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    SubjectRe: [PATCH v3 2/3] mutex: add support for wound/wait style locks, v3
    Hey,

    Op 30-04-13 21:14, Daniel Vetter schreef:
    > On Sun, Apr 28, 2013 at 07:04:07PM +0200, Maarten Lankhorst wrote:
    >> Changes since RFC patch v1:
    >> - Updated to use atomic_long instead of atomic, since the reservation_id was a long.
    >> - added mutex_reserve_lock_slow and mutex_reserve_lock_intr_slow
    >> - removed mutex_locked_set_reservation_id (or w/e it was called)
    >> Changes since RFC patch v2:
    >> - remove use of __mutex_lock_retval_arg, add warnings when using wrong combination of
    >> mutex_(,reserve_)lock/unlock.
    >> Changes since v1:
    >> - Add __always_inline to __mutex_lock_common, otherwise reservation paths can be
    >> triggered from normal locks, because __builtin_constant_p might evaluate to false
    >> for the constant 0 in that case. Tests for this have been added in the next patch.
    >> - Updated documentation slightly.
    >> Changes since v2:
    >> - Renamed everything to ww_mutex. (mlankhorst)
    >> - Added ww_acquire_ctx and ww_class. (mlankhorst)
    >> - Added a lot of checks for wrong api usage. (mlankhorst)
    >> - Documentation updates. (danvet)
    > While writing the kerneldoc I've carefully check that all restrictions are
    > enforced through debug checks somehow. I think that with full mutex debug
    > (including lockdep) enabled, plus the slowpath injector patch I've just
    > posted, _all_ interface abuse will be catched at runtime as long as all
    > the single-threaded/uncontended cases are exercises sufficiently.
    >
    > So I think we've fully achieved level 5 on the Rusty API safety scale
    > here. Higher levels seem pretty hard given that the concepts are rather
    > fancy, but I think with the new (and much more consitent) naming, plus the
    > explicit introduction as (more abstruct) structures for ww_class and
    > ww_acquire_context the interface is about as intuitive as it gets.
    >
    > So all together I'm pretty happy with what the interface looks like. And
    > one quick bikeshed below on the implementation.
    > -Daniel
    I included your fix below. I'm hoping to get this included in 3.11 through the drm tree, so
    I can convert ttm to use it, but I haven't received any further reply on the patch series.

    The 3.10 mutex improvement patches don't seem to cause any conflicts when merging
    linus' tree, so I'll use drm-next as a base.

    Are there any issues left? I included the patch you wrote for injecting -EDEADLK too
    in my tree. The overwhelming silence makes me think there are either none, or
    nobody cared enough to review it. :(

    >> +/*
    >> + * after acquiring lock with fastpath or when we lost out in contested
    >> + * slowpath, set ctx and wake up any waiters so they can recheck.
    >> + *
    >> + * This function is never called when CONFIG_DEBUG_LOCK_ALLOC is set,
    >> + * as the fastpath and opportunistic spinning are disabled in that case.
    >> + */
    >> +static __always_inline void
    >> +ww_mutex_set_context_fastpath(struct ww_mutex *lock,
    >> + struct ww_acquire_ctx *ctx)
    >> +{
    >> + unsigned long flags;
    >> + struct mutex_waiter *cur;
    >> +
    >> + ww_mutex_lock_acquired(lock, ctx, false);
    >> +
    >> + lock->ctx = ctx;
    >> + smp_mb__after_atomic_dec();
    > I think this should be
    >
    > + smp_mb__after_atomic_dec();
    > + lock->ctx = ctx;
    > + smp_mb();
    >
    > Also I wonder a bit how much this hurts the fastpath, and whether we
    > should just shovel the ctx into the atomic field with a cmpxcht, like the
    > rt mutex code does with the current pointer.
    >
    Fixed. I'm not sure if the second smp_mb is really needed. If there was a
    smp_mb__before_atomic_read it would have been sufficient.

    ~Maarten


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