linux-kernel - Re: [PATCH v3 2/3] mutex: add support for wound/wait style locks, v3

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Message-ID: <519CA976.9000109@canonical.com>
Date:	Wed, 22 May 2013 13:18:14 +0200
From:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
To:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	peterz@...radead.org, x86@...nel.org,
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
	robclark@...il.com, rostedt@...dmis.org, tglx@...utronix.de,
	mingo@...e.hu, linux-media@...r.kernel.org,
	Dave Airlie <airlied@...hat.com>
Subject: Re: [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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