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
>
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxx>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> ---
> Documentation/ww-mutex-design.txt | 322 +++++++++++++++++++++++++++
> include/linux/mutex-debug.h | 1
> include/linux/mutex.h | 257 +++++++++++++++++++++
> kernel/mutex.c | 445 ++++++++++++++++++++++++++++++++++++-
> lib/debug_locks.c | 2
> 5 files changed, 1010 insertions(+), 17 deletions(-)
> create mode 100644 Documentation/ww-mutex-design.txt
[snip]
> +/*
> + * 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.
> +
> + /*
> + * Check if lock is contended, if not there is nobody to wake up
> + */
> + if (likely(atomic_read(&lock->base.count) == 0))
> + return;
> +
> + /*
> + * Uh oh, we raced in fastpath, wake up everyone in this case,
> + * so they can see the new ctx
> + */
> + spin_lock_mutex(&lock->base.wait_lock, flags);
> + list_for_each_entry(cur, &lock->base.wait_list, list) {
> + debug_mutex_wake_waiter(&lock->base, cur);
> + wake_up_process(cur->task);
> + }
> + spin_unlock_mutex(&lock->base.wait_lock, flags);
> +}
> +
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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