Re: semantics of reader/writer semaphores in rt patch

[Peter Zijlstra]

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_)
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