From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
Date: Mon, 20 Jul 2020 17:55:18 +0200
Subject: dma-buf: Use sequence counter with associated wound/wait mutex
Patch-mainline: v5.9-rc1
Git-commit: cd29f22019ec4ab998d2e1e8c831c7c42db4aa7d
References: bsc#1176564 bsc#1162702

A sequence counter write side critical section must be protected by some
form of locking to serialize writers. If the serialization primitive is
not disabling preemption implicitly, preemption has to be explicitly
disabled before entering the sequence counter write side critical
section.

The dma-buf reservation subsystem uses plain sequence counters to manage
updates to reservations. Writer serialization is accomplished through a
wound/wait mutex.

Acquiring a wound/wait mutex does not disable preemption, so this needs
to be done manually before and after the write side critical section.

Use the newly-added seqcount_ww_mutex_t instead:

  - It associates the ww_mutex with the sequence count, which enables
    lockdep to validate that the write side critical section is properly
    serialized.

  - It removes the need to explicitly add preempt_disable/enable()
    around the write side critical section because the write_begin/end()
    functions for this new data type automatically do this.

If lockdep is disabled this ww_mutex lock association is compiled out
and has neither storage size nor runtime overhead.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://lkml.kernel.org/r/20200720155530.1173732-13-a.darwish@linutronix.de
[dwagner: context updated]
Acked-by: Daniel Wagner <dwagner@suse.de>
---
 drivers/dma-buf/dma-resv.c                    |    8 --------
 drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c |    2 --
 include/linux/dma-resv.h                      |    4 ++--
 3 files changed, 2 insertions(+), 12 deletions(-)

--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -97,7 +97,7 @@ void dma_resv_init(struct dma_resv *obj)
 {
 	ww_mutex_init(&obj->lock, &reservation_ww_class);
 
-	seqcount_init(&obj->seq);
+	seqcount_ww_mutex_init(&obj->seq, &obj->lock);
 	RCU_INIT_POINTER(obj->fence, NULL);
 	RCU_INIT_POINTER(obj->fence_excl, NULL);
 }
@@ -227,7 +227,6 @@ void dma_resv_add_shared_fence(struct dm
 	fobj = dma_resv_get_list(obj);
 	count = fobj->shared_count;
 
-	preempt_disable();
 	write_seqcount_begin(&obj->seq);
 
 	for (i = 0; i < count; ++i) {
@@ -249,7 +248,6 @@ replace:
 	smp_store_mb(fobj->shared_count, count);
 
 	write_seqcount_end(&obj->seq);
-	preempt_enable();
 	dma_fence_put(old);
 }
 EXPORT_SYMBOL(dma_resv_add_shared_fence);
@@ -276,14 +274,12 @@ void dma_resv_add_excl_fence(struct dma_
 	if (fence)
 		dma_fence_get(fence);
 
-	preempt_disable();
 	write_seqcount_begin(&obj->seq);
 	/* write_seqcount_begin provides the necessary memory barrier */
 	RCU_INIT_POINTER(obj->fence_excl, fence);
 	if (old)
 		old->shared_count = 0;
 	write_seqcount_end(&obj->seq);
-	preempt_enable();
 
 	/* inplace update, no shared fences */
 	while (i--)
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
@@ -252,11 +252,9 @@ static int amdgpu_amdkfd_remove_eviction
 	new->shared_count = k;
 
 	/* Install the new fence list, seqcount provides the barriers */
-	preempt_disable();
 	write_seqcount_begin(&resv->seq);
 	RCU_INIT_POINTER(resv->fence, new);
 	write_seqcount_end(&resv->seq);
-	preempt_enable();
 
 	/* Drop the references to the removed fences or move them to ef_list */
 	for (i = j, k = 0; i < old->shared_count; ++i) {
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -69,7 +69,7 @@ struct dma_resv_list {
  */
 struct dma_resv {
 	struct ww_mutex lock;
-	seqcount_t seq;
+	seqcount_ww_mutex_t seq;
 
 	struct dma_fence __rcu *fence_excl;
 	struct dma_resv_list __rcu *fence;