread);
> return last;
> }
>
> The way I see it, "prev" and "current" are almost always going to be
> pointing at the same thing; however, if a "need resched" happens,
> or there's a pre-emept or some-such, then prev and current won't be
> the same; in which case, finish_task_switch() will end up cleaning
> up the old current, instead of prev. This will result in dead processes
> hanging around, which will never be scheduled again, and will never
> get a chance to have put_task_struct() called on them.
Ok, thinking moer about this ... that will need maybe some help from
Ingo so I fully understand where schedule's are allowed ... We are
basically in the middle of the scheduler here, so I wonder how much of
the scheduler itself can be preempted or so ...
Basically, under which circumstances can prev and current be different ?
Ben.
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