On Wed, Feb 09, 2011 at 04:10:55PM -0800, Ira W. Snyder wrote: > On Wed, Feb 09, 2011 at 03:42:31PM -0800, Dmitry Torokhov wrote: > > On Wed, Feb 09, 2011 at 03:35:45PM -0800, Ira W. Snyder wrote: > > > On Wed, Feb 09, 2011 at 10:27:40AM -0800, Dmitry Torokhov wrote: > > > > > > [ snip stuff I've already fixed in the next version ] > > > > > > > > > > > > > The requirement is that the device stay open during reconfiguration. > > > > > This provides for that. Readers just block for as long as the device > > > > > is > > > > > not producing data. > > > > > > > > OK, you still need to make sure you do not touch free/used buffer while > > > > device is disabled. Also, you need to kick readers if you unbind the > > > > driver, so maybe a new flag priv->exists should be introduced and > > > > checked. > > > > > > > > > > I don't understand what you mean by "kick readers if you unbind the > > > driver". The kernel automatically increases the refcount on a module > > > when a process is using the module. This shows up in the "Used by" > > > column of lsmod's output. > > > > > > The kernel will not let you rmmod a module with a non-zero refcount. You > > > cannot get into the situation where you have rmmod'ed the module and a > > > reader is still blocking in read()/poll(). > > > > However you can still unbind the driver from the device by writing into > > driver's sysfs 'unbind' attribute. > > > > See drivers/base/bus.c::driver_unbind(). > > > > I was completely unaware of that "feature". I hunch that many drivers > are incapable of dealing with an unbind while they are still open.
Hmm, maybe older drivers... Anythig hotpluggable (USB, PCI, etc) should be in a better shape because they expect to be yanked at any time. > > Matter of fact, I don't see how this can EVER be safe. The driver core > automatically calls the data_of_remove() routine while there are still > blocked readers. This kfree()s the private data structure, which > contains the suggested priv->exists flag. What happens if the memory > allocator re-allocates that memory to a different driver before the > reader process is woken up to check the priv->exists flag? > > The only way to solve this is to count the number of open()s and > close()s, and block the unbind until all users have close()d the device. > Yes, you can kick readers and wait, or you can refcount that private structure and have readers grab a reference when they open your device and drop it in their fops->release() method. Your remove() should also drop reference instead of doing kfree() outright. Thanks. -- Dmitry _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
