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McKenney" Cc: LKML , rcu , Uladzislau Rezki , Neeraj Upadhyay , Boqun Feng , Joel Fernandes Subject: Re: [PATCH 1/4] rcu/nocb: Protect lazy shrinker against concurrent (de-)offloading Message-ID: References: <20230322194456.2331527-1-frederic@kernel.org> <20230322194456.2331527-2-frederic@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org Le Fri, Mar 24, 2023 at 03:51:54PM -0700, Paul E. McKenney a écrit : > On Fri, Mar 24, 2023 at 11:09:08PM +0100, Frederic Weisbecker wrote: > > Le Wed, Mar 22, 2023 at 04:18:24PM -0700, Paul E. McKenney a écrit : > > > > @@ -1336,13 +1336,25 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc) > > > > unsigned long flags; > > > > unsigned long count = 0; > > > > > > > > + /* > > > > + * Protect against concurrent (de-)offloading. Otherwise nocb locking > > > > + * may be ignored or imbalanced. > > > > + */ > > > > + mutex_lock(&rcu_state.barrier_mutex); > > > > > > I was worried about this possibly leading to out-of-memory deadlock, > > > but if I recall correctly, the (de-)offloading process never allocates > > > memory, so this should be OK? > > > > Good point. It _should_ be fine but like you, Joel and Hillf pointed out > > it's asking for trouble. > > > > We could try Joel's idea to use mutex_trylock() as a best effort, which > > should be fine as it's mostly uncontended. > > > > The alternative is to force nocb locking and check the offloading state > > right after. So instead of: > > > > rcu_nocb_lock_irqsave(rdp, flags); > > //flush stuff > > rcu_nocb_unlock_irqrestore(rdp, flags); > > > > Have: > > > > raw_spin_lock_irqsave(rdp->nocb_lock, flags); > > if (!rcu_rdp_is_offloaded(rdp)) > > raw_spin_unlock_irqrestore(rdp->nocb_lock, flags); > > continue; > > } > > //flush stuff > > rcu_nocb_unlock_irqrestore(rdp, flags); > > > > But it's not pretty and also disqualifies the last two patches as > > rcu_nocb_mask can't be iterated safely anymore. > > > > What do you think? > > The mutex_trylock() approach does have the advantage of simplicity, > and as you say should do well given low contention. > > Which reminds me, what sort of test strategy did you have in mind? > Memory exhaustion can have surprising effects. The best I can do is to trigger the count and scan callbacks through the shrinker debugfs and see if it crashes or not :-) > > > > > /* Snapshot count of all CPUs */ > > > > for_each_possible_cpu(cpu) { > > > > struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu); > > > > - int _count = READ_ONCE(rdp->lazy_len); > > > > + int _count; > > > > + > > > > + if (!rcu_rdp_is_offloaded(rdp)) > > > > + continue; > > > > > > If the CPU is offloaded, isn't ->lazy_len guaranteed to be zero? > > > > > > Or can it contain garbage after a de-offloading operation? > > > > If it's deoffloaded, ->lazy_len is indeed (supposed to be) guaranteed to be zero. > > Bypass is flushed and disabled atomically early on de-offloading and the > > flush resets ->lazy_len. > > Whew! At the moment, I don't feel strongly about whether or not > the following code should (1) read the value, (2) warn on non-zero, > (3) assume zero without reading, or (4) some other option that is not > occurring to me. Your choice! (2) looks like a good idea! Thanks.