Messages in this thread |  | | Date | Fri, 17 Jan 2014 13:01:49 -0500 (EST) | From | Alan Stern <> | Subject | Re: check && lockdep_no_validate (Was: lockdep: Introduce wait-type checks) |
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On Fri, 17 Jan 2014, Oleg Nesterov wrote:
> On 01/16, Alan Stern wrote: > > > > On Thu, 16 Jan 2014, Peter Zijlstra wrote: > > > > > On Thu, Jan 16, 2014 at 06:43:48PM +0100, Oleg Nesterov wrote: > > > > > > Perhaps we should change the meaning of lockdep_set_novalidate_class? > > > > (perhaps with rename). What do you think about the patch below? > > > > > > > > With this patch __lockdep_no_validate__ means "automatically nested", > > > > > > Yes, I suppose that might work, it would allow some validation. > > > > I haven't seen the patch, but I'm not so sure it will work. Suppose we > > have two devices, D1 and D2, and some other mutex, M. Then the locking > > pattern: > > > > lock(D1); > > lock(M); > > unlock(M); > > unlock(D1); > > > > generally should not conflict with: > > > > lock(M); > > lock(D2); > > unlock(D2); > > unlock(M); > > Yes, sure. This change assumes that the only problem in drivers/base is > dev->parent->mutex / dev->mutex dependency. If the locking is even more > "broken" (wrt lockdep), we can't replace lockdep_set_novalidate_class() > with lockdep_set_auto_nested().
I suspect it is even more "broken". But I can't point to specific examples.
> And, otoh, with this change lockdep can miss the real problems too, for > example: > > func1(dev) > { > device_lock(dev->parent); > mutex_lock(MUTEX); > device_lock(dev); > ... > } > > func2(dev) > { > device_lock(dev); > mutex_lock(MUTEX); > ... > } > > lockdep will only notice dev -> MUTEX dependency. > > I booted the kernel (under kvm) with this change and there is nothing > in dmesg, but of course this is not the real testing. > > So do you think that dev->mutex should not be validated at all ?
My guess is that if your change is deployed widely, there will be reports of violations. That's only a guess.
Still, you could go ahead and try it, just to see what happens.
> Just in case... Of course, if we actually add auto_nested we should not > use a single class unless dev->mutex will be the only user.
That's a good point. I don't know of any other classes using LOCKDEP_NO_VALIDATE, but there may be one or two.
Also, take a look at commit 356c05d58af0. It's a similar situation (not exactly the same).
Alan Stern
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