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SubjectRe: [PATCH v2 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes
Hi Thomas,

On Thu, Jun 14, 2018 at 09:29:21AM +0200, Thomas Hellstrom wrote:
> The current Wound-Wait mutex algorithm is actually not Wound-Wait but
> Wait-Die. Implement also Wound-Wait as a per-ww-class choice. Wound-Wait
> is, contrary to Wait-Die a preemptive algorithm and is known to generate
> fewer backoffs. Testing reveals that this is true if the
> number of simultaneous contending transactions is small.
> As the number of simultaneous contending threads increases, Wait-Wound
> becomes inferior to Wait-Die in terms of elapsed time.
> Possibly due to the larger number of held locks of sleeping transactions.
>
> Update documentation and callers.
>
> Timings using git://people.freedesktop.org/~thomash/ww_mutex_test
> tag patch-18-06-14
>
> Each thread runs 100000 batches of lock / unlock 800 ww mutexes randomly
> chosen out of 100000. Four core Intel x86_64:
>
> Algorithm #threads Rollbacks time
> Wound-Wait 4 ~100 ~17s.
> Wait-Die 4 ~150000 ~19s.
> Wound-Wait 16 ~360000 ~109s.
> Wait-Die 16 ~450000 ~82s.
>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Gustavo Padovan <gustavo@padovan.org>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Sean Paul <seanpaul@chromium.org>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Davidlohr Bueso <dave@stgolabs.net>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Kate Stewart <kstewart@linuxfoundation.org>
> Cc: Philippe Ombredanne <pombredanne@nexb.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: linux-doc@vger.kernel.org
> Cc: linux-media@vger.kernel.org
> Cc: linaro-mm-sig@lists.linaro.org
> Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
>
> ---
> v2:
> * Update API according to review comment by Greg Kroah-Hartman.
> * Address review comments by Peter Zijlstra:
> - Avoid _Bool in composites
> - Fix typo
> - Use __mutex_owner() where applicable
> - Rely on built-in barriers for the main loop exit condition,
> struct ww_acquire_ctx::wounded. Update code comments.
> - Explain unlocked use of list_empty().
> ---
> Documentation/locking/ww-mutex-design.txt | 54 ++++++++++++----
> drivers/dma-buf/reservation.c | 2 +-
> drivers/gpu/drm/drm_modeset_lock.c | 2 +-
> include/linux/ww_mutex.h | 19 ++++--
> kernel/locking/locktorture.c | 2 +-
> kernel/locking/mutex.c | 103 +++++++++++++++++++++++++++---
> kernel/locking/test-ww_mutex.c | 2 +-
> lib/locking-selftest.c | 2 +-
> 8 files changed, 156 insertions(+), 30 deletions(-)
>
> diff --git a/Documentation/locking/ww-mutex-design.txt b/Documentation/locking/ww-mutex-design.txt
> index 34c3a1b50b9a..b9597def9581 100644
> --- a/Documentation/locking/ww-mutex-design.txt
> +++ b/Documentation/locking/ww-mutex-design.txt
> @@ -1,4 +1,4 @@
> -Wait/Wound Deadlock-Proof Mutex Design
> +Wound/Wait Deadlock-Proof Mutex Design
> ======================================
>
> Please read mutex-design.txt first, as it applies to wait/wound mutexes too.
> @@ -32,10 +32,23 @@ the oldest task) wins, and the one with the higher reservation id (i.e. the
> younger task) unlocks all of the buffers that it has already locked, and then
> tries again.
>
> -In the RDBMS literature this deadlock handling approach is called wait/wound:
> -The older tasks waits until it can acquire the contended lock. The younger tasks
> -needs to back off and drop all the locks it is currently holding, i.e. the
> -younger task is wounded.
> +In the RDBMS literature, a reservation ticket is associated with a transaction.
> +and the deadlock handling approach is called Wait-Die. The name is based on
> +the actions of a locking thread when it encounters an already locked mutex.
> +If the transaction holding the lock is younger, the locking transaction waits.
> +If the transaction holding the lock is older, the locking transaction backs off
> +and dies. Hence Wait-Die.
> +There is also another algorithm called Wound-Wait:
> +If the transaction holding the lock is younger, the locking transaction
> +preempts the transaction holding the lock, requiring it to back off. It
> +Wounds the other transaction.
> +If the transaction holding the lock is older, it waits for the other
> +transaction. Hence Wound-Wait.
> +The two algorithms are both fair in that a transaction will eventually succeed.
> +However, the Wound-Wait algorithm is typically stated to generate fewer backoffs
> +compared to Wait-Die, but is, on the other hand, associated with more work than
> +Wait-Die when recovering from a backoff. Wound-Wait is also a preemptive
> +algorithm which requires a reliable way to preempt another transaction.
>
> Concepts
> --------
> @@ -47,10 +60,12 @@ Acquire context: To ensure eventual forward progress it is important the a task
> trying to acquire locks doesn't grab a new reservation id, but keeps the one it
> acquired when starting the lock acquisition. This ticket is stored in the
> acquire context. Furthermore the acquire context keeps track of debugging state
> -to catch w/w mutex interface abuse.
> +to catch w/w mutex interface abuse. An acquire context is representing a
> +transaction.
>
> W/w class: In contrast to normal mutexes the lock class needs to be explicit for
> -w/w mutexes, since it is required to initialize the acquire context.
> +w/w mutexes, since it is required to initialize the acquire context. The lock
> +class also specifies what algorithm to use, Wound-Wait or Wait-Die.
>
> Furthermore there are three different class of w/w lock acquire functions:
>
> @@ -90,6 +105,12 @@ provided.
> Usage
> -----
>
> +The algorithm (Wait-Die vs Wound-Wait) is chosen by using either
> +DEFINE_WW_CLASS_WDIE() for Wait-Die or DEFINE_WW_CLASS() for Wound-Wait.
> +As a rough rule of thumb, use Wound-Wait iff you typically expect the number
> +of simultaneous competing transactions to be small, and the rollback cost can
> +be substantial.
> +
> Three different ways to acquire locks within the same w/w class. Common
> definitions for methods #1 and #2:
>
> @@ -312,12 +333,23 @@ Design:
> 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) 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.
> + (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 Wound-Wait preemption is implemented with a lazy-preemption scheme:
> + The wounded status of the transaction is checked only when there is
> + contention for a new lock and hence a true chance of deadlock. In that
> + situation, if the transaction is wounded, it backs off, clears the
> + wounded status and retries. A great benefit of implementing preemption in
> + this way is that the wounded transaction can identify a contending lock to
> + wait for before restarting the transaction. Just blindly restarting the
> + transaction would likely make the transaction end up in a situation where
> + it would have to back off again.
>
> In general, not much contention is expected. The locks are typically used to
> - serialize access to resources for devices.
> + serialize access to resources for devices, and optimization focus should
> + therefore be directed towards the uncontended cases.
>
> Lockdep:
> Special care has been taken to warn for as many cases of api abuse
> diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
> index 314eb1071cce..b94a4bab2ecd 100644
> --- a/drivers/dma-buf/reservation.c
> +++ b/drivers/dma-buf/reservation.c
> @@ -46,7 +46,7 @@
> * write-side updates.
> */
>
> -DEFINE_WW_CLASS(reservation_ww_class);
> +DEFINE_WW_CLASS_WDIE(reservation_ww_class);
> EXPORT_SYMBOL(reservation_ww_class);
>
> struct lock_class_key reservation_seqcount_class;
> diff --git a/drivers/gpu/drm/drm_modeset_lock.c b/drivers/gpu/drm/drm_modeset_lock.c
> index 8a5100685875..ff00a814f617 100644
> --- a/drivers/gpu/drm/drm_modeset_lock.c
> +++ b/drivers/gpu/drm/drm_modeset_lock.c
> @@ -70,7 +70,7 @@
> * lists and lookup data structures.
> */
>
> -static DEFINE_WW_CLASS(crtc_ww_class);
> +static DEFINE_WW_CLASS_WDIE(crtc_ww_class);
>
> /**
> * drm_modeset_lock_all - take all modeset locks
> diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
> index 39fda195bf78..3880813b7db5 100644
> --- a/include/linux/ww_mutex.h
> +++ b/include/linux/ww_mutex.h
> @@ -8,6 +8,8 @@
> *
> * Wound/wait implementation:
> * Copyright (C) 2013 Canonical Ltd.
> + * Choice of algorithm:
> + * Copyright (C) 2018 WMWare Inc.
> *
> * This file contains the main data structure and API definitions.
> */
> @@ -23,15 +25,17 @@ struct ww_class {
> struct lock_class_key mutex_key;
> const char *acquire_name;
> const char *mutex_name;
> + unsigned int is_wait_die;
> };
>
> struct ww_acquire_ctx {
> struct task_struct *task;
> unsigned long stamp;
> unsigned acquired;
> + unsigned int wounded;
> + struct ww_class *ww_class;
> #ifdef CONFIG_DEBUG_MUTEXES
> unsigned done_acquire;
> - struct ww_class *ww_class;
> struct ww_mutex *contending_lock;
> #endif
> #ifdef CONFIG_DEBUG_LOCK_ALLOC
> @@ -58,17 +62,21 @@ struct ww_mutex {
> # define __WW_CLASS_MUTEX_INITIALIZER(lockname, class)
> #endif
>
> -#define __WW_CLASS_INITIALIZER(ww_class) \
> +#define __WW_CLASS_INITIALIZER(ww_class, _is_wait_die) \
> { .stamp = ATOMIC_LONG_INIT(0) \
> , .acquire_name = #ww_class "_acquire" \
> - , .mutex_name = #ww_class "_mutex" }
> + , .mutex_name = #ww_class "_mutex" \
> + , .is_wait_die = _is_wait_die }
>
> #define __WW_MUTEX_INITIALIZER(lockname, class) \
> { .base = __MUTEX_INITIALIZER(lockname.base) \
> __WW_CLASS_MUTEX_INITIALIZER(lockname, class) }
>
> #define DEFINE_WW_CLASS(classname) \
> - struct ww_class classname = __WW_CLASS_INITIALIZER(classname)
> + struct ww_class classname = __WW_CLASS_INITIALIZER(classname, 0)
> +
> +#define DEFINE_WW_CLASS_WDIE(classname) \
> + struct ww_class classname = __WW_CLASS_INITIALIZER(classname, 1)
>
> #define DEFINE_WW_MUTEX(mutexname, ww_class) \
> struct ww_mutex mutexname = __WW_MUTEX_INITIALIZER(mutexname, ww_class)
> @@ -123,8 +131,9 @@ static inline void ww_acquire_init(struct ww_acquire_ctx *ctx,
> ctx->task = current;
> ctx->stamp = atomic_long_inc_return_relaxed(&ww_class->stamp);
> ctx->acquired = 0;
> -#ifdef CONFIG_DEBUG_MUTEXES
> ctx->ww_class = ww_class;
> + ctx->wounded = false;
> +#ifdef CONFIG_DEBUG_MUTEXES
> ctx->done_acquire = 0;
> ctx->contending_lock = NULL;
> #endif
> diff --git a/kernel/locking/locktorture.c b/kernel/locking/locktorture.c
> index 6850ffd69125..e861c1bf0e1e 100644
> --- a/kernel/locking/locktorture.c
> +++ b/kernel/locking/locktorture.c
> @@ -365,7 +365,7 @@ static struct lock_torture_ops mutex_lock_ops = {
> };
>
> #include <linux/ww_mutex.h>
> -static DEFINE_WW_CLASS(torture_ww_class);
> +static DEFINE_WW_CLASS_WDIE(torture_ww_class);
> static DEFINE_WW_MUTEX(torture_ww_mutex_0, &torture_ww_class);
> static DEFINE_WW_MUTEX(torture_ww_mutex_1, &torture_ww_class);
> static DEFINE_WW_MUTEX(torture_ww_mutex_2, &torture_ww_class);
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index 2048359f33d2..ffa00b5aaf03 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -290,12 +290,49 @@ __ww_ctx_stamp_after(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b)
> (a->stamp != b->stamp || a > b);
> }
>
> +/*
> + * Wound the lock holder transaction if it's younger than the contending
> + * transaction, and there is a possibility of a deadlock.
> + * Also if the lock holder transaction isn't the current transaction,
> + * make sure it's woken up in case it's sleeping on another ww mutex.
> + */
> +static bool __ww_mutex_wound(struct mutex *lock,
> + struct ww_acquire_ctx *ww_ctx,
> + struct ww_acquire_ctx *hold_ctx)
> +{
> + struct task_struct *owner = __mutex_owner(lock);
> +
> + lockdep_assert_held(&lock->wait_lock);
> +
> + if (owner && hold_ctx && __ww_ctx_stamp_after(hold_ctx, ww_ctx) &&
> + ww_ctx->acquired > 0) {
> + hold_ctx->wounded = 1;
> +
> + /*
> + * wake_up_process() paired with set_current_state() inserts
> + * sufficient barriers to make sure @owner either sees it's
> + * wounded or has a wakeup pending to re-read the wounded
> + * state.

IIUC, "sufficient barriers" = full memory barriers (here). (You may
want to be more specific.)

> + *
> + * The value of hold_ctx->wounded in
> + * __ww_mutex_lock_check_stamp();

Missing parts/incomplete sentence?

Andrea


> + */
> + if (owner != current)
> + wake_up_process(owner);
> +
> + return true;
> + }
> +
> + return false;
> +}
> +
> /*
> * Wake up any waiters that may have to back off when the lock is held by the
> * given context.
> *
> * Due to the invariants on the wait list, this can only affect the first
> - * waiter with a context.
> + * waiter with a context, unless the Wound-Wait algorithm is used where
> + * also subsequent waiters with a context main wound the lock holder.
> *
> * The current task must not be on the wait list.
> */
> @@ -303,6 +340,7 @@ static void __sched
> __ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
> {
> struct mutex_waiter *cur;
> + unsigned int is_wait_die = ww_ctx->ww_class->is_wait_die;
>
> lockdep_assert_held(&lock->wait_lock);
>
> @@ -310,13 +348,14 @@ __ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
> if (!cur->ww_ctx)
> continue;
>
> - if (cur->ww_ctx->acquired > 0 &&
> + if (is_wait_die && cur->ww_ctx->acquired > 0 &&
> __ww_ctx_stamp_after(cur->ww_ctx, ww_ctx)) {
> debug_mutex_wake_waiter(lock, cur);
> wake_up_process(cur->task);
> }
>
> - break;
> + if (is_wait_die || __ww_mutex_wound(lock, cur->ww_ctx, ww_ctx))
> + break;
> }
> }
>
> @@ -338,12 +377,18 @@ ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
> * and keep spinning, or it will acquire wait_lock, add itself
> * to waiter list and sleep.
> */
> - smp_mb(); /* ^^^ */
> + smp_mb(); /* See comments above and below. */
>
> /*
> - * Check if lock is contended, if not there is nobody to wake up
> + * Check if lock is contended, if not there is nobody to wake up.
> + * We can use list_empty() unlocked here since it only compares a
> + * list_head field pointer to the address of the list head
> + * itself, similarly to how list_empty() can be considered RCU-safe.
> + * The memory barrier above pairs with the memory barrier in
> + * __ww_mutex_add_waiter and makes sure lock->ctx is visible before
> + * we check for waiters.
> */
> - if (likely(!(atomic_long_read(&lock->base.owner) & MUTEX_FLAG_WAITERS)))
> + if (likely(list_empty(&lock->base.wait_list)))
> return;
>
> /*
> @@ -653,6 +698,13 @@ __ww_mutex_lock_check_stamp(struct mutex *lock, struct mutex_waiter *waiter,
> struct ww_acquire_ctx *hold_ctx = READ_ONCE(ww->ctx);
> struct mutex_waiter *cur;
>
> + if (!ctx->ww_class->is_wait_die) {
> + if (ctx->wounded)
> + goto deadlock;
> + else
> + return 0;
> + }
> +
> if (hold_ctx && __ww_ctx_stamp_after(ctx, hold_ctx))
> goto deadlock;
>
> @@ -683,16 +735,21 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
> {
> struct mutex_waiter *cur;
> struct list_head *pos;
> + unsigned int is_wait_die;
>
> if (!ww_ctx) {
> list_add_tail(&waiter->list, &lock->wait_list);
> return 0;
> }
>
> + is_wait_die = ww_ctx->ww_class->is_wait_die;
> +
> /*
> * Add the waiter before the first waiter with a higher stamp.
> * Waiters without a context are skipped to avoid starving
> - * them.
> + * them. Wait-Die waiters may back off here. Wound-Wait waiters
> + * never back off here, but they are sorted in stamp order and
> + * may wound the lock holder.
> */
> pos = &lock->wait_list;
> list_for_each_entry_reverse(cur, &lock->wait_list, list) {
> @@ -701,7 +758,7 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
>
> if (__ww_ctx_stamp_after(ww_ctx, cur->ww_ctx)) {
> /* Back off immediately if necessary. */
> - if (ww_ctx->acquired > 0) {
> + if (is_wait_die && ww_ctx->acquired > 0) {
> #ifdef CONFIG_DEBUG_MUTEXES
> struct ww_mutex *ww;
>
> @@ -721,13 +778,28 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
> * Wake up the waiter so that it gets a chance to back
> * off.
> */
> - if (cur->ww_ctx->acquired > 0) {
> + if (is_wait_die && cur->ww_ctx->acquired > 0) {
> debug_mutex_wake_waiter(lock, cur);
> wake_up_process(cur->task);
> }
> }
>
> list_add_tail(&waiter->list, pos);
> + if (!is_wait_die) {
> + struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
> +
> + /*
> + * Make sure a racing lock taker sees a non-empty waiting list
> + * before we read ww->ctx, so that if we miss ww->ctx, the
> + * racing lock taker will see a non-empty list and call
> + * __ww_mutex_wake_up_for_backoff() and wound itself. The
> + * memory barrier pairs with the one in
> + * ww_mutex_set_context_fastpath().
> + */
> + smp_mb();
> + __ww_mutex_wound(lock, ww_ctx, ww->ctx);
> + }
> +
> return 0;
> }
>
> @@ -750,6 +822,14 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> if (use_ww_ctx && ww_ctx) {
> if (unlikely(ww_ctx == READ_ONCE(ww->ctx)))
> return -EALREADY;
> +
> + /*
> + * Reset the wounded flag after a backoff.
> + * No other process can race and wound us here since they
> + * can't have a valid owner pointer at this time
> + */
> + if (ww_ctx->acquired == 0)
> + ww_ctx->wounded = 0;
> }
>
> preempt_disable();
> @@ -858,6 +938,11 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> acquired:
> __set_current_state(TASK_RUNNING);
>
> + /* We stole the lock. Need to check wounded status. */
> + if (use_ww_ctx && ww_ctx && !ww_ctx->ww_class->is_wait_die &&
> + !__mutex_waiter_is_first(lock, &waiter))
> + __ww_mutex_wakeup_for_backoff(lock, ww_ctx);
> +
> mutex_remove_waiter(lock, &waiter, current);
> if (likely(list_empty(&lock->wait_list)))
> __mutex_clear_flag(lock, MUTEX_FLAGS);
> diff --git a/kernel/locking/test-ww_mutex.c b/kernel/locking/test-ww_mutex.c
> index 0e4cd64ad2c0..3413430611d8 100644
> --- a/kernel/locking/test-ww_mutex.c
> +++ b/kernel/locking/test-ww_mutex.c
> @@ -26,7 +26,7 @@
> #include <linux/slab.h>
> #include <linux/ww_mutex.h>
>
> -static DEFINE_WW_CLASS(ww_class);
> +static DEFINE_WW_CLASS_WDIE(ww_class);
> struct workqueue_struct *wq;
>
> struct test_mutex {
> diff --git a/lib/locking-selftest.c b/lib/locking-selftest.c
> index b5c1293ce147..d0abf65ba9ad 100644
> --- a/lib/locking-selftest.c
> +++ b/lib/locking-selftest.c
> @@ -29,7 +29,7 @@
> */
> static unsigned int debug_locks_verbose;
>
> -static DEFINE_WW_CLASS(ww_lockdep);
> +static DEFINE_WW_CLASS_WDIE(ww_lockdep);
>
> static int __init setup_debug_locks_verbose(char *str)
> {
> --
> 2.14.3
>

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