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    Subject[PATCH 5.10 147/563] rcu/exp: Mark current CPU as exp-QS in IPI loop second pass
    Date
    From: Frederic Weisbecker <frederic@kernel.org>

    [ Upstream commit 81f6d49cce2d2fe507e3fddcc4a6db021d9c2e7b ]

    Expedited RCU grace periods invoke sync_rcu_exp_select_node_cpus(), which
    takes two passes over the leaf rcu_node structure's CPUs. The first
    pass gathers up the current CPU and CPUs that are in dynticks idle mode.
    The workqueue will report a quiescent state on their behalf later.
    The second pass sends IPIs to the rest of the CPUs, but excludes the
    current CPU, incorrectly assuming it has been included in the first
    pass's list of CPUs.

    Unfortunately the current CPU may have changed between the first and
    second pass, due to the fact that the various rcu_node structures'
    ->lock fields have been dropped, thus momentarily enabling preemption.
    This means that if the second pass's CPU was not on the first pass's
    list, it will be ignored completely. There will be no IPI sent to
    it, and there will be no reporting of quiescent states on its behalf.
    Unfortunately, the expedited grace period will nevertheless be waiting
    for that CPU to report a quiescent state, but with that CPU having no
    reason to believe that such a report is needed.

    The result will be an expedited grace period stall.

    Fix this by no longer excluding the current CPU from consideration during
    the second pass.

    Fixes: b9ad4d6ed18e ("rcu: Avoid self-IPI in sync_rcu_exp_select_node_cpus()")
    Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
    Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
    Cc: Uladzislau Rezki <urezki@gmail.com>
    Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
    Cc: Boqun Feng <boqun.feng@gmail.com>
    Cc: Josh Triplett <josh@joshtriplett.org>
    Cc: Joel Fernandes <joel@joelfernandes.org>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/rcu/tree_exp.h | 1 +
    1 file changed, 1 insertion(+)

    diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
    index 0ffe185c1f46a..0dc16345e668c 100644
    --- a/kernel/rcu/tree_exp.h
    +++ b/kernel/rcu/tree_exp.h
    @@ -387,6 +387,7 @@ retry_ipi:
    continue;
    }
    if (get_cpu() == cpu) {
    + mask_ofl_test |= mask;
    put_cpu();
    continue;
    }
    --
    2.34.1


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