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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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