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 [email protected]
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