Messages in this thread | | | From | Oliver Neukum <> | Subject | Re: refcounting drivers' data structures used in sysfs buffers | Date | Mon, 12 Mar 2007 16:23:57 +0100 |
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Am Montag, 12. März 2007 15:57 schrieb Alan Stern:probably nece > On Mon, 12 Mar 2007, Oliver Neukum wrote: > > > > > Why? What's wrong with simply calling kref_get/put? > > > > > > It's the same old problem: the race between unbind and sysfs I/O. What > > > good does holding a reference to the private data structure do if the > > > show/store method gets called after the driver has been unbound from the > > > device? dev_get_drvdata() will no longer provide a valid pointer to the > > > private data, so the method will have no way to access it. Hence the > > > method needs another argument. > > > > It does half the job. You can make sure the driver is not asked to access > > freed memory. > > It is true that a driver will have to mark that device "disconnected" > > and return errors if that device's attributes are referenced, but this can > > be done internally. > > No, you're missing the point. Let's say driver A's disconnect() is > called, so the driver marks its private data structure as "disconnected" > and does dev_set_drvdata(NULL). Then driver B is probed and bound to the > device, and it does its own dev_set_drvdata(). Then a user still holding > an open sysfs file reference for driver A calls a show() or store() > method. The method will do dev_get_drvdata(), receiving the pointer to > driver B's private data. Now you're in trouble, because A's method will > think it owns B's private data!
Yes, I was missing the point. In consequence, drivers must not use dev_get_drvdata() to get their references to their private data. It's probably necessary to store it in struct sysfs_buffer and include that in the store/show callbacks. (The same does apply to interfaces of course)
> > Yes, this is a bit more complicated. > > {rant mode} > > Who came up with the idea of making life simpler by adding a code path? > > All these problems were already solved for device nodes. Ioctl is ugly, but > > at least a known code path. > > {rant off} > > I'll let Greg give the complete answer. :-) Bear in mind, however, that > the aim was probably to make life simpler for userspace -- which does not > mean making life simpler for the kernel.
That doesn't mean that the method needed to be thrown out. Sysfs could simply pass through the syscalls for a device, like it is done in character devices. I am tempted to recommend such radical surgery.
> (Incidentally, I'm not so sure that all these problems really were solved > by ioctl on device nodes. I bet you could find plenty of cases where > ioctl races with disconnect if you looked.)
I will look. Death to all race conditions.
Regards Oliver - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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