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    Subject[PATCH] rcu: Fix dynticks_nmi_nesting underflow check in rcu_is_cpu_rrupt_from_idle
    Date
    For the smp_call_function() optimization, where callbacks can run from
    idle context, in commit 806f04e9fd2c ("rcu: Allow for smp_call_function()
    running callbacks from idle"), an additional check is added in
    rcu_is_cpu_rrupt_from_idle(), for dynticks_nmi_nesting value being 0,
    for these smp_call_function() callbacks running from idle loop.
    However, this commit missed updating a preexisting underflow check
    of dynticks_nmi_nesting, which checks for a non zero positive value.
    Fix this warning and while at it, read the counter only once.

    Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
    ---

    Hi,

    I was not able to get this warning, with scftorture.

    RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 0,
    "RCU dynticks_nmi_nesting counter underflow/zero!");

    Not sure if idle loop smp_call_function() optimization is already present
    in mainline?

    Another thing, which I am not sure of is, maybe lockdep gets disabled
    in the idle loop contexts, where rcu_is_cpu_rrupt_from_idle() is called?
    Was this the original intention, to keep the lockdep based
    RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 0
    check separate from idle task context nesting value
    WARN_ON_ONCE(!nesting && !is_idle_task(current)) check?


    Thanks
    Neeraj

    kernel/rcu/tree.c | 5 ++---
    1 file changed, 2 insertions(+), 3 deletions(-)

    diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
    index bc8b489..c3037cf 100644
    --- a/kernel/rcu/tree.c
    +++ b/kernel/rcu/tree.c
    @@ -457,11 +457,10 @@ static int rcu_is_cpu_rrupt_from_idle(void)
    /* Check for counter underflows */
    RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nesting) < 0,
    "RCU dynticks_nesting counter underflow!");
    - RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 0,
    - "RCU dynticks_nmi_nesting counter underflow/zero!");
    + nesting = __this_cpu_read(rcu_data.dynticks_nmi_nesting);
    + RCU_LOCKDEP_WARN(nesting < 0, "RCU dynticks_nmi_nesting counter underflow!");

    /* Are we at first interrupt nesting level? */
    - nesting = __this_cpu_read(rcu_data.dynticks_nmi_nesting);
    if (nesting > 1)
    return false;

    --
    The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
    a Linux Foundation Collaborative Project
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