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    SubjectRe: [PATCH 1/9] ipmi: use refcount in message handler
    Paul E. McKenney wrote:

    >On Sun, Oct 23, 2005 at 11:42:17PM -0500, Matt Domsch wrote:
    >
    >
    >>On Sun, Oct 23, 2005 at 07:19:32PM -0700, Paul E. McKenney wrote:
    >>
    >>
    >>>My guess is that this read-side critical section can be invoked from and
    >>>SMI, and that SMIs can occur even if interrupts are disabled. If my guess
    >>>is wrong, please enlighten me. And feel free to ignore the next few
    >>>paragraphs in that case, along with a number of my suggested changes,
    >>>since they all depend critically on my guess being correct.
    >>>
    >>>
    >>Paul, it took me a bit to figure this out too, but Corey uses the TLA
    >>"SMI" to mean "Systems Management Interface", not "Systems Management
    >>Interrupt". From Documentation/IPMI.txt:
    >>
    >>ipmi_msghandler - This is the central piece of software for the IPMI
    >>system. It handles all messages, message timing, and responses. The
    >>IPMI users tie into this, and the IPMI physical interfaces (called
    >>System Management Interfaces, or SMIs) also tie in here.
    >>
    >>
    >>There are at least 4 basic types of physical hardware interfaces (BT,
    >>SMIC, KCS, and I2C), which may (or more often, may not) have their own
    >>hardware interrupt lines, but these are normal interrupts, not
    >>CPU-magic "systems management interrupts". So I think this isn't a
    >>problem.
    >>
    >>
    >
    >OK, thank you for the tutorial on the "other SMI"!
    >
    >
    Yeah, I've really misnamed this, unfortunately. Too many TLAs.

    >The comments about turning synchronize_rcu() into synchronize_sched()
    >and rcu_read_lock() into preempt_disable() do not apply, please ignore.
    >
    >However, I still do not understand how using RCU on cmd_rcvrs helps,
    >given that all of the accesses that I could see were already protected
    >by cmd_rcvrs_lock.
    >
    >Any further enlightenment available?
    >
    >
    The calls in handle_ipmb_get_msg_cmd and handle_lan_get_msg_cmd don't
    need spinlock protection, just an RCU read lock. Kind of the point of
    the RCU list. Thanks for spotting this.

    Thanks,

    -Corey
    -
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