Messages in this thread | | | From | Joel Fernandes <> | Date | Sun, 23 Sep 2018 22:25:44 -0400 | Subject | Re: Question about ->head field of rcu_segcblist |
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On Sun, Sep 23, 2018 at 7:54 PM Paul E. McKenney <paulmck@linux.ibm.com> wrote: > > On Sun, Sep 23, 2018 at 07:30:30PM -0400, Joel Fernandes wrote: > > Hi Paul, > > > > I was parsing the Data-Structures document and had a question about > > the following "Important note" text. > > > > Could it be clarified in the below text better why "remaining > > callbacks are placed back on the RCU_DONE_TAIL segment", is a reason > > for not depending on ->head for determining if no callbacks are > > associated with the rcu_segcblist? If callbacks are added back to the > > DONE_TAIL segment, then I would think rcu_head should be != NULL. > > Infact the "rsclp->head = *rsclp->tails[RCU_DONE_TAIL];" in > > rcu_segcblist_extract_done_cbs should set the ->head to NULL if I > > understand correctly. > > The rcu_segcblist_extract_done_cbs() function will set rsclp->head > to NULL only if there were no non-done callbacks on the rsclp list. > Otherwise, if there are non-done callbacks, then rsclp->head will > be set to the first non-done callback. > > Either way, the problem is that the done callbacks can be removed > and re-added, but the count is not adjusted until the re-add. So > you have to look at the count to see if there are callbacks. > > Testing rsclp->head fails because it can be temporarily NULL, even > though there are callbacks hanging off of a pointer in rcu_do_batch()'s > stack frame. > > Or am I misunderstanding your question?
Thanks yes that clears it up, I see what you mean that that ->head field is temporarily volatile and really the ->len tells the real story :-)
- Joel
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