Am 26.10.18 um 10:03 schrieb Chris Wilson:
> We need to serialise the addition of a new fence into the shared list
> such that the fence is visible before we claim it is there. Otherwise a
> concurrent reader of the shared fence list will see an uninitialised
> fence slot before it is set.
>
> <4> [109.613162] general protection fault: 0000 [#1] PREEMPT SMP PTI
> <4> [109.613177] CPU: 1 PID: 1357 Comm: gem_busy Tainted: G U 4.19.0-rc8-CI-CI_DRM_5035+ #1
> <4> [109.613189] Hardware name: Dell Inc. XPS 8300 /0Y2MRG, BIOS A06 10/17/2011
> <4> [109.613252] RIP: 0010:i915_gem_busy_ioctl+0x146/0x380 [i915]
> <4> [109.613261] Code: 0b 43 04 49 83 c6 08 4d 39 e6 89 43 04 74 6d 4d 8b 3e e8 5d 54 f4 e0 85 c0 74 0d 80 3d 08 71 1d 00 00
> 0f 84 bb 00 00 00 31 c0 <49> 81 7f 08 20 3a 2c a0 75 cc 41 8b 97 50 02 00 00 49 8b 8f a8 00
> <4> [109.613283] RSP: 0018:ffffc9000044bcf8 EFLAGS: 00010246
> <4> [109.613292] RAX: 0000000000000000 RBX: ffffc9000044bdc0 RCX: 0000000000000001
> <4> [109.613302] RDX: 0000000000000000 RSI: 00000000ffffffff RDI: ffffffff822474a0
> <4> [109.613311] RBP: ffffc9000044bd28 R08: ffff88021e158680 R09: 0000000000000001
> <4> [109.613321] R10: 0000000000000040 R11: 0000000000000000 R12: ffff88021e1641b8
> <4> [109.613331] R13: 0000000000000003 R14: ffff88021e1641b0 R15: 6b6b6b6b6b6b6b6b
> <4> [109.613341] FS: 00007f9c9fc84980(0000) GS:ffff880227a40000(0000) knlGS:0000000000000000
> <4> [109.613352] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> <4> [109.613360] CR2: 00007f9c9fcb8000 CR3: 00000002247d4005 CR4: 00000000000606e0
>
> Fixes: 27836b641c1b ("dma-buf: remove shared fence staging in reservation object")
> Testcase: igt/gem_busy/close-race
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Christian König <christian.koenig@xxxxxxx>
> Cc: Junwei Zhang <Jerry.Zhang@xxxxxxx>
> Cc: Huang Rui <ray.huang@xxxxxxx>
> Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
Reviewed-by: Christian König <christian.koenig@xxxxxxx>
> ---
> drivers/dma-buf/reservation.c | 14 +++++++-------
> 1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
> index 5fb4fd461908..c1618335ca99 100644
> --- a/drivers/dma-buf/reservation.c
> +++ b/drivers/dma-buf/reservation.c
> @@ -147,16 +147,17 @@ void reservation_object_add_shared_fence(struct reservation_object *obj,
> struct dma_fence *fence)
> {
> struct reservation_object_list *fobj;
> - unsigned int i;
> + unsigned int i, count;
>
> dma_fence_get(fence);
>
> fobj = reservation_object_get_list(obj);
> + count = fobj->shared_count;
>
> preempt_disable();
> write_seqcount_begin(&obj->seq);
>
> - for (i = 0; i < fobj->shared_count; ++i) {
> + for (i = 0; i < count; ++i) {
> struct dma_fence *old_fence;
>
> old_fence = rcu_dereference_protected(fobj->shared[i],
> @@ -169,14 +170,13 @@ void reservation_object_add_shared_fence(struct reservation_object *obj,
> }
>
> BUG_ON(fobj->shared_count >= fobj->shared_max);
> - fobj->shared_count++;
> + count++;
>
> replace:
> - /*
> - * memory barrier is added by write_seqcount_begin,
> - * fobj->shared_count is protected by this lock too
> - */
> RCU_INIT_POINTER(fobj->shared[i], fence);
> + /* pointer update must be visible before we extend the shared_count */
> + smp_store_mb(fobj->shared_count, count);
> +
> write_seqcount_end(&obj->seq);
> preempt_enable();
> }
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