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

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Date:   Tue, 4 Feb 2020 11:26:56 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 2/4/20 11:12 AM, Waiman Long wrote:
> On 2/4/20 10:42 AM, Peter Zijlstra wrote:
>> 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:
>>
> Well, I think that will eliminate the 1-entry chains for the process
> context. However, we can still have 1-entry chain in the irq context, I
> think, as long as there are process context locks in front of it.
>
> I think this fix is still worthwhile as it will eliminate some of the
> 1-entry chains.

Sorry, I think I mis-read the code. This patch will eliminate some
cross-context check. How  about something like

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 32406ef0d6a2..d746897b638f 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -2931,7 +2931,7 @@ static int validate_chain(struct task_struct *curr,
         * (If lookup_chain_cache_add() return with 1 it acquires
         * graph_lock for us)
         */
-       if (!hlock->trylock && hlock->check &&
+       if ((chain_head != 1) && !hlock->trylock && hlock->check &&
            lookup_chain_cache_add(curr, hlock, chain_key)) {
                /*
                 * Check whether last held lock:
@@ -3937,7 +3937,7 @@ static int __lock_acquire(struct lockdep_map
*lock, unsign
        hlock->prev_chain_key = chain_key;
        if (separate_irq_context(curr, hlock)) {
                chain_key = INITIAL_CHAIN_KEY;
-               chain_head = 1;
+               chain_head = 2; /* Head of irq context chain */
        }
        chain_key = iterate_chain_key(chain_key, class_idx);
 
Cheers,
Longman