• Robert Love's avatar
    [PATCH] kernel preemption bits (2/2) · 4d0b85ea
    Robert Love authored
    In both preempt_schedule in sched.c and resume_kernel in entry.S, it is
    possible to return with need_resched set and thus a pending preemption
    but not service that preemption for some time.
    
    Consider:
    
    	- return from schedule() to preempt_schedule
    	- interrupt occurs, sets need_resched
    	- we cannot preempt since preempt_count = PREEMPT_ACTIVE
    	- back in preempt_schedule, set preempt_count = 0
    
    Now we again can preempt, but we will not.  Instead we return and
    continue executing.  On the next interrupt, we will redo the whole
    fiasco which is a waste since we could of reentered schedule while we
    were there.  Worse, if we acquire a lock before the next interrupt we
    can potentially delay the pending reschedule a very long time.  This is
    not acceptable.
    
    The solution is to check for and loop on need_resched on resume_kernel
    and preempt_schedule like schedule itself does.
    4d0b85ea
sched.c 43.6 KB