Re: [PATCH] dma-buf: Update reservation shared_count after adding the new fence

["Koenig, Christian" <Christian.Koenig@amd.com>]

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@chris-wilson.co.uk>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Junwei Zhang <Jerry.Zhang@amd.com>
> Cc: Huang Rui <ray.huang@amd.com>
> Cc: Sumit Semwal <sumit.semwal@linaro.org>

Reviewed-by: Christian König <christian.koenig@amd.com>

> ---
>   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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