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From: "Paul E. McKenney" <firstname.lastname@example.org> To: Andrea Parri <email@example.com> Cc: Joel Fernandes <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, Josh Triplett <email@example.com>, Steven Rostedt <firstname.lastname@example.org>, Mathieu Desnoyers <email@example.com>, Lai Jiangshan <firstname.lastname@example.org>, Jonathan Corbet <email@example.com>, firstname.lastname@example.org Subject: Re: [PATCH] doc/rcu: Correct field_count field naming in examples Date: Sat, 11 May 2019 17:41:31 -0700 [thread overview] Message-ID: <20190512004131.GE3923@linux.ibm.com> (raw) In-Reply-To: <20190511221126.GA3984@andrea> On Sun, May 12, 2019 at 12:11:26AM +0200, Andrea Parri wrote: > Hi Paul, Joel, > > > > > On the other hand, would you have ideas for more modern replacement > > > > examples? > > > > > > There are 3 cases I can see in listRCU.txt: > > > (1) action taken outside of read_lock (can tolerate stale data), no in-place update. > > > this is the best possible usage of RCU. > > > (2) action taken outside of read_lock, in-place updates > > > this is good as long as not too many in-place updates. > > > involves copying creating new list node and replacing the > > > node being updated with it. > > > (3) cannot tolerate stale data: here a deleted or obsolete flag can be used > > > protected by a per-entry lock. reader > > > aborts if object is stale. > > > > > > Any replacement example must make satisfy (3) too? > > > > It would be OK to have a separate example for (3). It would of course > > be nicer to have one example for all three, but not all -that- important. > > > > > The only example for (3) that I know of is sysvipc sempahores which you also > > > mentioned in the paper. Looking through this code, it hasn't changed > > > conceptually and it could be a fit for an example (ipc_valid_object() checks > > > for whether the object is stale). > > > > That is indeed the classic canonical example. ;-) > > > > > The other example could be dentry look up which uses seqlocks for the > > > RCU-walk case? But that could be too complex. This is also something I first > > > learnt from the paper and then the excellent path-lookup.rst document in > > > kernel sources. > > > > This is a great example, but it would need serious simplification for > > use in the Documentation/RCU directory. Note that dcache uses it to > > gain very limited and targeted consistency -- only a few types of updates > > acquire the write-side of that seqlock. > > > > Might be quite worthwhile to have a simplified example, though! > > Perhaps a trivial hash table where write-side sequence lock is acquired > > only when moving an element from one chain to another? > > Sorry to take you down here..., but what do you mean by "the paper"? ;-/ One or both of these two: http://www2.rdrop.com/~paulmck/techreports/survey.2012.09.17a.pdf http://www2.rdrop.com/~paulmck/techreports/RCUUsage.2013.02.24a.pdf Thanx, Paul
next prev parent reply other threads:[~2019-05-12 0:41 UTC|newest] Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-05-05 2:03 [PATCH] doc/rcu: Correct field_count field naming in examples Joel Fernandes (Google) 2019-05-07 0:04 ` Paul E. McKenney 2019-05-08 16:26 ` Joel Fernandes 2019-05-08 18:16 ` Paul E. McKenney 2019-05-11 22:11 ` Andrea Parri 2019-05-12 0:41 ` Paul E. McKenney [this message] 2019-05-12 1:09 ` Andrea Parri 2019-05-13 3:43 ` Joel Fernandes 2019-05-14 22:13 ` Paul E. McKenney 2019-05-25 10:07 ` Joel Fernandes
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