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    SubjectRe: per cpun+ spin locks coexistence?
    Peter Teoh a écrit :
    > Thanks for the explanation, much apologies for this newbie discussion.
    > But I still find it inexplicable:
    >
    > On Mon, Mar 17, 2008 at 4:20 AM, Johannes Weiner <hannes@saeurebad.de> wrote:
    >
    >> A per-cpu variable is basically an array the size of the number of
    >> possible CPUs in the system. get_cpu_var() checks what current CPU we
    >> are running on and gets the array-element corresponding to this CPU.
    >>
    >> So, really oversimplified, get_cpu_var(foo) translates to something like
    >> foo[smp_processor_id()].
    >>
    >>
    >
    > Ok, so calling get_cpu_var() always return the array-element for the
    > current CPU, and since by design, only the current CPU can
    > modify/write to this array element (this is my assumption - correct?),
    > and the other CPU will just read it (using the per_cpu construct).
    > So far correct? So why do u still need to spin_lock() to lock other
    > CPU from accessing - the other CPU will always just READ it, so just
    > go ahead and let them read it. Seemed like it defeats the purpose of
    > get_cpu_var()'s design?
    >
    > But supposed u really want to put a spin_lock(), just to be sure
    > nobody is even reading it, or modifying it, so then what is the
    > original purpose of get_cpu_var() - is it not to implement something
    > that can be parallelized among different CPU, without affecting each
    > other, and using no locks?
    >
    > The dual use of spin_lock+get_cpu_var() confuses me here :-). (not
    > the per_cpu(), which I agree is supposed to be callabe from all the
    > different CPU, for purpose of enumeration or data collection).
    >
    >
    You are right Peter, that fs/file.c contains some leftover from previous
    implementation of defer queue,
    that was using a timer.

    So we can probably provide a patch that :

    - Use spin_lock() & spin_unlock() instead of spin_lock_bh() &
    spin_unlock_bh() in free_fdtable_work()
    since we dont anymore use a softirq (timer) to reschedule the workqueue.

    ( this timer was deleted by the following patch :
    http://readlist.com/lists/vger.kernel.org/linux-kernel/50/251040.html


    But, you cannot avoid use of spin_lock()/spin_unlock() because
    schedule_work() makes no garantee that the work will be done by this cpu.

    (free_fdtable_work() can be called to flush the fd defer queue of CPU X
    on behalf CPU Y, with X != Y .
    You then can have a corruption because CPU X is inside
    free_fdtable_rcu() and CPU Y is inside free_fdtable_work() )

    So both spin_lock() and get_cpu_var() are necessary :

    - One to get the precpu data for optimal performance on SMP (but not
    mandatory)
    - One to protect the data from corruption on SMP.




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