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Date:   Wed, 10 Jul 2019 13:30:02 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Yuyang Du <duyuyang@...il.com>
Cc:     peterz@...radead.org, will.deacon@....com, mingo@...nel.org,
        bvanassche@....org, ming.lei@...hat.com, frederic@...nel.org,
        tglx@...utronix.de, linux-kernel@...r.kernel.org,
        longman@...hat.com, paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH v3 30/30] locking/lockdep: Remove irq-safe to irq-unsafe
 read check

On Fri, Jun 28, 2019 at 05:15:28PM +0800, Yuyang Du wrote:
> We have a lockdep warning:
> 
>   ========================================================
>   WARNING: possible irq lock inversion dependency detected
>   5.1.0-rc7+ #141 Not tainted
>   --------------------------------------------------------
>   kworker/8:2/328 just changed the state of lock:
>   0000000007f1a95b (&(&host->lock)->rlock){-...}, at: ata_bmdma_interrupt+0x27/0x1c0 [libata]
>   but this lock took another, HARDIRQ-READ-unsafe lock in the past:
>    (&trig->leddev_list_lock){.+.?}
> 
> and interrupts could create inverse lock ordering between them.
> 
> other info that might help us debug this:
>    Possible interrupt unsafe locking scenario:
> 
>          CPU0                    CPU1
>          ----                    ----
>     lock(&trig->leddev_list_lock);
>                                  local_irq_disable();
>                                  lock(&(&host->lock)->rlock);
>                                  lock(&trig->leddev_list_lock);
>     <Interrupt>
>       lock(&(&host->lock)->rlock);
> 
>  *** DEADLOCK ***
> 
> This splat is a false positive, which is enabled by the addition of

If so, I think the better way is to reorder this patch before recursive
read lock suppport, for better bisect-ability.

Regards,
Boqun

> recursive read locks in the graph. Specifically, trig->leddev_list_lock is a
> rwlock_t type, which was not in the graph before recursive read lock support
> was added in lockdep.
> 
> This false positve is caused by a "false-positive" check in IRQ usage check.
> 
> In mark_lock_irq(), the following checks are currently performed:
> 
>    ----------------------------------
>   |   ->      | unsafe | read unsafe |
>   |----------------------------------|
>   | safe      |  F  B  |    F* B*    |
>   |----------------------------------|
>   | read safe |  F* B* |      -      |
>    ----------------------------------
> 
> Where:
> F: check_usage_forwards
> B: check_usage_backwards
> *: check enabled by STRICT_READ_CHECKS
> 
> But actually the safe -> unsafe read dependency does not create a deadlock
> scenario.
> 
> Fix this by simply removing those two checks, and since safe read -> unsafe
> is indeed a problem, these checks are not actually strict per se, so remove
> the macro STRICT_READ_CHECKS, and we have the following checks:
> 
>    ----------------------------------
>   |   ->      | unsafe | read unsafe |
>   |----------------------------------|
>   | safe      |  F  B  |      -      |
>   |----------------------------------|
>   | read safe |  F  B  |      -      |
>    ----------------------------------
> 
> Signed-off-by: Yuyang Du <duyuyang@...il.com>
> ---
>  kernel/locking/lockdep.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index c7ba647..d12ab0e 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -3558,8 +3558,6 @@ static int SOFTIRQ_verbose(struct lock_class *class)
>  	return 0;
>  }
>  
> -#define STRICT_READ_CHECKS	1
> -
>  static int (*state_verbose_f[])(struct lock_class *class) = {
>  #define LOCKDEP_STATE(__STATE) \
>  	__STATE##_verbose,
> @@ -3605,7 +3603,7 @@ typedef int (*check_usage_f)(struct task_struct *, struct held_lock *,
>  	 * Validate that the lock dependencies don't have conflicting usage
>  	 * states.
>  	 */
> -	if ((!read || STRICT_READ_CHECKS) &&
> +	if ((!read || !dir) &&
>  			!usage(curr, this, excl_bit, state_name(new_bit & ~LOCK_USAGE_READ_MASK)))
>  		return 0;
>  
> @@ -3616,7 +3614,7 @@ typedef int (*check_usage_f)(struct task_struct *, struct held_lock *,
>  		if (!valid_state(curr, this, new_bit, excl_bit + LOCK_USAGE_READ_MASK))
>  			return 0;
>  
> -		if (STRICT_READ_CHECKS &&
> +		if (dir &&
>  			!usage(curr, this, excl_bit + LOCK_USAGE_READ_MASK,
>  				state_name(new_bit + LOCK_USAGE_READ_MASK)))
>  			return 0;
> -- 
> 1.8.3.1
> 

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