Re: [PATCH v2 02/17] locking/lockdep: Add read-write type for dependency

Boqun Feng
2019-05-29T11:37:58Z

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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:
>
> lock_type[1] (u16), lock_type[0] (u16)
>
> or:
>
> dep_type (int)
>
> where value:
>
> 0: exclusive / write
> 1: read
> 2: recursive read
>
> Note that (int) dep_type value may vary with different architectural
> endianness, so use helper functions to operate on these types.
>
> 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(-)
>
> 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

> + };
>
> /*
> * 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 = links_to;
> entry->distance = distance;
> entry->trace = *trace;
> + entry->dep_type = 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 lock_class *class)
> return ret;
> }
>
> +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] = { 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 = 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));
>
> if (!ret)
> return 0;
>
> ret = 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;
>
> 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_MASK))
>
> +#define LOCK_TYPE_BITS 16
> +#define LOCK_TYPE_MASK 0xFFFF
> +
> /*
> * Usage-state bitmasks:
> */
> --
> 1.8.3.1
>

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