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    SubjectRe: [PATCH 0/7] isdn4linux: add drivers for Siemens Gigaset ISDN DECT PABX
    On Fri, 03 Mar 2006 07:53:28 +0100, Arjan van de Ven wrote:

    > On Fri, 2006-03-03 at 00:03 +0100, Tilman Schmidt wrote:

    >> So you are saying that, for example
    >>
    >> spin_lock_irqsave(&cs->ev_lock, flags);
    >> head = cs->ev_head;
    >> tail = cs->ev_tail;
    >> spin_unlock_irqrestore(&cs->ev_lock, flags);
    >>
    >> is (mutatis mutandis) actually cheaper than
    >>
    >> head = atomic_read(&cs->ev_head);
    >> tail = atomic_read(&cs->ev_tail);
    >
    > atomic_read is special since it's not actually an atomic operation ;)
    > but.. think about it: you do 2 atomic reads, however there is ZERO
    > guarantee that the reads are atomic with respect to eachother; eg your
    > head and tail are not an atomic "snapshot" of these 2 variables!

    That's not a problem. It's a ringbuffer. It doesn't need an atomic
    snapshot of the reading and writing pointers together. Nothing breaks
    if a reader advances the read pointer while a writer is holding a
    local copy of it, or vice versa. The only thing we have to guard
    against is the result of an individual read operation being corrupted
    by a parallel write.

    So what's better in that case? Should we change these from atomic to
    spinlocked or not?

    [#define IFNULL*]
    >> Ok, these were mainly debugging aids. We'll just drop them and let the
    >> oops mechanism catch the (hopefully non-existent) remaining cases of
    >> pointers being unexpectedly NULL.
    >
    > you can also use WARN_ON() and BUG_ON() for that, you then get a more
    > readable oops message (with filename and line information)

    Actually, we won't. The IFNULL* macros were not only printing a message,
    but also taking evasive action in order to avoid dereferencing the NULL
    pointer. To achieve the same with WARN_ON() would require four lines of
    code for each occurrence, which IMHO is too much code clutter for a class
    of bugs which should be largely eradicated by now anyway.

    >> >> +void gigaset_dbg_buffer(enum debuglevel level, const unsigned char *msg,
    >> >> + size_t len, const unsigned char *buf, int from_user)
    >> >
    >> > such "from_user" parameter is highly evil, and also breaks sparse and
    >> > friends.. (btw please run sparse on the code and fix all warnings)
    >>
    >> Are you referring to anything in particular? We do run sparse regularly,
    >> and it did not emit any warnings for the submitted version, not even for
    >> this function. (But heaps of them for other parts of the kernel, if you
    >> pardon the remark.)
    >
    > msg should have the __user atribute here since it can be in userspace...
    > sometimes. It is the "sometimes" that is the bad idea!

    That's understood and will be fixed. I was just wondering whether your
    remark in parentheses was prompted by any particular sparse warnings you
    wanted us to fix and which for some reason we hadn't seen?

    > (GFP_ATOMIC is like borrowing from the VM, the VM will be in slight
    > imbalance afterwards. With GFP_KERNEL you allow the kernel to fix this
    > imbalance. A slight imbalance is fine and not a problem. Especially if
    > you give it the memory back soon. But if the imbalance can accumulate,
    > for example because you keep allocating and free the memory much later,
    > it can become a problem)

    Thanks muchly for that very lucid explanation. I see much clearer now
    in that area! :-)

    Regards
    Tilman

    --
    Tilman Schmidt E-Mail: tilman@imap.cc
    Bonn, Germany
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