On Tue, 26 Jun 2018 10:22:01 +0200,
Shreyas NC wrote:
>
> > > +/**
> > > + * sdw_acquire_bus_lock: Acquire bus lock for all Master runtime(s)
> > > + *
> > > + * @stream: SoundWire stream
> > > + *
> > > + * Acquire bus_lock for each of the master runtime(m_rt) part of this
> > > + * stream to reconfigure the bus.
> > > + */
> > > +static void sdw_acquire_bus_lock(struct sdw_stream_runtime *stream)
> > > +{
> > > + struct sdw_master_runtime *m_rt = NULL;
> > > + struct sdw_bus *bus = NULL;
> > > +
> > > + /* Iterate for all Master(s) in Master list */
> > > + list_for_each_entry(m_rt, &stream->master_list, stream_node) {
> > > + bus = m_rt->bus;
> > > +
> > > + mutex_lock(&bus->bus_lock);
> > > + }
> > > +}
> >
> > So it's nested locks? Then you'd need some more trick to deal with
> > the lockdep. I guess you'll get the false-positive deadlock detection
> > by this code when the mutex lock debug is enabled.
> >
> > Also, is the linked order assured not to lead to a real deadlock?
> >
>
> Hi Takashi,
>
> Thanks for the review :)
>
> A multi link SoundWire stream consists of a list of Master runtimes and
> more importantly only one master runtime per SoundWire bus instance.
>
> So, these mutexes are actually different mutex locks(one per bus instance)
> and are not nested.
You take a mutex lock inside a mutex lock, so they are nested.
If they take the very same lock, it's called a "deadlock" instead.
> In SDW we have a bus instance per Master (link). In multi-link case, a
> stream may have multiple Masters, thus we need to lock all bus instances
> before we operate on them.
>
> Now since these are invoked from a stream (pcm ops) they will be always
> serialized and DPCM ensures we are never racing.
>
> We did add this note here and in Documentation to make it explicit.
Well, my question is whether the order to take the multiple locks is
always assured. You're calling like:
list_for_each_entry(m_rt, &stream->master_list, stream_node)
mutex_lock();
And it's a linked-list. If a stream has a link of masters like
M1->M2->M3 while another stream has a link like M2->M1->M3, it'll lead
to a deadlock with the concurrent calls above.
> > > +/**
> > > + * sdw_release_bus_lock: Release bus lock for all Master runtime(s)
> > > + *
> > > + * @stream: SoundWire stream
> > > + *
> > > + * Release the previously held bus_lock after reconfiguring the bus.
> > > + */
> > > +static void sdw_release_bus_lock(struct sdw_stream_runtime *stream)
> > > +{
> > > + struct sdw_master_runtime *m_rt = NULL;
> > > + struct sdw_bus *bus = NULL;
> > > +
> > > + /* Iterate for all Master(s) in Master list */
> > > + list_for_each_entry(m_rt, &stream->master_list, stream_node) {
> > > + bus = m_rt->bus;
> > > + mutex_unlock(&bus->bus_lock);
> > > + }
> >
> > ... and this looks bad. The loop for unlocking should be traversed
> > reversely.
> >
>
> Yes in principle I agree locking should be in reverse, but as explained
> above in this case, it does not matter much :)
It does matter when you dealing with the multiple nested mutexes...
thanks,
Takashi
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