Messages in this thread Patch in this message | | | Date | Thu, 03 Mar 2005 22:36:38 +0300 | From | "Eugeny S. Mints" <> | Subject | Re: [patch] Real-Time Preemption, deactivate() scheduling issue |
| |
please consider the following scenario for full RT kernel.
Task A is running then an irq is occured which in turn wakes up irq related thread (B) of a higher priority than A.
my current understanding that actual context switch between A and B will occure at preempt_schedule_irq() on the "return form irq " path.
in this case the following "if" statement in __schedule() always returns false since preempt_schedule_irq() always sets up PREEMPT_ACTIVE before __schedule() call.
if ((prev->state & ~TASK_RUNNING_MUTEX) && !(preempt_count() & PREEMPT_ACTIVE)) {
as result the deactivate() is never called for preempted task A in this scenario. BUt if the task A is preempted while not in TASK_RUNNING state such behaviour seems incorrect since we get a task in not TASK_RUNNING state linked into a run queue.
An example:
drivers/net/irda/sir_dev.c: 76 (2.6.10 kernel)
spin_lock_irqsave(&dev->tx_lock, flags); /* serialize th other tx operations */ while (dev->tx_buff.len > 0) { /* wait until tx idle */ spin_unlock_irqrestore(&dev->tx_lock, flags); 76: set_current_state(TASK_UNINTERRUPTIBLE); schedule_timeout(msecs_to_jiffies(10)); spin_lock_irqsave(&dev->tx_lock, flags); }
At line 76 irqs are enabled, preemption is enabled. Let assume the task A executes this code and gets preempted right after line 76. Task state is TASK_UNINTERRUPTIBLE but it will not be deactevated. Of cource this is the bug in set_current_state() utilization in this particular driver but schedule stuff should be robust to such bugs I believe. There are a lot such bugs in the kernel I believe.
Not sure what the actual reason for !(preempt_count() & PREEMPT_ACTIVE)) condition is but if it's just a sort of optimization (not remove a task from run queue if it was preemped in TASK_RUNNING state) then probably it should be removed in order to save correctness. Patch attached.
Eugeny
--- sched.c.orig 2005-03-03 22:35:16.000000000 +0300 +++ sched.c 2005-03-03 22:34:58.000000000 +0300 @@ -2891,8 +2891,7 @@ spin_lock_irq(&rq->lock); switch_count = &prev->nvcsw; // TODO: temporary - to see it in vmstat - if ((prev->state & ~TASK_RUNNING_MUTEX) && - !(preempt_count() & PREEMPT_ACTIVE)) { + if ((prev->state & ~TASK_RUNNING_MUTEX)) { switch_count = &prev->nvcsw; if (unlikely((prev->state & TASK_INTERRUPTIBLE) && unlikely(signal_pending(prev)))) | |