From b2fe00bbb2b6bf06929dc300a4e3e91d9c12a3c9 Mon Sep 17 00:00:00 2001 From: Chris Wilson Date: Thu, 14 Jan 2021 13:56:09 +0000 Subject: [PATCH] drm/i915: Drop i915_request.lock serialisation around await_start 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 Cc: Tvrtko Ursulin Reviewed-by: Andi Shyti Link: https://patchwork.freedesktop.org/patch/msgid/20210114135612.13210-2-chris@chris-wilson.co.uk --- drivers/gpu/drm/i915/i915_request.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c index 784c05a..973ecea 100644 --- a/drivers/gpu/drm/i915/i915_request.c +++ b/drivers/gpu/drm/i915/i915_request.c @@ -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; -- 2.7.4