On Sun, Oct 26, 2014 at 08:42:57AM +0100, Peter Zijlstra wrote: > On Sun, Oct 26, 2014 at 12:21:31AM +0200, Thomas Gleixner wrote: > > 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? > > There were patches by ego that fix lockdep's read side tracking. I need > to find a few spare days to look at those :/ Hmm, that's only for the rwlock_t because that has 'creative' locking rules, I'm not sure why rwsem would need a distinction between read and write at all; so something simple like the below might just be all we need. Entirely untested.. --- include/linux/lockdep.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h index 74ab23176e9b..10606beb5672 100644 --- a/include/linux/lockdep.h +++ b/include/linux/lockdep.h @@ -505,7 +505,7 @@ static inline void print_irqtrace_events(struct task_struct *curr) #define rwsem_acquire(l, s, t, i) lock_acquire_exclusive(l, s, t, NULL, i) #define rwsem_acquire_nest(l, s, t, n, i) lock_acquire_exclusive(l, s, t, n, i) -#define rwsem_acquire_read(l, s, t, i) lock_acquire_shared(l, s, t, NULL, i) +#define rwsem_acquire_read(l, s, t, i) lock_acquire_exclusive(l, s, t, NULL, i) #define rwsem_release(l, n, i) lock_release(l, n, i) #define lock_map_acquire(l) lock_acquire_exclusive(l, 0, 0, NULL, _THIS_IP_) -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
