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rcu: Move rcu_dynticks_eqs_online() to rcu_cpu_starting() 

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2065994

commit 2caebefb00f03b5ba13d44aa6cc3723759b43822
Author: Paul E. McKenney <paulmck@kernel.org>
Date:   Wed, 28 Jul 2021 12:38:42 -0700

    rcu: Move rcu_dynticks_eqs_online() to rcu_cpu_starting()

    The purpose of rcu_dynticks_eqs_online() is to adjust the ->dynticks
    counter of an incoming CPU when required.  It is currently invoked
    from rcutree_prepare_cpu(), which runs before the incoming CPU is
    running, and thus on some other CPU.  This makes the per-CPU accesses in
    rcu_dynticks_eqs_online() iffy at best, and it all "works" only because
    the running CPU cannot possibly be in dyntick-idle mode, which means
    that rcu_dynticks_eqs_online() never has any effect.

    It is currently OK for rcu_dynticks_eqs_online() to have no effect, but
    only because the CPU-offline process just happens to leave ->dynticks in
    the correct state.  After all, if ->dynticks were in the wrong state on a
    just-onlined CPU, rcutorture would complain bitterly the next time that
    CPU went idle, at least in kernels built with CONFIG_RCU_EQS_DEBUG=y,
    for example, those built by rcutorture scenario TREE04.  One could
    argue that this means that rcu_dynticks_eqs_online() is unnecessary,
    however, removing it would make the CPU-online process vulnerable to
    slight changes in the CPU-offline process.

    One could also ask why it is safe to move the rcu_dynticks_eqs_online()
    call so late in the CPU-online process.  Indeed, there was a time when it
    would not have been safe, which does much to explain its current location.
    However, the marking of a CPU as online from an RCU perspective has long
    since moved from rcutree_prepare_cpu() to rcu_cpu_starting(), and all
    that is required is that ->dynticks be set correctly by the time that
    the CPU is marked as online from an RCU perspective.  After all, the RCU
    grace-period kthread does not check to see if offline CPUs are also idle.
    (In case you were curious, this is one reason why there is quiescent-state
    reporting as part of the offlining process.)

    This commit therefore moves the call to rcu_dynticks_eqs_online() from
    rcutree_prepare_cpu() to rcu_cpu_starting(), this latter being guaranteed
    to be running on the incoming CPU.  The call to this function must of
    course be placed before this rcu_cpu_starting() announces this CPU's
    presence to RCU.

    Reported-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Signed-off-by: Waiman Long <longman@redhat.com>
This commit is contained in:
Waiman Long 2022-03-24 17:15:42 -04:00
parent 5375177f45
commit 86f4f43dc8
1 changed files with 1 additions and 1 deletions
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2
kernel/rcu/tree.c
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@ -4130,7 +4130,6 @@ int rcutree_prepare_cpu(unsigned int cpu)
rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs);
rdp->blimit = blimit;
rdp->dynticks_nesting = 1; /* CPU not up, no tearing. */
rcu_dynticks_eqs_online();
raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
/*
@ -4250,6 +4249,7 @@ void rcu_cpu_starting(unsigned int cpu)
mask = rdp->grpmask;
WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);
WARN_ON_ONCE(!(rnp->ofl_seq & 0x1));
rcu_dynticks_eqs_online();
smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().
raw_spin_lock_irqsave_rcu_node(rnp, flags);
WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);