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drm/i915: Drop i915_request.lock serialisation around await_start

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Originally, we used the signal->lock as a means of following the
previous link in its timeline and peeking at the previous fence.
However, we have replaced the explicit serialisation with a series of
very careful probes that anticipate the links being deleted and the
fences recycled before we are able to acquire a strong reference to it.
We do not need the signal->lock crutch anymore, nor want the contention.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210114135612.13210-2-chris@chris-wilson.co.uk
This commit is contained in:
Chris Wilson 2021-01-14 13:56:09 +00:00
parent 163433e5c5
commit b2fe00bbb2
1 changed files with 8 additions and 2 deletions
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10
drivers/gpu/drm/i915/i915_request.c
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@ -969,9 +969,16 @@ i915_request_await_start(struct i915_request *rq, struct i915_request *signal)
if (i915_request_started(signal))
return 0;
/*
* The caller holds a reference on @signal, but we do not serialise
* against it being retired and removed from the lists.
*
* We do not hold a reference to the request before @signal, and
* so must be very careful to ensure that it is not _recycled_ as
* we follow the link backwards.
*/
fence = NULL;
rcu_read_lock();
spin_lock_irq(&signal->lock);
do {
struct list_head *pos = READ_ONCE(signal->link.prev);
struct i915_request *prev;
@ -1002,7 +1009,6 @@ i915_request_await_start(struct i915_request *rq, struct i915_request *signal)
fence = &prev->fence;
} while (0);
spin_unlock_irq(&signal->lock);
rcu_read_unlock();
if (!fence)
return 0;