Hi,
On Tue, 9 Oct 2018 11:24:29 +0200
Juri Lelli <juri.lelli@xxxxxxxxxx> wrote:
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>
> Track the blocked-on relation for mutexes, this allows following this
> relation at schedule time. Add blocked_task to track the inverse
> relation.
>
> ,-> task
> | | blocked-on
> | v
> blocked-task | mutex
> | | owner
> | v
> `-- task
I was a little bit confused by this description, because (if I
understand the code well) blocked_task does not actually track the
inverse of the "blocked_on" relationship, but just points to the task
that is _currently_ acting as a proxy for a given task.
In theory, we could have multiple tasks blocked on "mutex" (which is
owned by "task"), so if "blocked_task" tracked the inverse of
"blocked_on" it should have been a list (or a data structure containing
pointers to multiple task structures), no?
I would propose to change "blocked_task" into something like
"current_proxy", or similar, which should be more clear (unless I
completely misunderstood this stuff... In that case, sorry about the
noise)
Also, I suspect that this "blocked_task" (or "current_proxy") field
should be introcuced in patch 5 (same for the "task_is_blocked()"
function from patch 4... Should it go in patch 5?)
Luca
>
> This patch only enables blocked-on relation, blocked-task will be
> enabled in a later patch implementing proxy().
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> [minor changes while rebasing]
> Signed-off-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
> ---
> include/linux/sched.h | 6 ++----
> kernel/fork.c | 6 +++---
> kernel/locking/mutex-debug.c | 7 +++----
> kernel/locking/mutex.c | 3 +++
> 4 files changed, 11 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 977cb57d7bc9..a35e8ab3eef1 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -907,10 +907,8 @@ struct task_struct {
> struct rt_mutex_waiter *pi_blocked_on;
> #endif
>
> -#ifdef CONFIG_DEBUG_MUTEXES
> - /* Mutex deadlock detection: */
> - struct mutex_waiter *blocked_on;
> -#endif
> + struct task_struct *blocked_task; /* task
> that's boosting us */
> + struct mutex *blocked_on; /* lock
> we're blocked on */
> #ifdef CONFIG_TRACE_IRQFLAGS
> unsigned int irq_events;
> diff --git a/kernel/fork.c b/kernel/fork.c
> index f0b58479534f..ef27a675b0d7 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1827,9 +1827,9 @@ static __latent_entropy struct task_struct
> *copy_process( lockdep_init_task(p);
> #endif
>
> -#ifdef CONFIG_DEBUG_MUTEXES
> - p->blocked_on = NULL; /* not blocked yet */
> -#endif
> + p->blocked_task = NULL; /* nobody is boosting us yet*/
> + p->blocked_on = NULL; /* not blocked yet */
> +
> #ifdef CONFIG_BCACHE
> p->sequential_io = 0;
> p->sequential_io_avg = 0;
> diff --git a/kernel/locking/mutex-debug.c
> b/kernel/locking/mutex-debug.c index a660d38b6c29..6605e083a3e9 100644
> --- a/kernel/locking/mutex-debug.c
> +++ b/kernel/locking/mutex-debug.c
> @@ -53,8 +53,8 @@ void debug_mutex_add_waiter(struct mutex *lock,
> struct mutex_waiter *waiter, {
> SMP_DEBUG_LOCKS_WARN_ON(!raw_spin_is_locked(&lock->wait_lock));
>
> - /* Mark the current thread as blocked on the lock: */
> - task->blocked_on = waiter;
> + /* Current thread can't be alredy blocked (since it's
> executing!) */
> + DEBUG_LOCKS_WARN_ON(task->blocked_on);
> }
>
> void mutex_remove_waiter(struct mutex *lock, struct mutex_waiter
> *waiter, @@ -62,8 +62,7 @@ void mutex_remove_waiter(struct mutex
> *lock, struct mutex_waiter *waiter, {
> DEBUG_LOCKS_WARN_ON(list_empty(&waiter->list));
> DEBUG_LOCKS_WARN_ON(waiter->task != task);
> - DEBUG_LOCKS_WARN_ON(task->blocked_on != waiter);
> - task->blocked_on = NULL;
> + DEBUG_LOCKS_WARN_ON(task->blocked_on != lock);
>
> list_del_init(&waiter->list);
> waiter->task = NULL;
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index f37402cd8496..76b59b555da3 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -979,6 +979,7 @@ __mutex_lock_common(struct mutex *lock, long
> state, unsigned int subclass, }
>
> waiter.task = current;
> + current->blocked_on = lock;
>
> set_current_state(state);
> for (;;) {
> @@ -1047,6 +1048,8 @@ __mutex_lock_common(struct mutex *lock, long
> state, unsigned int subclass, }
>
> mutex_remove_waiter(lock, &waiter, current);
> + current->blocked_on = NULL;
> +
> if (likely(list_empty(&lock->wait_list)))
> __mutex_clear_flag(lock, MUTEX_FLAGS);
>
