From 0b1570b7ffe68dfefa07cb092a0723f898bb8184 Mon Sep 17 00:00:00 2001 From: Chris Wilson Date: Thu, 27 Feb 2020 08:57:14 +0000 Subject: [PATCH] drm/i915: Protect i915_request_await_start from early waits We need to be extremely careful inside i915_request_await_start() as it needs to walk the list of requests in the foreign timeline with very little protection. As we hold our own timeline mutex, we can not nest inside the signaler's timeline mutex, so all that remains is our RCU protection. However, to be safe we need to tell the compiler that we may be traversing the list only under RCU protection, and furthermore we need to start declaring requests as elements of the timeline from their construction. Fixes: 9ddc8ec027a3 ("drm/i915: Eliminate the trylock for awaiting an earlier request") Fixes: 6a79d848403d ("drm/i915: Lock signaler timeline while navigating") Signed-off-by: Chris Wilson Reviewed-by: Tvrtko Ursulin Link: https://patchwork.freedesktop.org/patch/msgid/20200227085723.1961649-11-chris@chris-wilson.co.uk (cherry picked from commit d22d2d073ef859b346bc32cb25299262e3973769) Signed-off-by: Jani Nikula --- drivers/gpu/drm/i915/i915_request.c | 41 ++++++++++++++++++++--------- 1 file changed, 28 insertions(+), 13 deletions(-) diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c index f56b046a32de..dcaa85a91090 100644 --- a/drivers/gpu/drm/i915/i915_request.c +++ b/drivers/gpu/drm/i915/i915_request.c @@ -275,7 +275,7 @@ bool i915_request_retire(struct i915_request *rq) spin_unlock_irq(&rq->lock); remove_from_client(rq); - list_del(&rq->link); + list_del_rcu(&rq->link); intel_context_exit(rq->context); intel_context_unpin(rq->context); @@ -721,6 +721,8 @@ __i915_request_create(struct intel_context *ce, gfp_t gfp) rq->infix = rq->ring->emit; /* end of header; start of user payload */ intel_context_mark_active(ce); + list_add_tail_rcu(&rq->link, &tl->requests); + return rq; err_unwind: @@ -777,13 +779,23 @@ i915_request_await_start(struct i915_request *rq, struct i915_request *signal) GEM_BUG_ON(i915_request_timeline(rq) == rcu_access_pointer(signal->timeline)); + if (i915_request_started(signal)) + return 0; + fence = NULL; rcu_read_lock(); spin_lock_irq(&signal->lock); - if (!i915_request_started(signal) && - !list_is_first(&signal->link, - &rcu_dereference(signal->timeline)->requests)) { - struct i915_request *prev = list_prev_entry(signal, link); + do { + struct list_head *pos = READ_ONCE(signal->link.prev); + struct i915_request *prev; + + /* Confirm signal has not been retired, the link is valid */ + if (unlikely(i915_request_started(signal))) + break; + + /* Is signal the earliest request on its timeline? */ + if (pos == &rcu_dereference(signal->timeline)->requests) + break; /* * Peek at the request before us in the timeline. That @@ -791,13 +803,18 @@ i915_request_await_start(struct i915_request *rq, struct i915_request *signal) * after acquiring a reference to it, confirm that it is * still part of the signaler's timeline. */ - if (i915_request_get_rcu(prev)) { - if (list_next_entry(prev, link) == signal) - fence = &prev->fence; - else - i915_request_put(prev); + prev = list_entry(pos, typeof(*prev), link); + if (!i915_request_get_rcu(prev)) + break; + + /* After the strong barrier, confirm prev is still attached */ + if (unlikely(READ_ONCE(prev->link.next) != &signal->link)) { + i915_request_put(prev); + break; } - } + + fence = &prev->fence; + } while (0); spin_unlock_irq(&signal->lock); rcu_read_unlock(); if (!fence) @@ -1242,8 +1259,6 @@ __i915_request_add_to_timeline(struct i915_request *rq) 0); } - list_add_tail(&rq->link, &timeline->requests); - /* * Make sure that no request gazumped us - if it was allocated after * our i915_request_alloc() and called __i915_request_add() before -- 2.30.2