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References: <20180619082445.11062-1-thellstrom@vmware.com> <20180619082445.11062-3-thellstrom@vmware.com>
In-Reply-To: <20180619082445.11062-3-thellstrom@vmware.com>
From:   Rob Clark <robdclark@gmail.com>
Date:   Wed, 16 Jan 2019 09:24:10 -0500
Message-ID: <CAF6AEGubXRVL-PA7TUvW-oF3tTL=oaQ4F+AD_AiX=wUjxCov1g@mail.gmail.com>
Subject: Re: [PATCH v4 2/3] locking: Implement an algorithm choice for
 Wound-Wait mutexes
To:     Thomas Hellstrom <thellstrom@vmware.com>
Cc:     dri-devel <dri-devel@lists.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        VMware Graphics <linux-graphics-maintainer@vmware.com>,
        pv-drivers@vmware.com, Ingo Molnar <mingo@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Gustavo Padovan <gustavo@padovan.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Sean Paul <seanpaul@chromium.org>,
        David Airlie <airlied@linux.ie>,
        Davidlohr Bueso <dave@stgolabs.net>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Kate Stewart <kstewart@linuxfoundation.org>,
        Philippe Ombredanne <pombredanne@nexb.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-doc@vger.kernel.org, linux-media@vger.kernel.org,
        linaro-mm-sig@lists.linaro.org
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So, I guess this is to do w/ the magic of merge commits, but it looks
like the hunk changing the crtc_ww_class got lost:

=EE=82=B0 ~/src/linux =EE=82=B0 =EE=82=A0 master =EE=82=B0 git show --prett=
y=3Dshort
08295b3b5beec9aac0f7a9db86f0fc3792039da3
drivers/gpu/drm/drm_modeset_lock.c
commit 08295b3b5beec9aac0f7a9db86f0fc3792039da3
Author: Thomas Hellstrom <thellstrom@vmware.com>

    locking: Implement an algorithm choice for Wound-Wait mutexes

diff --git a/drivers/gpu/drm/drm_modeset_lock.c
b/drivers/gpu/drm/drm_modeset_lock.c
index 8a5100685875..638be2eb67b4 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_WD_CLASS(crtc_ww_class);

 /**
  * drm_modeset_lock_all - take all modeset locks
=EE=82=B0 ~/src/linux =EE=82=B0 =EE=82=A0 master =EE=82=B0 git log --pretty=
=3Dformat:%H
drivers/gpu/drm/drm_modeset_lock.c  | grep
08295b3b5beec9aac0f7a9db86f0fc3792039da3
=EE=82=B0 ~/src/linux =EE=82=B0 =EE=82=A0 master =EE=82=B0 1 =EE=82=B0


BR,
-R


On Tue, Jun 19, 2018 at 4:29 AM Thomas Hellstrom <thellstrom@vmware.com> wr=
ote:
>
> 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-18-06-15
>
> 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: 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
> Co-authored-by: Peter Zijlstra <peterz@infradead.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().
> v3:
> * Adapt to and incorporate cleanup by Peter Zijlstra
> * Remove unlocked use of list_empty().
> v4:
> * Move code related to adding a waiter to the lock waiter list to a
>   separate function.
> ---
>  Documentation/locking/ww-mutex-design.txt |  57 +++++++++--
>  drivers/dma-buf/reservation.c             |   2 +-
>  drivers/gpu/drm/drm_modeset_lock.c        |   2 +-
>  include/linux/ww_mutex.h                  |  17 ++-
>  kernel/locking/locktorture.c              |   2 +-
>  kernel/locking/mutex.c                    | 165 ++++++++++++++++++++++++=
+++---
>  kernel/locking/test-ww_mutex.c            |   2 +-
>  lib/locking-selftest.c                    |   2 +-
>  8 files changed, 213 insertions(+), 36 deletions(-)
>
> diff --git a/Documentation/locking/ww-mutex-design.txt b/Documentation/lo=
cking/ww-mutex-design.txt
> index 2fd7f2a2af21..f0ed7c30e695 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
>  =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>
>  Please read mutex-design.txt first, as it applies to wait/wound mutexes =
too.
> @@ -32,10 +32,26 @@ the oldest task) wins, and the one with the higher re=
servation 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/d=
ie:
> -The older tasks waits until it can acquire the contended lock. The young=
er tasks
> -needs to back off and drop all the locks it is currently holding, i.e. t=
he
> -younger task dies.
> +In the RDBMS literature, a reservation ticket is associated with a trans=
action.
> +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 mut=
ex.
> +If the transaction holding the lock is younger, the locking transaction =
waits.
> +If the transaction holding the lock is older, the locking transaction ba=
cks 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
> +wounds the transaction holding the lock, requesting it to die.
> +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 s=
ucceed.
> +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 wo=
rk than
> +Wait-Die when recovering from a backoff. Wound-Wait is also a preemptive
> +algorithm in that transactions are wounded by other transactions, and th=
at
> +requires a reliable way to pick up up the wounded condition and preempt =
the
> +running transaction. Note that this is not the same as process preemptio=
n. A
> +Wound-Wait transaction is considered preempted when it dies (returning
> +-EDEADLK) following a wound.
>
>  Concepts
>  --------
> @@ -47,10 +63,12 @@ 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 th=
e
>  acquire context. Furthermore the acquire context keeps track of debuggin=
g 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 expl=
icit 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 function=
s:
>
> @@ -90,6 +108,12 @@ provided.
>  Usage
>  -----
>
> +The algorithm (Wait-Die vs Wound-Wait) is chosen by using either
> +DEFINE_WW_CLASS() (Wound-Wait) or DEFINE_WD_CLASS() (Wait-Die)
> +As a rough rule of thumb, use Wound-Wait iff you
> +expect the number of simultaneous competing transactions to be typically=
 small,
> +and you want to reduce the number of rollbacks.
> +
>  Three different ways to acquire locks within the same w/w class. Common
>  definitions for methods #1 and #2:
>
> @@ -312,12 +336,23 @@ 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 loc=
ks
> -      acquired already (ctx->acquired > 0). Note that this waiter may co=
me
> -      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 w=
aiter
> +      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 loc=
k to
> +  wait for before restarting the transaction. Just blindly restarting th=
e
> +  transaction would likely make the transaction end up in a situation wh=
ere
> +  it would have to back off again.
>
>    In general, not much contention is expected. The locks are typically u=
sed to
> -  serialize access to resources for devices.
> +  serialize access to resources for devices, and optimization focus shou=
ld
> +  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..20bf90f4ee63 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_WD_CLASS(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_mod=
eset_lock.c
> index 8a5100685875..638be2eb67b4 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_WD_CLASS(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 f82fce2229c8..3af7c0e03be5 100644
> --- a/include/linux/ww_mutex.h
> +++ b/include/linux/ww_mutex.h
> @@ -8,6 +8,8 @@
>   *
>   * Wait/Die implementation:
>   *  Copyright (C) 2013 Canonical Ltd.
> + * Choice of algorithm:
> + *  Copyright (C) 2018 WMWare Inc.
>   *
>   * This file contains the main data structure and API definitions.
>   */
> @@ -23,12 +25,15 @@ 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 int acquired;
> +       unsigned short wounded;
> +       unsigned short is_wait_die;
>  #ifdef CONFIG_DEBUG_MUTEXES
>         unsigned int done_acquire;
>         struct ww_class *ww_class;
> @@ -58,17 +63,21 @@ 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 =3D ATOMIC_LONG_INIT(0) \
>                 , .acquire_name =3D #ww_class "_acquire" \
> -               , .mutex_name =3D #ww_class "_mutex" }
> +               , .mutex_name =3D #ww_class "_mutex" \
> +               , .is_wait_die =3D _is_wait_die }
>
>  #define __WW_MUTEX_INITIALIZER(lockname, class) \
>                 { .base =3D  __MUTEX_INITIALIZER(lockname.base) \
>                 __WW_CLASS_MUTEX_INITIALIZER(lockname, class) }
>
> +#define DEFINE_WD_CLASS(classname) \
> +       struct ww_class classname =3D __WW_CLASS_INITIALIZER(classname, 1=
)
> +
>  #define DEFINE_WW_CLASS(classname) \
> -       struct ww_class classname =3D __WW_CLASS_INITIALIZER(classname)
> +       struct ww_class classname =3D __WW_CLASS_INITIALIZER(classname, 0=
)
>
>  #define DEFINE_WW_MUTEX(mutexname, ww_class) \
>         struct ww_mutex mutexname =3D __WW_MUTEX_INITIALIZER(mutexname, w=
w_class)
> @@ -123,6 +132,8 @@ static inline void ww_acquire_init(struct ww_acquire_=
ctx *ctx,
>         ctx->task =3D current;
>         ctx->stamp =3D atomic_long_inc_return_relaxed(&ww_class->stamp);
>         ctx->acquired =3D 0;
> +       ctx->wounded =3D false;
> +       ctx->is_wait_die =3D ww_class->is_wait_die;
>  #ifdef CONFIG_DEBUG_MUTEXES
>         ctx->ww_class =3D ww_class;
>         ctx->done_acquire =3D 0;
> diff --git a/kernel/locking/locktorture.c b/kernel/locking/locktorture.c
> index 6850ffd69125..907e0325892c 100644
> --- a/kernel/locking/locktorture.c
> +++ b/kernel/locking/locktorture.c
> @@ -365,7 +365,7 @@ static struct lock_torture_ops mutex_lock_ops =3D {
>  };
>
>  #include <linux/ww_mutex.h>
> -static DEFINE_WW_CLASS(torture_ww_class);
> +static DEFINE_WD_CLASS(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 412b4fc08235..8ca83a5e3d24 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -172,6 +172,21 @@ static inline bool __mutex_waiter_is_first(struct mu=
tex *lock, struct mutex_wait
>         return list_first_entry(&lock->wait_list, struct mutex_waiter, li=
st) =3D=3D waiter;
>  }
>
> +/*
> + * Add @waiter to a given location in the lock wait_list and set the
> + * FLAG_WAITERS flag if it's the first waiter.
> + */
> +static void __sched
> +__mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter,
> +                  struct list_head *list)
> +{
> +       debug_mutex_add_waiter(lock, waiter, current);
> +
> +       list_add_tail(&waiter->list, list);
> +       if (__mutex_waiter_is_first(lock, waiter))
> +               __mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
> +}
> +
>  /*
>   * Give up ownership to a specific task, when @task =3D NULL, this is eq=
uivalent
>   * to a regular unlock. Sets PICKUP on a handoff, clears HANDOF, preserv=
es
> @@ -248,6 +263,11 @@ EXPORT_SYMBOL(mutex_lock);
>   *   The newer transactions are killed when:
>   *     It (the new transaction) makes a request for a lock being held
>   *     by an older transaction.
> + *
> + * Wound-Wait:
> + *   The newer transactions are wounded when:
> + *     An older transaction makes a request for a lock being held by
> + *     the newer transaction.
>   */
>
>  /*
> @@ -319,6 +339,9 @@ static bool __sched
>  __ww_mutex_die(struct mutex *lock, struct mutex_waiter *waiter,
>                struct ww_acquire_ctx *ww_ctx)
>  {
> +       if (!ww_ctx->is_wait_die)
> +               return false;
> +
>         if (waiter->ww_ctx->acquired > 0 &&
>                         __ww_ctx_stamp_after(waiter->ww_ctx, ww_ctx)) {
>                 debug_mutex_wake_waiter(lock, waiter);
> @@ -328,13 +351,65 @@ __ww_mutex_die(struct mutex *lock, struct mutex_wai=
ter *waiter,
>         return true;
>  }
>
> +/*
> + * Wound-Wait; wound a younger @hold_ctx if it holds the lock.
> + *
> + * Wound the lock holder if there are waiters with older transactions th=
an
> + * the lock holders. Even if multiple waiters may wound the lock holder,
> + * it's sufficient that only one does.
> + */
> +static bool __ww_mutex_wound(struct mutex *lock,
> +                            struct ww_acquire_ctx *ww_ctx,
> +                            struct ww_acquire_ctx *hold_ctx)
> +{
> +       struct task_struct *owner =3D __mutex_owner(lock);
> +
> +       lockdep_assert_held(&lock->wait_lock);
> +
> +       /*
> +        * Possible through __ww_mutex_add_waiter() when we race with
> +        * ww_mutex_set_context_fastpath(). In that case we'll get here a=
gain
> +        * through __ww_mutex_check_waiters().
> +        */
> +       if (!hold_ctx)
> +               return false;
> +
> +       /*
> +        * Can have !owner because of __mutex_unlock_slowpath(), but if o=
wner,
> +        * it cannot go away because we'll have FLAG_WAITERS set and hold
> +        * wait_lock.
> +        */
> +       if (!owner)
> +               return false;
> +
> +       if (ww_ctx->acquired > 0 && __ww_ctx_stamp_after(hold_ctx, ww_ctx=
)) {
> +               hold_ctx->wounded =3D 1;
> +
> +               /*
> +                * wake_up_process() paired with set_current_state()
> +                * inserts sufficient barriers to make sure @owner either=
 sees
> +                * it's wounded in __ww_mutex_lock_check_stamp() or has a
> +                * wakeup pending to re-read the wounded state.
> +                */
> +               if (owner !=3D current)
> +                       wake_up_process(owner);
> +
> +               return true;
> +       }
> +
> +       return false;
> +}
> +
>  /*
>   * We just acquired @lock under @ww_ctx, if there are later contexts wai=
ting
> - * behind us on the wait-list, check if they need to die.
> + * behind us on the wait-list, check if they need to die, or wound us.
>   *
>   * See __ww_mutex_add_waiter() for the list-order construction; basicall=
y the
>   * list is ordered by stamp, smallest (oldest) first.
>   *
> + * This relies on never mixing wait-die/wound-wait on the same wait-list=
;
> + * which is currently ensured by that being a ww_class property.
> + *
>   * The current task must not be on the wait list.
>   */
>  static void __sched
> @@ -348,7 +423,8 @@ __ww_mutex_check_waiters(struct mutex *lock, struct w=
w_acquire_ctx *ww_ctx)
>                 if (!cur->ww_ctx)
>                         continue;
>
> -               if (__ww_mutex_die(lock, cur, ww_ctx))
> +               if (__ww_mutex_die(lock, cur, ww_ctx) ||
> +                   __ww_mutex_wound(lock, cur->ww_ctx, ww_ctx))
>                         break;
>         }
>  }
> @@ -369,17 +445,23 @@ 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
> +        * [W] ww->ctx =3D ctx        [W] MUTEX_FLAG_WAITERS
> +        *     MB                       MB
> +        * [R] MUTEX_FLAG_WAITERS   [R] ww->ctx
> +        *
> +        * The memory barrier above pairs with the memory barrier in
> +        * __ww_mutex_add_waiter() and makes sure we either observe ww->c=
tx
> +        * and/or !empty list.
>          */
>         if (likely(!(atomic_long_read(&lock->base.owner) & MUTEX_FLAG_WAI=
TERS)))
>                 return;
>
>         /*
>          * Uh oh, we raced in fastpath, check if any of the waiters need =
to
> -        * die.
> +        * die or wound us.
>          */
>         spin_lock(&lock->base.wait_lock);
>         __ww_mutex_check_waiters(&lock->base, ctx);
> @@ -681,7 +763,9 @@ __ww_mutex_kill(struct mutex *lock, struct ww_acquire=
_ctx *ww_ctx)
>
>
>  /*
> - * Check whether we need to kill the transaction for the current lock ac=
quire.
> + * Check the wound condition for the current lock acquire.
> + *
> + * Wound-Wait: If we're wounded, kill ourself.
>   *
>   * Wait-Die: If we're trying to acquire a lock already held by an older
>   *           context, kill ourselves.
> @@ -700,6 +784,13 @@ __ww_mutex_check_kill(struct mutex *lock, struct mut=
ex_waiter *waiter,
>         if (ctx->acquired =3D=3D 0)
>                 return 0;
>
> +       if (!ctx->is_wait_die) {
> +               if (ctx->wounded)
> +                       return __ww_mutex_kill(lock, ctx);
> +
> +               return 0;
> +       }
> +
>         if (hold_ctx && __ww_ctx_stamp_after(ctx, hold_ctx))
>                 return __ww_mutex_kill(lock, ctx);
>
> @@ -726,7 +817,8 @@ __ww_mutex_check_kill(struct mutex *lock, struct mute=
x_waiter *waiter,
>   * Waiters without context are interspersed in FIFO order.
>   *
>   * Furthermore, for Wait-Die kill ourself immediately when possible (the=
re are
> - * older contexts already waiting) to avoid unnecessary waiting.
> + * older contexts already waiting) to avoid unnecessary waiting and for
> + * Wound-Wait ensure we wound the owning context when it is younger.
>   */
>  static inline int __sched
>  __ww_mutex_add_waiter(struct mutex_waiter *waiter,
> @@ -735,16 +827,21 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
>  {
>         struct mutex_waiter *cur;
>         struct list_head *pos;
> +       bool is_wait_die;
>
>         if (!ww_ctx) {
> -               list_add_tail(&waiter->list, &lock->wait_list);
> +               __mutex_add_waiter(lock, waiter, &lock->wait_list);
>                 return 0;
>         }
>
> +       is_wait_die =3D ww_ctx->is_wait_die;
> +
>         /*
>          * Add the waiter before the first waiter with a higher stamp.
>          * Waiters without a context are skipped to avoid starving
> -        * them. Wait-Die waiters may die here.
> +        * them. Wait-Die waiters may die here. Wound-Wait waiters
> +        * never die here, but they are sorted in stamp order and
> +        * may wound the lock holder.
>          */
>         pos =3D &lock->wait_list;
>         list_for_each_entry_reverse(cur, &lock->wait_list, list) {
> @@ -757,10 +854,12 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
>                          * is no point in queueing behind it, as we'd hav=
e to
>                          * die the moment it would acquire the lock.
>                          */
> -                       int ret =3D __ww_mutex_kill(lock, ww_ctx);
> +                       if (is_wait_die) {
> +                               int ret =3D __ww_mutex_kill(lock, ww_ctx)=
;
>
> -                       if (ret)
> -                               return ret;
> +                               if (ret)
> +                                       return ret;
> +                       }
>
>                         break;
>                 }
> @@ -771,7 +870,23 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
>                 __ww_mutex_die(lock, cur, ww_ctx);
>         }
>
> -       list_add_tail(&waiter->list, pos);
> +       __mutex_add_waiter(lock, waiter, pos);
> +
> +       /*
> +        * Wound-Wait: if we're blocking on a mutex owned by a younger co=
ntext,
> +        * wound that such that we might proceed.
> +        */
> +       if (!is_wait_die) {
> +               struct ww_mutex *ww =3D container_of(lock, struct ww_mute=
x, base);
> +
> +               /*
> +                * See ww_mutex_set_context_fastpath(). Orders setting
> +                * MUTEX_FLAG_WAITERS vs the ww->ctx load,
> +                * such that either we or the fastpath will wound @ww->ct=
x.
> +                */
> +               smp_mb();
> +               __ww_mutex_wound(lock, ww_ctx, ww->ctx);
> +       }
>
>         return 0;
>  }
> @@ -795,6 +910,14 @@ __mutex_lock_common(struct mutex *lock, long state, =
unsigned int subclass,
>         if (use_ww_ctx && ww_ctx) {
>                 if (unlikely(ww_ctx =3D=3D READ_ONCE(ww->ctx)))
>                         return -EALREADY;
> +
> +               /*
> +                * Reset the wounded flag after a kill. No other process =
can
> +                * race and wound us here since they can't have a valid o=
wner
> +                * pointer if we don't have any locks held.
> +                */
> +               if (ww_ctx->acquired =3D=3D 0)
> +                       ww_ctx->wounded =3D 0;
>         }
>
>         preempt_disable();
> @@ -828,7 +951,8 @@ __mutex_lock_common(struct mutex *lock, long state, u=
nsigned int subclass,
>
>         if (!use_ww_ctx) {
>                 /* add waiting tasks to the end of the waitqueue (FIFO): =
*/
> -               list_add_tail(&waiter.list, &lock->wait_list);
> +               __mutex_add_waiter(lock, &waiter, &lock->wait_list);
> +
>
>  #ifdef CONFIG_DEBUG_MUTEXES
>                 waiter.ww_ctx =3D MUTEX_POISON_WW_CTX;
> @@ -847,9 +971,6 @@ __mutex_lock_common(struct mutex *lock, long state, u=
nsigned int subclass,
>
>         waiter.task =3D current;
>
> -       if (__mutex_waiter_is_first(lock, &waiter))
> -               __mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
> -
>         set_current_state(state);
>         for (;;) {
>                 /*
> @@ -906,6 +1027,16 @@ __mutex_lock_common(struct mutex *lock, long state,=
 unsigned int subclass,
>  acquired:
>         __set_current_state(TASK_RUNNING);
>
> +       if (use_ww_ctx && ww_ctx) {
> +               /*
> +                * Wound-Wait; we stole the lock (!first_waiter), check t=
he
> +                * waiters as anyone might want to wound us.
> +                */
> +               if (!ww_ctx->is_wait_die &&
> +                   !__mutex_waiter_is_first(lock, &waiter))
> +                       __ww_mutex_check_waiters(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_mute=
x.c
> index 0e4cd64ad2c0..5b915b370d5a 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_WD_CLASS(ww_class);
>  struct workqueue_struct *wq;
>
>  struct test_mutex {
> diff --git a/lib/locking-selftest.c b/lib/locking-selftest.c
> index b5c1293ce147..1e1bbf171eca 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_WD_CLASS(ww_lockdep);
>
>  static int __init setup_debug_locks_verbose(char *str)
>  {
> --
> 2.14.3
>