Re: [PATCH v3 7/9] liblockdep: Support using LD_PRELOAD

[Peter Zijlstra <peterz@infradead.org>]

On Fri, May 10, 2013 at 12:06:46PM -0400, Sasha Levin wrote:
> On 05/10/2013 09:57 AM, Peter Zijlstra wrote:
> > So you're doing instance tracking and not creating classes like the kernel
> > lockdep does? While that reduces false positives it also greatly reduces the
> > effectiveness of lockdep.
> > 
> > The power of lock-classes is that it increases the chance of catching potential
> > deadlocks without there ever actually being a deadlock.
> 
> Originally I had classes working as you've pointed out, until the first time I've
> tried running lockdep on qemu.
> 
> They appear to have wrappers for every api call known to man, including all the
> posix locking apis.
> 
> Basically, instead of directly calling pthread_mutex_lock() for example, there's
> a wrapper named qemu_mutex_lock() that calls the api above:
> 
> 	void qemu_mutex_lock(QemuMutex *mutex)
> 	{
> 	    int err;
> 	
> 	    err = pthread_mutex_lock(&mutex->lock);
> 	    if (err)
> 	        error_exit(err, __func__);
> 	}
> 
> So as you might imagine, the first time I ran it my log exploded with warnings.
> 
> I've poked around the source of other big projects, and the example above is
> somewhat common with projects that wrap everything to be compatible with different
> architectures or apis - which is something that doesn't happen in the kernel.

Urgh.. yes that might be a problem. Still it is something that should at least
be clearly stated somewhere (the Changelog for one).

Not being able to do classes sucks though :/

Hmm, we could do something like:

  $ LIBLOCKDEP_CLASS_DEPTH=n LD_PRELOAD=liblockdep.so my_app

where an @n of -1 would indicate per-instance classes and 0+ would be the
__builtin_return_address(n). That way, the above qemu thing should work with 1;
which should be the return address of the wrapper.

Of course, projects mixing different wrapper depths will be immense 'fun' :/

We could make it even worse and make the depth depend on the DSO name.. /me
runs like crazeh :-)