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Date:   Wed, 29 May 2019 19:37:45 +0800
From:   Boqun Feng <boqun.feng@gmail.com>
To:     Yuyang Du <duyuyang@gmail.com>
Cc:     peterz@infradead.org, will.deacon@arm.com, mingo@kernel.org,
        bvanassche@acm.org, ming.lei@redhat.com, frederic@kernel.org,
        tglx@linutronix.de, paulmck@linux.ibm.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 02/17] locking/lockdep: Add read-write type for
 dependency
Message-ID: <20190529112321.GB5929@tardis>
References: <20190516080015.16033-1-duyuyang@gmail.com>
 <20190516080015.16033-3-duyuyang@gmail.com>
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On Thu, May 16, 2019 at 04:00:00PM +0800, Yuyang Du wrote:
> Direct dependency needs to keep track of its locks' read-write types. A
> union field is added to lock_list struct so the type is stored there as
> this:
>=20
> 	lock_type[1] (u16), lock_type[0] (u16)
>=20
> 			or:
>=20
> 	dep_type (int)
>=20
> where value:
>=20
>  0: exclusive / write
>  1: read
>  2: recursive read
>=20
> Note that (int) dep_type value may vary with different architectural
> endianness, so use helper functions to operate on these types.
>=20
> Signed-off-by: Yuyang Du <duyuyang@gmail.com>
> ---
>  include/linux/lockdep.h            | 12 ++++++++++++
>  kernel/locking/lockdep.c           | 34 +++++++++++++++++++++++++++++++-=
--
>  kernel/locking/lockdep_internals.h |  3 +++
>  3 files changed, 46 insertions(+), 3 deletions(-)
>=20
> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> index 1009c47..bc09c85 100644
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -195,6 +195,18 @@ struct lock_list {
>  	struct lock_class		*links_to;
>  	struct lock_trace		trace;
>  	int				distance;
> +	/*
> +	 * This field keeps track of the read-write type of this dependency.
> +	 *
> +	 * With L1 -> L2:
> +	 *
> +	 * lock_type[0] stores the type of L1, while lock_type[1] stores the
> +	 * type of L2.
> +	 */
> +	union {
> +		int	dep_type;
> +		u16	lock_type[2];

This takes extra space for storing the type of dependencies. In my
previous patchset, only 2bits are used, because you only need to care
about four types of dependencies:

1.	Exclusive -> Recursive
2.	Shared (read or recursive read) -> Recursive
3.	Exclusize -> Non-Recursive (write or non-recursive readers)
4.	Shared (read or recursive read) -> Non-Recursive

To understand why, here are my slides for Linux Plumber Conference last
year:

	https://www.linuxplumbersconf.org/event/2/contributions/74/attachments/67/=
78/Recursive_read_deadlocks_and_Where_to_find_them.pdf

And we can steal two bits from "distance", since 2^30 would be enough.
I think it's important to save the size of lock_list since we will have
a lot of them.

Regards,
Boqun

> +	};
> =20
>  	/*
>  	 * The parent field is used to implement breadth-first search, and the
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index fb6be63..e7eedbf 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -1225,7 +1225,7 @@ static struct lock_list *alloc_list_entry(void)
>  static int add_lock_to_list(struct lock_class *this,
>  			    struct lock_class *links_to, struct list_head *head,
>  			    unsigned long ip, int distance,
> -			    struct lock_trace *trace)
> +			    struct lock_trace *trace, int dep_type)
>  {
>  	struct lock_list *entry;
>  	/*
> @@ -1240,6 +1240,8 @@ static int add_lock_to_list(struct lock_class *this,
>  	entry->links_to =3D links_to;
>  	entry->distance =3D distance;
>  	entry->trace =3D *trace;
> +	entry->dep_type =3D dep_type;
> +
>  	/*
>  	 * Both allocation and removal are done under the graph lock; but
>  	 * iteration is under RCU-sched; see look_up_lock_class() and
> @@ -1677,6 +1679,30 @@ unsigned long lockdep_count_backward_deps(struct l=
ock_class *class)
>  	return ret;
>  }
> =20
> +static inline int get_dep_type(struct held_lock *lock1, struct held_lock=
 *lock2)
> +{
> +	/*
> +	 * With dependency lock1 -> lock2:
> +	 *
> +	 * lock_type[0] is lock1, while lock_type[1] is lock2.
> +	 *
> +	 * Avoid architectural endianness difference composing dep_type.
> +	 */
> +	u16 type[2] =3D { lock1->read, lock2->read };
> +
> +	return *(int *)type;
> +}
> +
> +static inline int get_lock_type1(struct lock_list *lock)
> +{
> +	return lock->lock_type[0];
> +}
> +
> +static inline int get_lock_type2(struct lock_list *lock)
> +{
> +	return lock->lock_type[1];
> +}
> +
>  /*
>   * Check that the dependency graph starting at <src> can lead to
>   * <target> or not. Print an error and return 0 if it does.
> @@ -2446,14 +2472,16 @@ static inline void inc_chains(void)
>  	 */
>  	ret =3D add_lock_to_list(hlock_class(next), hlock_class(prev),
>  			       &hlock_class(prev)->locks_after,
> -			       next->acquire_ip, distance, trace);
> +			       next->acquire_ip, distance, trace,
> +			       get_dep_type(prev, next));
> =20
>  	if (!ret)
>  		return 0;
> =20
>  	ret =3D add_lock_to_list(hlock_class(prev), hlock_class(next),
>  			       &hlock_class(next)->locks_before,
> -			       next->acquire_ip, distance, trace);
> +			       next->acquire_ip, distance, trace,
> +			       get_dep_type(next, prev));
>  	if (!ret)
>  		return 0;
> =20
> diff --git a/kernel/locking/lockdep_internals.h b/kernel/locking/lockdep_=
internals.h
> index 150ec3f..c287bcb 100644
> --- a/kernel/locking/lockdep_internals.h
> +++ b/kernel/locking/lockdep_internals.h
> @@ -26,6 +26,9 @@ enum lock_usage_bit {
>  #define LOCK_USAGE_DIR_MASK  2
>  #define LOCK_USAGE_STATE_MASK (~(LOCK_USAGE_READ_MASK | LOCK_USAGE_DIR_M=
ASK))
> =20
> +#define LOCK_TYPE_BITS	16
> +#define LOCK_TYPE_MASK	0xFFFF
> +
>  /*
>   * Usage-state bitmasks:
>   */
> --=20
> 1.8.3.1
>=20

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