On Sun, 26 Oct 2014, Thomas Gleixner wrote:
> On Thu, 23 Oct 2014, Chris Friesen wrote:
> > 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.
>
> So RT does not violate ANY of the existing mainline semantics, it just
> imposes a performance impact of not allowing multiple readers.
@peterz: It might be worthwhile to have a CONFIG_LOCKDEP=y dependent
mode which restricts concurrent readers to 1 in mainline to catch this
kind of stuff. Hmm?
Thanks,
tglx
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