[PATCH 2/5] drm: Add Reusable task barrier.

[Christian König]

Am 11.12.19 um 21:19 schrieb Alex Deucher:
> On Wed, Dec 11, 2019 at 3:07 PM Andrey Grodzovsky
> <andrey.grodzovsky at amd.com> wrote:
>> It is used to synchronize N threads at a rendevouz point before execution
>> of critical code that has to be started by all the threads at approximatly
>> the same time.
>>
>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky at amd.com>
> You should resend to dri-devel since this task barrier is being added
> to common code.

Additional to that this whole thing has the potential to raise lockdep 
warnings and if I'm not completely mistaken doesn't even work correctly.

See Linux kernel semaphores don't allow negative values (the count field 
in struct semaphore is unsigned).

Regards,
Christian.

>
> Alex
>
>> ---
>>   include/drm/task_barrier.h | 106 +++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 106 insertions(+)
>>   create mode 100644 include/drm/task_barrier.h
>>
>> diff --git a/include/drm/task_barrier.h b/include/drm/task_barrier.h
>> new file mode 100644
>> index 0000000..81fb0f7
>> --- /dev/null
>> +++ b/include/drm/task_barrier.h
>> @@ -0,0 +1,106 @@
>> +/*
>> + * Copyright 2019 Advanced Micro Devices, Inc.
>> + *
>> + * Permission is hereby granted, free of charge, to any person obtaining a
>> + * copy of this software and associated documentation files (the "Software"),
>> + * to deal in the Software without restriction, including without limitation
>> + * the rights to use, copy, modify, merge, publish, distribute, sublicense,
>> + * and/or sell copies of the Software, and to permit persons to whom the
>> + * Software is furnished to do so, subject to the following conditions:
>> + *
>> + * The above copyright notice and this permission notice shall be included in
>> + * all copies or substantial portions of the Software.
>> + *
>> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
>> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
>> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
>> + * THE COPYRIGHT HOLDER(S) OR AUTHOR(S) BE LIABLE FOR ANY CLAIM, DAMAGES OR
>> + * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
>> + * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
>> + * OTHER DEALINGS IN THE SOFTWARE.
>> + *
>> + */
>> +#include <linux/semaphore.h>
>> +#include <linux/atomic.h>
>> +
>> +/*
>> + * Reusable 2 PHASE task barrier (randevouz point) implementation for N tasks.
>> + * Based on the Little book of sempahores - https://greenteapress.com/wp/semaphores/
>> + */
>> +
>> +
>> +
>> +#ifndef DRM_TASK_BARRIER_H_
>> +#define DRM_TASK_BARRIER_H_
>> +
>> +/*
>> + * Represents an instance of a task barrier.
>> + */
>> +struct task_barrier {
>> +       unsigned int n;
>> +       atomic_t count;
>> +       struct semaphore enter_turnstile;
>> +       struct semaphore exit_turnstile;
>> +};
>> +
>> +static inline void task_barrier_signal_turnstile(struct semaphore *turnstile,
>> +                                                unsigned int n)
>> +{
>> +       int i;
>> +
>> +       for (i = 0 ; i < n; i++)
>> +               up(turnstile);
>> +}
>> +
>> +static inline void task_barrier_init(struct task_barrier *tb)
>> +{
>> +       tb->n = 0;
>> +       atomic_set(&tb->count, 0);
>> +       sema_init(&tb->enter_turnstile, 0);
>> +       sema_init(&tb->exit_turnstile, 0);
>> +}
>> +
>> +static inline void task_barrier_add_task(struct task_barrier *tb)
>> +{
>> +       tb->n++;
>> +}
>> +
>> +static inline void task_barrier_rem_task(struct task_barrier *tb)
>> +{
>> +       tb->n--;
>> +}
>> +
>> +/*
>> + * Lines up all the threads BEFORE the critical point.
>> + *
>> + * When all thread passed this code the entry barrier is back to locked state.
>> + */
>> +static inline void task_barrier_enter(struct task_barrier *tb)
>> +{
>> +       if (atomic_inc_return(&tb->count) == tb->n)
>> +               task_barrier_signal_turnstile(&tb->enter_turnstile, tb->n);
>> +
>> +       down(&tb->enter_turnstile);
>> +}
>> +
>> +/*
>> + * Lines up all the threads AFTER the critical point.
>> + *
>> + * This function is used to avoid any one thread running ahead of the reset if
>> + * the barrier is used in a loop (repeatedly) .
>> + */
>> +static inline void task_barrier_exit(struct task_barrier *tb)
>> +{
>> +       if (atomic_dec_return(&tb->count) == 0)
>> +               task_barrier_signal_turnstile(&tb->exit_turnstile, tb->n);
>> +
>> +       down(&tb->exit_turnstile);
>> +}
>> +
>> +static inline void task_barrier_full(struct task_barrier *tb)
>> +{
>> +       task_barrier_enter(tb);
>> +       task_barrier_exit(tb);
>> +}
>> +
>> +#endif
>> --
>> 2.7.4
>>
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