linux-kernel - Re: [PATCH v5 6/7] locking/lockdep: Reuse freed chain

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Date:   Tue, 4 Feb 2020 16:42:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH v5 6/7] locking/lockdep: Reuse freed chain_hlocks entries

On Mon, Feb 03, 2020 at 11:41:46AM -0500, Waiman Long wrote:
> +	/*
> +	 * We require a minimum of 2 (u16) entries to encode a freelist
> +	 * 'pointer'.
> +	 */
> +	req = max(req, 2);


Would something simple like the below not avoid that whole 1 entry
'chain' nonsense?

It boots and passes the selftests, so it must be perfect :-)

--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3163,7 +3163,7 @@ static int validate_chain(struct task_st
 	 * (If lookup_chain_cache_add() return with 1 it acquires
 	 * graph_lock for us)
 	 */
-	if (!hlock->trylock && hlock->check &&
+	if (!chain_head && !hlock->trylock && hlock->check &&
 	    lookup_chain_cache_add(curr, hlock, chain_key)) {
 		/*
 		 * Check whether last held lock: