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 <[email protected]> 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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