linux-kernel - Re: [PATCH v6 00/16] locking/lockdep: Add support for dynamic keys

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Date:   Sun, 3 Feb 2019 09:36:38 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, tj@...nel.org, longman@...hat.com,
        johannes.berg@...el.com, linux-kernel@...r.kernel.org,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v6 00/16] locking/lockdep: Add support for dynamic keys

On 2/1/19 4:15 AM, Peter Zijlstra wrote:
> On Fri, Jan 18, 2019 at 06:34:20PM -0800, Bart Van Assche wrote:
>> I agree with what you wrote. The only code I know of that accesses list
>> entries using RCU is the __bfs() function. In that function I found the
>> following loop:
>>
>> 	list_for_each_entry_rcu(entry, head, entry) { [ ... ] }
> 
> Thing is; I can't seem to find any __bfs() usage outside of graph_lock.
> 
>    count_{fwd,bwd}_deps() - takes graph lock
> 
>    check_{noncircular,redudant}() - called from check_prev_add() <-
>    check_prevs_add() <- validate_chain() which takes graph lock
> 
>    find_usage{,_fwd,_bwd}
>      <- check_usage() <- check_irq_usage() <- check_prev_add_irq() <-
>      check_prev_add <- check_prevs_add() <- validate_chain() which takes
>      graph lock
> 
>      <- check_usage_{fwd,bdw}() <- mark_lock_irq() <- mark_lock() which
>      takes graph lock
> 
> Or did I miss something? If there are no __bfs() users outside of graph
> lock, then we can simply remove that _rcu from the iteration, and
> simplify all that.

Every time I make a single change to the lockdep code I have to rerun my 
test case for a week to make sure that no regressions have been 
introduced. In other words, I can make further changes but that could 
take some time. Do you want me to look into this simplification now or 
after this patch series went upstream?

>> Since zap_class() calls list_del_rcu(&entry->entry), since a grace period
>> occurs between the call_rcu() invocation and the RCU callback function,
>> since at least an RCU reader lock must be held around RCU loops and since
>> sleeping is not allowed while holding an RCU read lock I think there is
>> no risk that __bfs() will examine a list entry after it has been freed.
> 
> So you agree that list_entry_being_freed() should only check the current
> pf?

Sorry if I wasn't clear enough. In a previous e-mail I tried to explain 
that both pf's have to be checked. Another way to explain that is as 
follows:
- Each list entry has one of the following states: free, in use or being
   freed.
- "Free" means that the corresponding bit in the list_entries_in_use
   bitmap has not been set.
- "In use" means that the corresponding bit in the list_entries_in_use
   bitmap has been set and that none of the corresponding bits in the
   list_entries_being_freed bitmaps have been set.
- "Being freed" means that the corresponding bit in one of the
   list_entries_being_freed bitmaps has been set.

Since it can happen that multiple elements of the pending_free[] array 
are in the state where call_rcu() has been called but the RCU callback 
function has not yet been called, I think that zap_class() must check 
the list_entries_being_freed bitmaps in all pending_free[] array elements.

Thanks,

Bart.