From mboxrd@z Thu Jan 1 00:00:00 1970 From: f@zz.de (Florian Lohoff) Date: Mon, 3 Feb 2014 08:52:59 +0100 Subject: [lttng-dev] URCU accessing rcu_head->func In-Reply-To: <582363683.17541.1391398658096.JavaMail.zimbra@efficios.com> References: <20140131201632.GA32482@pax.zz.de> <582363683.17541.1391398658096.JavaMail.zimbra@efficios.com> Message-ID: <20140203075259.GA25614@pax.zz.de> 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 -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 828 bytes Desc: Digital signature URL: