Re: [PATCH 5/6] drm/ttm: replace dma_resv object on deleted BOs v2

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Am 11.02.20 um 16:02 schrieb Pan, Xinhui:

2020年2月11日 22:14,Daniel Vetter <daniel@xxxxxxxx> 写道:

On Mon, Feb 10, 2020 at 04:09:06PM +0100, Christian König wrote:
When non-imported BOs are resurrected for delayed delete we replace
the dma_resv object to allow for easy reclaiming of the resources.

v2: move that to ttm_bo_individualize_resv

Signed-off-by: Christian König <christian.koenig@xxxxxxx>
---
drivers/gpu/drm/ttm/ttm_bo.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
index d0624685f5d2..4d161038de98 100644
--- a/drivers/gpu/drm/ttm/ttm_bo.c
+++ b/drivers/gpu/drm/ttm/ttm_bo.c
@@ -393,6 +393,14 @@ static int ttm_bo_individualize_resv(struct ttm_buffer_object *bo)

	r = dma_resv_copy_fences(&bo->base._resv, bo->base.resv);
	dma_resv_unlock(&bo->base._resv);
+	if (r)
+		return r;
+
+	if (bo->type != ttm_bo_type_sg) {
+		spin_lock(&ttm_bo_glob.lru_lock);
+		bo->base.resv = &bo->base._resv;
Having the dma_resv pointer be protected by the lru_lock for ttm internal
stuff, but invariant everywhere else is really confusing. Not sure that's
I think this is reader VS writer.
To avoid any internal functions using the old resv,  using an existing spin lock is acceptable.
Maybe RCU is better? That will need a lot of effort.
Anyway, ttm sucks. We HAS done a lot of work on it to make it better running on modern system.

Yeah that summarize my recent presentation about TTM pretty much :)

Double checked that and the only reason we have the lock is that in ttm_mem_evict_first() we trylock first and then grab a reference.

So we should probably rework that code as well and then we can also drop that lock here, but that should come later.

Christian.



a great idea, I've just chased some ttm code around freaking out about
that.
-Daniel

+		spin_unlock(&ttm_bo_glob.lru_lock);
+	}

	return r;
}
@@ -720,7 +728,7 @@ static bool ttm_bo_evict_swapout_allowable(struct ttm_buffer_object *bo,

	if (bo->base.resv == ctx->resv) {
		dma_resv_assert_held(bo->base.resv);
-		if (ctx->flags & TTM_OPT_FLAG_ALLOW_RES_EVICT || bo->deleted)
+		if (ctx->flags & TTM_OPT_FLAG_ALLOW_RES_EVICT)
			ret = true;
		*locked = false;
		if (busy)
--
2.17.1

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--
Daniel Vetter
Software Engineer, Intel Corporation
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