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    SubjectRe: [RFC/PATCH 0/22] W1: sysfs, lifetime and other fixes
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    On Tue, 2005-04-26 at 01:50 -0500, Dmitry Torokhov wrote:
    > On Tuesday 26 April 2005 01:43, Evgeniy Polyakov wrote:
    > > On Mon, 2005-04-25 at 15:22 -0500, Dmitry Torokhov wrote:
    > > > On 4/25/05, Evgeniy Polyakov <johnpol@2ka.mipt.ru> wrote:
    > > > > While thinking about locking schema
    > > > with respect to sysfs files I recalled,
    > > > > why I implemented such a logic -
    > > > > now one can _always_ remove _any_ module
    > > > > [corresponding object is removed from accessible
    > > > > pathes and waits untill all exsting users are gone],
    > > > > which is very good - I really like it in networking model,
    > > > > while with whole device driver model
    > > > > if we will read device's file very quickly
    > > > > in several threads we may end up not unloading it at all.
    > > >
    > > > I am sorrry, that is complete bull*. sysfs also allows removing
    > > > modules at an arbitrary time (and usually without annoying "waiting
    > > > for refcount" at that)... You just seem to not understand how driver
    > > > code works, thus the need of inventing your own schema.
    > >
    > > Ok, let's try again - now with explanation,
    > > since it looks like you did not even try to understand what I said.
    > > If you will remove objects from ->remove() callback
    > > you may end up with rmmod being stuck.
    > > Explanation: each read still gets reference counter,
    > > while in rmmod path there is a wait until it is zero.
    > > If there are too many simultaneous reads - even
    > > if each will put reference counter at the end, we still can have
    > > non zero refcnt each time we check it in rmmod path.
    > > That is why object must be removed from accessible pathes
    > > first, and only freed in ->remove() callback.
    >
    > Please try to read the code. device_unregister and kobject_unregister
    > do not require caller to wait for the last reference to drop, they rely
    > on availability of release method to clean up the object when last user
    > is gone. driver_unregister is blocking (like your family code) but
    > teardown takes no time. If driver is in use (attributes are open) then
    > module refcount is non-zero and instead of (possibly endless) "waiting for
    > refcount to drop" message you will get nice -EBUSY.
    >
    > If you program so that you wait in module_exit for object release - you
    > get what you deserve.

    But we can remove objects not from rmmod path.
    You pointed right example in one previous e-mail.

    Using above "waiting for device..." message is for debug only.

    > > > BTW, I am looking at the connector code ATM and I am just amazed at
    > > > all wied refounting stuff that is going on there. what a single
    > > > actomic_dec_and_test() call without checkng reurn vaue is supposed to
    > > > do again?
    > >
    > > It has explicit barrieres, which guarantees that
    > > there will not be atomic operation vs. non atomic
    > > reordering.
    >
    > And you can't use explicit barriers - why exactly?

    I used them - code was following:
    smp_mb__before_atomic_dec();
    atomic_dec();
    smp_mb__after_atomic_dec();

    I think simple atomic_dec_and_test() or even atomic_dec_and_lock()
    is better.

    --
    Evgeniy Polyakov

    Crash is better than data corruption -- Arthur Grabowski
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