linux-kernel - Re: [PATCH RESEND] lockdep: Pass lockdep expression to RCU lists

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Date:   Mon, 17 Feb 2020 16:12:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Amol Grover <frextrite@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Joel Fernandes <joel@...lfernandes.org>,
        Madhuparna Bhowmik <madhuparnabhowmik04@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH RESEND] lockdep: Pass lockdep expression to RCU lists

On Sun, Feb 16, 2020 at 01:16:36PM +0530, Amol Grover wrote:
> Data is traversed using hlist_for_each_entry_rcu outside an
> RCU read-side critical section but under the protection
> of either lockdep_lock or with irqs disabled.
> 
> Hence, add corresponding lockdep expression to silence false-positive
> lockdep warnings, and harden RCU lists. Also add macro for
> corresponding lockdep expression.
> 
> Two things to note:
> - RCU traversals protected under both, irqs disabled and
> graph lock, have both the checks in the lockdep expression.
> - RCU traversals under the protection of just disabled irqs
> don't have a corresponding lockdep expression as it is implicitly
> checked for.
> 
> Signed-off-by: Amol Grover <frextrite@...il.com>
> ---
>  kernel/locking/lockdep.c | 21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 32282e7112d3..696ad5d4daed 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -85,6 +85,8 @@ module_param(lock_stat, int, 0644);
>   * code to recurse back into the lockdep code...
>   */
>  static arch_spinlock_t lockdep_lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED;
> +#define graph_lock_held() \
> +	arch_spin_is_locked(&lockdep_lock)
>  static struct task_struct *lockdep_selftest_task_struct;
>  
>  static int graph_lock(void)
> @@ -1009,7 +1011,7 @@ static bool __check_data_structures(void)
>  	/* Check the chain_key of all lock chains. */
>  	for (i = 0; i < ARRAY_SIZE(chainhash_table); i++) {
>  		head = chainhash_table + i;
> -		hlist_for_each_entry_rcu(chain, head, entry) {
> +		hlist_for_each_entry_rcu(chain, head, entry, graph_lock_held()) {
>  			if (!check_lock_chain_key(chain))
>  				return false;
>  		}

URGH.. this patch combines two horribles to create a horrific :/

 - spin_is_locked() is an abomination
 - this RCU list stuff is just plain annoying

I'm tempted to do something like:

#define STFU (true)

	hlist_for_each_entry_rcu(chain, head, entry, STFU) {

Paul, are we going a little over-board with this stuff? Do we really
have to annotate all of this?