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    Subject[patch 59/59] sched: fix cond_resched_softirq() offset
    -stable review patch.  If anyone has any objections, please let us know.
    ------------------

    From: Ingo Molnar <mingo@elte.hu>

    Remove the __resched_legal() check: it is conceptually broken. The biggest
    problem it had is that it can mask buggy cond_resched() calls. A
    cond_resched() call is only legal if we are not in an atomic context, with
    two narrow exceptions:

    - if the system is booting
    - a reacquire_kernel_lock() down() done while PREEMPT_ACTIVE is set

    But __resched_legal() hid this and just silently returned whenever
    these primitives were called from invalid contexts. (Same goes for
    cond_resched_locked() and cond_resched_softirq()).

    Furthermore, the __legal_resched(0) call was buggy in that it caused
    unnecessarily long softirq latencies via cond_resched_softirq(). (which is
    only called from softirq-off sections, hence the code did nothing.)

    The fix is to resurrect the efficiency of the might_sleep checks and to
    only allow the narrow exceptions.

    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    Signed-off-by: Andrew Morton <akpm@osdl.org>
    Signed-off-by: Linus Torvalds <torvalds@osdl.org>
    [chrisw: backport to 2.6.19.2]
    Signed-off-by: Chris Wright <chrisw@sous-sol.org>
    ---
    kernel/sched.c | 16 ++++------------
    1 file changed, 4 insertions(+), 12 deletions(-)

    --- linux-2.6.19.2.orig/kernel/sched.c
    +++ linux-2.6.19.2/kernel/sched.c
    @@ -4524,15 +4524,6 @@ asmlinkage long sys_sched_yield(void)
    return 0;
    }

    -static inline int __resched_legal(int expected_preempt_count)
    -{
    - if (unlikely(preempt_count() != expected_preempt_count))
    - return 0;
    - if (unlikely(system_state != SYSTEM_RUNNING))
    - return 0;
    - return 1;
    -}
    -
    static void __cond_resched(void)
    {
    #ifdef CONFIG_DEBUG_SPINLOCK_SLEEP
    @@ -4552,7 +4543,8 @@ static void __cond_resched(void)

    int __sched cond_resched(void)
    {
    - if (need_resched() && __resched_legal(0)) {
    + if (need_resched() && !(preempt_count() & PREEMPT_ACTIVE) &&
    + system_state == SYSTEM_RUNNING) {
    __cond_resched();
    return 1;
    }
    @@ -4578,7 +4570,7 @@ int cond_resched_lock(spinlock_t *lock)
    ret = 1;
    spin_lock(lock);
    }
    - if (need_resched() && __resched_legal(1)) {
    + if (need_resched() && system_state == SYSTEM_RUNNING) {
    spin_release(&lock->dep_map, 1, _THIS_IP_);
    _raw_spin_unlock(lock);
    preempt_enable_no_resched();
    @@ -4594,7 +4586,7 @@ int __sched cond_resched_softirq(void)
    {
    BUG_ON(!in_softirq());

    - if (need_resched() && __resched_legal(0)) {
    + if (need_resched() && system_state == SYSTEM_RUNNING) {
    raw_local_irq_disable();
    _local_bh_enable();
    raw_local_irq_enable();
    --
    -
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