Re: semantics of reader/writer semaphores in rt patch

[Chris Friesen]

On 10/25/2014 04:19 PM, Thomas Gleixner wrote:
> On Thu, 23 Oct 2014, Chris Friesen wrote:
>
> > I recently noticed that when CONFIG_PREEMPT_RT_FULL is enabled we the
> > semantics change.  From "include/linux/rwsem_rt.h":
> >
> >   * Note that the semantics are different from the usual
> >   * Linux rw-sems, in PREEMPT_RT mode we do not allow
> >   * multiple readers to hold the lock at once, we only allow
> >   * a read-lock owner to read-lock recursively. This is
> >   * better for latency, makes the implementation inherently
> >   * fair and makes it simpler as well.
> >
> > How is this valid?  It seems to me that there are any number of code paths
> > that could depend on having multiple threads of execution be able to hold the
> > reader lock simultaneously.  Something as simple as:
> >
> > thread A:
> > take rw_semaphore X for reading
> > take lock Y, modify data, release lock Y
> > wake up thread B
> > wait on conditional protected by lock Y
> > free rw_semaphore X
> >
> > thread B:
> > take rw_semaphore X for reading
> > wait on conditional protected by lock Y
> > send message to wake up thread A
> > free rw_semaphore X
>
> I don't see why B should wake A without changing the conditional. A
> won't make progress by being woken by B as the conditional does not
> magically change just because B wakes A.
>
> So what you wanted to say is:
>
>    thread B:
>    take rw_semaphore X for reading
>    wait on conditional protected by lock Y
> + take lock Y, modify data, release lock Y
>    send message to wake up thread A
>    free rw_semaphore X
>
> Otherwise your example does not make any sense at all. And that has
> some serious non RT related implications.


Yes, your reformulated version is what I meant to say.  Sorry for any 
confusion.


> > Does the RT kernel just disallow this sort of algorithm?
>
> Yes. For a good reason. Let's add thread C
>
> A    	   	B		C
> down_read(X)
> 				down_write(X)
> lock(Y)
> modify data
> unlock(Y)
> wake(B)
> 		down_read(X)
>
> Due to the mainline rwsem fairness semantics:
>
> A holds X, C is blocked on A and B is blocked on A.
>
> Deadlock, without RT and the single reader restriction being involved.


Crap, I had forgotten about the fairness semantics stuff.  That makes 
perfect sense.

Thanks for the explanation.

Chris

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