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Date:   Thu, 9 Jun 2022 14:53:02 +0200
From:   Marco Elver <elver@google.com>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        linux-perf-users@vger.kernel.org, x86@kernel.org,
        linux-sh@vger.kernel.org, kasan-dev@googlegroups.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/8] perf/hw_breakpoint: Optimize list of per-task
 breakpoints
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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().