[lttng-dev] URCU accessing rcu_head->func

[Florian Lohoff]

Hi, Mathieu,

On Mon, Feb 03, 2014 at 03:37:38AM +0000, Mathieu Desnoyers wrote:
> > 
> > I am in the read side critical section so my structure
> > cant appear under my feet but the above can be racy at
> > best.
> 
> I guess you meant "disappear" here ?

Yep

> > What would be the best way to achieve something like:
> > 
> > 	"queue for call_rcu if not already queued"
> > 
> > Another flag with an atomic inc or something?
> 
> Indeed, the scheme you describe above seems to be racy, since there could be
> two or more read-side critical sections accessing the same object concurrently,
> and doing two call_rcu() enqueuing the same object (same linked list node) twice
> into the call_rcu lists will simply corrupt those lists.
> 
> Let me first rephrase your intent here just to make sure I understand: you use
> RCU read-side to provide existence guarantee on the "rs" object, and you want
> to enqueue the object rcuhead with call_rcu() for deletion (free_obj) after a
> grace period has elapsed.
> 
> Assuming you care about keeping this code path wait-free or lock-free (at least),
> using uatomic_xchg() would allow you to set an atomic flag and know, from the
> returned value, whether you "own" the object reclaim or not. You can then decide
> whether or not call_rcu() needs to be called based on the value read by
> uatomic_xchg().
> 
> You could do something similar with mutexes, but then you would lose the
> wait-freedom characteristic of this code path, which I am guessing is one
> key reason why you use RCU read-side lock and call_rcu() in the first place.
> 
> Does it make sense ?

I solved it a little different. I am using a RCU hash for keeping the 
data structures and i simply took the reasult of cds_lfht_del
or cds_lfht_add_replace.

If i successfully remove my structure from the hash i queue for later
destruction. As the removal should only happen successfully once i should
be done.

Its just a little bit more tricky to get correct. 

Flo
-- 
Florian Lohoff                                                 f at zz.de
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