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    Subject[PATCH 4.9 83/92] sched/core: Allow __sched_setscheduler() in interrupts when PI is not used
    Date
    4.9-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Steven Rostedt (VMware) <rostedt@goodmis.org>

    commit 896bbb2522587e3b8eb2a0d204d43ccc1042a00d upstream.

    When priority inheritance was added back in 2.6.18 to sched_setscheduler(), it
    added a path to taking an rt-mutex wait_lock, which is not IRQ safe. As PI
    is not a common occurrence, lockdep will likely never trigger if
    sched_setscheduler was called from interrupt context. A BUG_ON() was added
    to trigger if __sched_setscheduler() was ever called from interrupt context
    because there was a possibility to take the wait_lock.

    Today the wait_lock is irq safe, but the path to taking it in
    sched_setscheduler() is the same as the path to taking it from normal
    context. The wait_lock is taken with raw_spin_lock_irq() and released with
    raw_spin_unlock_irq() which will indiscriminately enable interrupts,
    which would be bad in interrupt context.

    The problem is that normalize_rt_tasks, which is called by triggering the
    sysrq nice-all-RT-tasks was changed to call __sched_setscheduler(), and this
    is done from interrupt context!

    Now __sched_setscheduler() takes a "pi" parameter that is used to know if
    the priority inheritance should be called or not. As the BUG_ON() only cares
    about calling the PI code, it should only bug if called from interrupt
    context with the "pi" parameter set to true.

    Reported-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
    Tested-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Andrew Morton <akpm@osdl.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Fixes: dbc7f069b93a ("sched: Use replace normalize_task() with __sched_setscheduler()")
    Link: http://lkml.kernel.org/r/20170308124654.10e598f2@gandalf.local.home
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>


    ---
    kernel/sched/core.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -4087,8 +4087,8 @@ static int __sched_setscheduler(struct t
    int queue_flags = DEQUEUE_SAVE | DEQUEUE_MOVE;
    struct rq *rq;

    - /* may grab non-irq protected spin_locks */
    - BUG_ON(in_interrupt());
    + /* The pi code expects interrupts enabled */
    + BUG_ON(pi && in_interrupt());
    recheck:
    /* double check policy once rq lock held */
    if (policy < 0) {

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