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    SubjectRe: [net-next-2.6 PATCH RFC] TCPCT part 1d: generate Responder Cookie
    William Allen Simpson a écrit :
    > Paul E. McKenney wrote:
    >> On Tue, Nov 03, 2009 at 05:38:10PM -0500, William Allen Simpson wrote:
    >>> Documentation/RCU/checklist.txt #7 says:
    >>> One exception to this rule: rcu_read_lock() and rcu_read_unlock()
    >>> may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
    >>> in cases where local bottom halves are already known to be
    >>> disabled, for example, in irq or softirq context. Commenting
    >>> such cases is a must, of course! And the jury is still out on
    >>> whether the increased speed is worth it.
    >> I strongly suggest using the matching primitives unless you have a
    >> really strong reason not to.
    > Eric gave contrary advice. But he also suggested (in an earlier message)
    > clearing the secrets with a timer, which could be a separate context --
    > although much later in time.
    > As you suggest, I'll use the _bh suffix everywhere until every i is dotted
    > and t is crossed. Then, check for efficiency later after thorough
    > analysis by experts such as yourself.
    > This code will be hit on every SYN and SYNACK that has a cookie option.
    > But it's just prior to a CPU intensive sha_transform -- in comparison,
    > it's trivial.

    I think you misunderstood my advice ;)

    In the same function, you *cannot* use both variants like your last patch did :




    Reasoning is :

    If you need _bh() for the rcu_read_lock_bh(), thats because you know
    soft irq can happen anytime (they are not masked).

    Then you also need _bh for the spin_lock() call, or risk deadlock.

    -> tcp_cookie_generator();
    -> interrupt -> softirq -> SYN frame received -> tcp_cookie_generator() -> spin_lock(); hang

    Your choices are :

    1) Caller took care of disabling softirqs (or is only called from softirq handler),
    then _bh suffixes are not necessary in tcp_cookie_generator().
    -> spin_lock() & rcu_read_lock();

    2) You dont know what called you (process context or softirq context)
    -> you MUST use _bh prefixes on spin_lock_bh() & rcu_read_lock_bh();

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      Last update: 2009-11-05 14:21    [W:0.026 / U:5.064 seconds]
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