On 06/14/2018 12:51 PM, Peter Zijlstra wrote:
> On Wed, Jun 13, 2018 at 04:05:43PM +0200, Thomas Hellstrom wrote:
>> In short, with Wait-Die (before the patch) it's the process _taking_ the
>> contended lock that backs off if necessary. No preemption required. With
>> Wound-Wait, it's the process _holding_ the contended lock that gets wounded
>> (preempted), and it needs to back off at its own discretion but no later
>> than when it's going to sleep on another ww mutex. That point is where we
>> intercept the preemption request. We're preempting the transaction rather
>> than the process.
> This:
>
> Wait-die:
> The newer transactions are killed when:
> It (= the newer transaction) makes a reqeust for a lock being held
> by an older transactions
>
> Wound-wait:
> The newer transactions are killed when:
> An older transaction makes a request for a lock being held by the
> newer transactions
>
> Would make for an excellent comment somewhere. No talking about
> preemption, although I think I know what you mean with it, that is not
> how preemption is normally used.
Ok. I'll incorporate something along this line. Unfortunately that last
statement is not fully true. It should read
"The newer transactions are wounded when:", not "killed" when.
The literature makes a distinction between "killed" and "wounded". In
our context, "Killed" is when a transaction actually receives an
-EDEADLK and needs to back off. "Wounded" is when someone (typically
another transaction) requests a transaction to kill itself. A wound will
often, but not always, lead to a kill. If the wounded transaction has
finished its locking sequence, or has the opportunity to grab
uncontended ww mutexes or steal contended (non-handoff) ww mutexes to
finish its transaction it will do so and never kill itself.
>
> In scheduling speak preemption is when we pick a runnable (but !running)
> task to run instead of the current running task. In this case however,
> our T2 is blocked on a lock acquisition (one owned by our T1) and T1 is
> the only runnable task. Only when T1's progress is inhibited by T2 (T1
> wants a lock held by T2) do we wound/wake T2.
Indeed. The preemption spoken about in the Wound-Wait litterature means
that a transaction preempts another transaction when it wounds it. In
distributed computing my understanding is that the preempted transaction
is aborted instantly and restarted after a random delay. Of course, we
have no means of mapping wounding to process preemption in the linux
kernel, so that's why I referred to it as "lazy preemption". In process
analogy "wounded" wound roughly correspond to (need_resched() == true),
and returning -EDEADLK would correspond to voluntary preemption.
>
> In any case, I had a little look at the current ww_mutex code and ended
> up with the below patch that hopefully clarifies things a little.
>
> ---
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index f44f658ae629..a20c04619b2a 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -244,6 +244,10 @@ void __sched mutex_lock(struct mutex *lock)
> EXPORT_SYMBOL(mutex_lock);
> #endif
>
> +/*
> + * Associate the ww_mutex @ww with the context @ww_ctx under which we acquired
> + * it.
> + */
IMO use of "acquire_context" or "context" is a little unfortunate when
the literature uses "transaction",
but otherwise fine.
> static __always_inline void
> ww_mutex_lock_acquired(struct ww_mutex *ww, struct ww_acquire_ctx *ww_ctx)
> {
> @@ -282,26 +286,36 @@ ww_mutex_lock_acquired(struct ww_mutex *ww, struct ww_acquire_ctx *ww_ctx)
> DEBUG_LOCKS_WARN_ON(ww_ctx->ww_class != ww->ww_class);
> #endif
> ww_ctx->acquired++;
> + lock->ctx = ctx;
> }
>
> +/*
> + * Determine if context @a is 'after' context @b. IOW, @a should be wounded in
> + * favour of @b.
> + */
So "wounded" should never really be used with Wait-Die
"Determine whether context @a represents a younger transaction than
context @b"?
> static inline bool __sched
> __ww_ctx_stamp_after(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b)
> {
> - return a->stamp - b->stamp <= LONG_MAX &&
> - (a->stamp != b->stamp || a > b);
> +
> + return (signed long)(a->stamp - b->stamp) > 0;
> }
>
> /*
> - * Wake up any waiters that may have to back off when the lock is held by the
> - * given context.
> + * We just acquired @lock under @ww_ctx, if there are later contexts waiting
> + * behind us on the wait-list, wake them up so they can wound themselves.
Actually for Wait-Die, Back off or "Die" is the correct terminology.
> *
> - * Due to the invariants on the wait list, this can only affect the first
> - * waiter with a context.
> + * See __ww_mutex_add_waiter() for the list-order construction; basically the
> + * list is ordered by stamp smallest (oldest) first, so if there is a later
> + * (younger) stamp on the list behind us, wake it so it can wound itself.
> + *
> + * Because __ww_mutex_add_waiter() and __ww_mutex_check_stamp() wake any
> + * but the earliest context, this can only affect the first waiter (with a
> + * context).
The wait list invariants are stated in
Documentation/locking/ww-mutex-design.txt.
Perhaps we could copy those into the code to make the comment more
understandable:
" We maintain the following invariants for the wait list:
(1) Waiters with an acquire context are sorted by stamp order; waiters
without an acquire context are interspersed in FIFO order.
(2) For Wait-Die, among waiters with contexts, only the first one can
have
other locks acquired already (ctx->acquired > 0). Note that this
waiter
may come after other waiters without contexts in the list."
> *
> * The current task must not be on the wait list.
> */
> static void __sched
> -__ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
> +__ww_mutex_wakeup_for_wound(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
Again, "wound" is unsuitable for Wait-Die. + numerous additional places.
Thanks,
Thomas
@@ -244,6 +244,10 @@ void __sched mutex_lock(struct mutex *lock)
EXPORT_SYMBOL(mutex_lock);
#endif
+/*
+ * Associate the ww_mutex @ww with the context @ww_ctx under which we acquired
+ * it.
+ */
static __always_inline void
ww_mutex_lock_acquired(struct ww_mutex *ww, struct ww_acquire_ctx *ww_ctx)
{
@@ -282,26 +286,36 @@ ww_mutex_lock_acquired(struct ww_mutex *ww, struct ww_acquire_ctx *ww_ctx)
DEBUG_LOCKS_WARN_ON(ww_ctx->ww_class != ww->ww_class);
#endif
ww_ctx->acquired++;
+ lock->ctx = ctx;
}
+/*
+ * Determine if context @a is 'after' context @b. IOW, @a should be wounded in
+ * favour of @b.
+ */
static inline bool __sched
__ww_ctx_stamp_after(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b)
{
- return a->stamp - b->stamp <= LONG_MAX &&
- (a->stamp != b->stamp || a > b);
+
+ return (signed long)(a->stamp - b->stamp) > 0;
}
/*
- * Wake up any waiters that may have to back off when the lock is held by the
- * given context.
+ * We just acquired @lock under @ww_ctx, if there are later contexts waiting
+ * behind us on the wait-list, wake them up so they can wound themselves.
*
- * Due to the invariants on the wait list, this can only affect the first
- * waiter with a context.
+ * See __ww_mutex_add_waiter() for the list-order construction; basically the
+ * list is ordered by stamp smallest (oldest) first, so if there is a later
+ * (younger) stamp on the list behind us, wake it so it can wound itself.
+ *
+ * Because __ww_mutex_add_waiter() and __ww_mutex_check_stamp() wake any
+ * but the earliest context, this can only affect the first waiter (with a
+ * context).
*
* The current task must not be on the wait list.
*/
static void __sched
-__ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
+__ww_mutex_wakeup_for_wound(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
{
struct mutex_waiter *cur;
@@ -322,16 +336,14 @@ __ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
}
/*
- * After acquiring lock with fastpath or when we lost out in contested
- * slowpath, set ctx and wake up any waiters so they can recheck.
+ * After acquiring lock with fastpath, where we do not hold wait_lock, set ctx
+ * and wake up any waiters so they can recheck.
*/
static __always_inline void
ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
{
ww_mutex_lock_acquired(lock, ctx);
- lock->ctx = ctx;
-
/*
* The lock->ctx update should be visible on all cores before
* the atomic read is done, otherwise contended waiters might be
@@ -352,25 +364,10 @@ ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
* so they can see the new lock->ctx.
*/
spin_lock(&lock->base.wait_lock);
- __ww_mutex_wakeup_for_backoff(&lock->base, ctx);
+ __ww_mutex_wakeup_for_wound(&lock->base, ctx);
spin_unlock(&lock->base.wait_lock);
}
-/*
- * After acquiring lock in the slowpath set ctx.
- *
- * Unlike for the fast path, the caller ensures that waiters are woken up where
- * necessary.
- *
- * Callers must hold the mutex wait_lock.
- */
-static __always_inline void
-ww_mutex_set_context_slowpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
-{
- ww_mutex_lock_acquired(lock, ctx);
- lock->ctx = ctx;
-}
-
#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
static inline
@@ -646,20 +643,30 @@ void __sched ww_mutex_unlock(struct ww_mutex *lock)
}
EXPORT_SYMBOL(ww_mutex_unlock);
+/*
+ * Check the wound condition for the current lock acquire. If we're trying to
+ * acquire a lock already held by an older context, wound ourselves.
+ *
+ * Since __ww_mutex_add_waiter() orders the wait-list on stamp, we only have to
+ * look at waiters before us in the wait-list.
+ */
static inline int __sched
-__ww_mutex_lock_check_stamp(struct mutex *lock, struct mutex_waiter *waiter,
+__ww_mutex_check_stamp(struct mutex *lock, struct mutex_waiter *waiter,
struct ww_acquire_ctx *ctx)
{
struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
struct ww_acquire_ctx *hold_ctx = READ_ONCE(ww->ctx);
struct mutex_waiter *cur;
+ if (ctx->acquired == 0)
+ return 0;
+
if (hold_ctx && __ww_ctx_stamp_after(ctx, hold_ctx))
goto deadlock;
/*
* If there is a waiter in front of us that has a context, then its
- * stamp is earlier than ours and we must back off.
+ * stamp is earlier than ours and we must wound ourself.
*/
cur = waiter;
list_for_each_entry_continue_reverse(cur, &lock->wait_list, list) {
@@ -677,6 +684,14 @@ __ww_mutex_lock_check_stamp(struct mutex *lock, struct mutex_waiter *waiter,
return -EDEADLK;
}
+/*
+ * Add @waiter to the wait-list, keep the wait-list ordered by stamp, smallest
+ * first. Such that older contexts are preferred to acquire the lock over
+ * younger contexts.
+ *
+ * Furthermore, wound ourself immediately when possible (there are older
+ * contexts already waiting) to avoid unnecessary waiting.
+ */
static inline int __sched
__ww_mutex_add_waiter(struct mutex_waiter *waiter,
struct mutex *lock,
@@ -700,8 +715,12 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
if (!cur->ww_ctx)
continue;
+ /*
+ * If we find an older context waiting, there is no point in
+ * queueing behind it, as we'd have to wound ourselves the
+ * moment it would acquire the lock.
+ */
if (__ww_ctx_stamp_after(ww_ctx, cur->ww_ctx)) {
- /* Back off immediately if necessary. */
if (ww_ctx->acquired > 0) {
#ifdef CONFIG_DEBUG_MUTEXES
struct ww_mutex *ww;
@@ -719,8 +738,9 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
pos = &cur->list;
/*
- * Wake up the waiter so that it gets a chance to back
- * off.
+ * When we enqueued an older context, wake all younger
+ * contexts such that they can wound themselves, see
+ * __ww_mutex_check_stamp().
*/
if (cur->ww_ctx->acquired > 0) {
debug_mutex_wake_waiter(lock, cur);
@@ -772,7 +792,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
*/
if (__mutex_trylock(lock)) {
if (use_ww_ctx && ww_ctx)
- __ww_mutex_wakeup_for_backoff(lock, ww_ctx);
+ __ww_mutex_wakeup_for_wound(lock, ww_ctx);
goto skip_wait;
}
@@ -790,10 +810,10 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
waiter.ww_ctx = MUTEX_POISON_WW_CTX;
#endif
} else {
- /* Add in stamp order, waking up waiters that must back off. */
+ /* Add in stamp order, waking up waiters that must wound themselves. */
ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);
if (ret)
- goto err_early_backoff;
+ goto err_early_wound;
waiter.ww_ctx = ww_ctx;
}
@@ -824,8 +844,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
goto err;
}
- if (use_ww_ctx && ww_ctx && ww_ctx->acquired > 0) {
- ret = __ww_mutex_lock_check_stamp(lock, &waiter, ww_ctx);
+ if (use_ww_ctx && ww_ctx) {
+ ret = __ww_mutex_check_stamp(lock, &waiter, ww_ctx);
if (ret)
goto err;
}
@@ -870,7 +890,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
lock_acquired(&lock->dep_map, ip);
if (use_ww_ctx && ww_ctx)
- ww_mutex_set_context_slowpath(ww, ww_ctx);
+ ww_mutex_lock_acquired(ww, ww_ctx);
spin_unlock(&lock->wait_lock);
preempt_enable();
@@ -879,7 +899,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
err:
__set_current_state(TASK_RUNNING);
mutex_remove_waiter(lock, &waiter, current);
-err_early_backoff:
+err_early_wound:
spin_unlock(&lock->wait_lock);
debug_mutex_free_waiter(&waiter);
mutex_release(&lock->dep_map, 1, ip);