Messages in this thread | | | From | Dmitry Vyukov <> | Date | Thu, 9 Jun 2022 15:05:40 +0200 | Subject | Re: [PATCH 1/8] perf/hw_breakpoint: Optimize list of per-task breakpoints |
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On Thu, 9 Jun 2022 at 14:53, Marco Elver <elver@google.com> wrote: > > On Thu, Jun 09, 2022 at 02:30PM +0200, Dmitry Vyukov wrote: > [...] > > > + rcu_read_lock(); > > > > Why do we need rcu_read_lock() here? > > The patch does not change anything with respect to locking, so all > > accesses to the container should still be protected by nr_bp_mutex. > > Similarly for the rcu variant of for_each below. > [...] > > > + head = rhltable_lookup(&task_bps_ht, &bp->hw.target, task_bps_ht_params); > > > + if (!head) > > > + goto out; > > > + > > > + rhl_for_each_entry_rcu(iter, pos, head, hw.bp_list) { > > It's part of rhashtable's interface requirements: > > /** > * rhltable_lookup - search hash list table > * @hlt: hash table > * @key: the pointer to the key > * @params: hash table parameters > * > * Computes the hash value for the key and traverses the bucket chain looking > * for a entry with an identical key. All matching entries are returned > * in a list. > * > * This must only be called under the RCU read lock. > * > * Returns the list of entries that match the given key. > */ > > Beyond that, even though there might not appear to be any concurrent > rhashtable modifications, it'll be allowed in patch 6/8. Furthermore, > rhashtable actually does concurrent background compactions since I > selected 'automatic_shrinking = true' (so we don't leak tons of memory > after starting and killing those 1000s of tasks) -- there's this > call_rcu() in lib/rhashtable.c that looks like that's when it's used. > This work is done in a deferred work by rht_deferred_worker().
I see. Thanks.
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
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