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    SubjectRe: [PATCH V2 2/7] rcu: cleanup rcu_preempt_deferred_qs()
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    On 2019/11/3 10:01 上午, Boqun Feng wrote:
    > Hi Jiangshan,
    >
    >
    > I haven't checked the correctness of this patch carefully, but..
    >
    >
    > On Sat, Nov 02, 2019 at 12:45:54PM +0000, Lai Jiangshan wrote:
    >> Don't need to set ->rcu_read_lock_nesting negative, irq-protected
    >> rcu_preempt_deferred_qs_irqrestore() doesn't expect
    >> ->rcu_read_lock_nesting to be negative to work, it even
    >> doesn't access to ->rcu_read_lock_nesting any more.
    >
    > rcu_preempt_deferred_qs_irqrestore() will report RCU qs, and may
    > eventually call swake_up() or its friends to wake up, say, the gp
    > kthread, and the wake up functions could go into the scheduler code
    > path which might have RCU read-side critical section in it, IOW,
    > accessing ->rcu_read_lock_nesting.

    Sure, thank you for pointing it out.

    I should rewrite the changelog in next round. Like this:

    rcu: cleanup rcu_preempt_deferred_qs()

    IRQ-protected rcu_preempt_deferred_qs_irqrestore() itself doesn't
    expect ->rcu_read_lock_nesting to be negative to work.

    There might be RCU read-side critical section in it (from wakeup()
    or so), 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once)
    will ensure that ->rcu_read_unlock_special is zero and these RCU
    read-side critical sections will not call rcu_read_unlock_special().

    Thanks
    Lai

    ===
    PS: Were 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once)
    not applied earlier, it will be protected by previous patch (patch1)
    in this series
    "rcu: use preempt_count to test whether scheduler locks is held"
    when rcu_read_unlock_special() is called.



    >
    > Again, haven't checked closely, but this argument in the commit log
    > seems untrue.
    >
    > Regards,
    > Boqun
    >
    >>
    >> It is true that NMI over rcu_preempt_deferred_qs_irqrestore()
    >> may access to ->rcu_read_lock_nesting, but it is still safe
    >> since rcu_read_unlock_special() can protect itself from NMI.
    >>
    >> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
    >> ---
    >> kernel/rcu/tree_plugin.h | 5 -----
    >> 1 file changed, 5 deletions(-)
    >>
    >> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
    >> index aba5896d67e3..2fab8be2061f 100644
    >> --- a/kernel/rcu/tree_plugin.h
    >> +++ b/kernel/rcu/tree_plugin.h
    >> @@ -552,16 +552,11 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
    >> static void rcu_preempt_deferred_qs(struct task_struct *t)
    >> {
    >> unsigned long flags;
    >> - bool couldrecurse = t->rcu_read_lock_nesting >= 0;
    >>
    >> if (!rcu_preempt_need_deferred_qs(t))
    >> return;
    >> - if (couldrecurse)
    >> - t->rcu_read_lock_nesting -= RCU_NEST_BIAS;
    >> local_irq_save(flags);
    >> rcu_preempt_deferred_qs_irqrestore(t, flags);
    >> - if (couldrecurse)
    >> - t->rcu_read_lock_nesting += RCU_NEST_BIAS;
    >> }
    >>
    >> /*
    >> --
    >> 2.20.1
    >>

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