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    Subject[patch 18/23] cond_resched() fix

    -stable review patch. If anyone has any objections, please let us know.

    ------------------
    From: Andrew Morton <akpm@osdl.org>

    [PATCH] cond_resched() fix

    Fix a bug identified by Zou Nan hai <nanhai.zou@intel.com>:

    If the system is in state SYSTEM_BOOTING, and need_resched() is true,
    cond_resched() returns true even though it didn't reschedule. Consequently
    need_resched() remains true and JBD locks up.

    Fix that by teaching cond_resched() to only return true if it really did call
    schedule().

    cond_resched_lock() and cond_resched_softirq() have a problem too. If we're
    in SYSTEM_BOOTING state and need_resched() is true, these functions will drop
    the lock and will then try to call schedule(), but the SYSTEM_BOOTING state
    will prevent schedule() from being called. So on return, need_resched() will
    still be true, but cond_resched_lock() has to return 1 to tell the caller that
    the lock was dropped. The caller will probably lock up.

    Bottom line: if these functions dropped the lock, they _must_ call schedule()
    to clear need_resched(). Make it so.

    Also, uninline __cond_resched(). It's largeish, and slowpath.

    Acked-by: Ingo Molnar <mingo@elte.hu>
    Signed-off-by: Andrew Morton <akpm@osdl.org>
    Signed-off-by: Linus Torvalds <torvalds@osdl.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    kernel/sched.c | 25 +++++++++++++------------
    1 file changed, 13 insertions(+), 12 deletions(-)

    --- linux-2.6.17.7.orig/kernel/sched.c
    +++ linux-2.6.17.7/kernel/sched.c
    @@ -4044,17 +4044,22 @@ asmlinkage long sys_sched_yield(void)
    return 0;
    }

    -static inline void __cond_resched(void)
    +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)
    {
    /*
    * The BKS might be reacquired before we have dropped
    * PREEMPT_ACTIVE, which could trigger a second
    * cond_resched() call.
    */
    - if (unlikely(preempt_count()))
    - return;
    - if (unlikely(system_state != SYSTEM_RUNNING))
    - return;
    do {
    add_preempt_count(PREEMPT_ACTIVE);
    schedule();
    @@ -4064,13 +4069,12 @@ static inline void __cond_resched(void)

    int __sched cond_resched(void)
    {
    - if (need_resched()) {
    + if (need_resched() && __resched_legal(0)) {
    __cond_resched();
    return 1;
    }
    return 0;
    }
    -
    EXPORT_SYMBOL(cond_resched);

    /*
    @@ -4091,7 +4095,7 @@ int cond_resched_lock(spinlock_t *lock)
    ret = 1;
    spin_lock(lock);
    }
    - if (need_resched()) {
    + if (need_resched() && __resched_legal(1)) {
    _raw_spin_unlock(lock);
    preempt_enable_no_resched();
    __cond_resched();
    @@ -4100,14 +4104,13 @@ int cond_resched_lock(spinlock_t *lock)
    }
    return ret;
    }
    -
    EXPORT_SYMBOL(cond_resched_lock);

    int __sched cond_resched_softirq(void)
    {
    BUG_ON(!in_softirq());

    - if (need_resched()) {
    + if (need_resched() && __resched_legal(0)) {
    __local_bh_enable();
    __cond_resched();
    local_bh_disable();
    @@ -4115,10 +4118,8 @@ int __sched cond_resched_softirq(void)
    }
    return 0;
    }
    -
    EXPORT_SYMBOL(cond_resched_softirq);

    -
    /**
    * yield - yield the current processor to other threads.
    *
    --
    -
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