Re: [PATCH 1/2] lockdep: Avoid to modify chain keys in validate_chain()

[Boqun Feng]

Hi Chris,

Could you try this to see if it fixes the problem? Thanks!

Regards,
Boqun

On Mon, Nov 02, 2020 at 01:37:41PM +0800, Boqun Feng wrote:
> Chris Wilson reported a problem spotted by check_chain_key(): a chain
> key got changed in validate_chain() because we modify the ->read in
> validate_chain() to skip checks for dependency adding, and ->read is
> taken into calculation for chain key since commit f611e8cf98ec
> ("lockdep: Take read/write status in consideration when generate
> chainkey").
> 
> Fix this by avoiding to modify ->read in validate_chain() based on two
> facts: a) since we now support recursive read lock detection, there is
> no need to skip checks for dependency adding for recursive readers, b)
> since we have a), there is only one case left (nest_lock) where we want
> to skip checks in validate_chain(), we simply remove the modification
> for ->read and rely on the return value of check_deadlock() to skip the
> dependency adding.
> 
> Reported-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> ---
> Peter,
> 
> I managed to get a reproducer for the problem Chris reported, please see
> patch #2. With this patch, that problem gets fixed.
> 
> This small patchset is based on your locking/core, patch #2 actually
> relies on your "s/raw_spin/spin" changes, thanks for taking care of that
> ;-)
> 
> Regards,
> Boqun
> 
>  kernel/locking/lockdep.c | 19 +++++++++----------
>  1 file changed, 9 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 3e99dfef8408..a294326fd998 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -2765,7 +2765,9 @@ print_deadlock_bug(struct task_struct *curr, struct held_lock *prev,
>   * (Note that this has to be done separately, because the graph cannot
>   * detect such classes of deadlocks.)
>   *
> - * Returns: 0 on deadlock detected, 1 on OK, 2 on recursive read
> + * Returns: 0 on deadlock detected, 1 on OK, 2 if another lock with the same
> + * lock class is held but nest_lock is also held, i.e. we rely on the
> + * nest_lock to avoid the deadlock.
>   */
>  static int
>  check_deadlock(struct task_struct *curr, struct held_lock *next)
> @@ -2788,7 +2790,7 @@ check_deadlock(struct task_struct *curr, struct held_lock *next)
>  		 * lock class (i.e. read_lock(lock)+read_lock(lock)):
>  		 */
>  		if ((next->read == 2) && prev->read)
> -			return 2;
> +			continue;
>  
>  		/*
>  		 * We're holding the nest_lock, which serializes this lock's
> @@ -3592,16 +3594,13 @@ static int validate_chain(struct task_struct *curr,
>  
>  		if (!ret)
>  			return 0;
> -		/*
> -		 * Mark recursive read, as we jump over it when
> -		 * building dependencies (just like we jump over
> -		 * trylock entries):
> -		 */
> -		if (ret == 2)
> -			hlock->read = 2;
>  		/*
>  		 * Add dependency only if this lock is not the head
> -		 * of the chain, and if it's not a secondary read-lock:
> +		 * of the chain, and if the new lock introduces no more
> +		 * lock dependency (because we already hold a lock with the
> +		 * same lock class) nor deadlock (because the nest_lock
> +		 * serializes nesting locks), see the comments for
> +		 * check_deadlock().
>  		 */
>  		if (!chain_head && ret != 2) {
>  			if (!check_prevs_add(curr, hlock))
> -- 
> 2.28.0
>