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From: Jason Gunthorpe <jgg@ziepe.ca>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: "Leon Romanovsky" <leon@kernel.org>,
	"linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	"Doug Ledford" <dledford@redhat.com>,
	"Vetter, Daniel" <daniel.vetter@intel.com>,
	"Christian König" <christian.koenig@amd.com>,
	"Xiong, Jianxin" <jianxin.xiong@intel.com>
Subject: Re: [RFC PATCH v2 0/3] RDMA: add dma-buf support
Date: Fri, 3 Jul 2020 09:03:35 -0300
Message-ID: <20200703120335.GT25301@ziepe.ca> (raw)
In-Reply-To: <20200702181540.GC3278063@phenom.ffwll.local>

On Thu, Jul 02, 2020 at 08:15:40PM +0200, Daniel Vetter wrote:
> > > > 3. rdma driver worker gets busy to restart rx:
> > > > 	1. lock all dma-buf that are currently in use (dma_resv_lock).
> > > > 	thanks to ww_mutex deadlock avoidance this is possible
> > > Why all? Why not just lock the one that was invalidated to restore the
> > > mappings? That is some artifact of the GPU approach?
> > 
> > No, but you must make sure that mapping one doesn't invalidate others you
> > need.
> > 
> > Otherwise you can end up in a nice live lock :)
> 
> Also if you don't have pagefaults, but have to track busy memory at a
> context level, you do need to grab all locks of all buffers you need, or
> you'd race. There's nothing stopping a concurrent ->notify_move on some
> other buffer you'll need otherwise, and if you try to be clever and roll
> you're own locking, you'll anger lockdep - you're own lock will have to be
> on both sides of ww_mutex or it wont work, and that deadlocks.

So you are worried about atomically building some multi buffer
transaction? I don't think this applies to RDMA which isn't going to
be transcational here..

> > > And why is this done with work queues and locking instead of a
> > > callback saying the buffer is valid again?
> > 
> > You can do this as well, but a work queue is usually easier to handle than a
> > notification in an interrupt context of a foreign driver.
> 
> Yeah you can just install a dma_fence callback but
> - that's hardirq context
> - if you don't have per-page hw faults you need the multi-lock ww_mutex
>   dance anyway to avoid races.

It is still best to avoid the per-page faults and preload the new
mapping once it is ready.

Jason
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Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <1593451903-30959-1-git-send-email-jianxin.xiong@intel.com>
     [not found] ` <20200629185152.GD25301@ziepe.ca>
     [not found]   ` <MW3PR11MB4555A99038FA0CFC3ED80D3DE56F0@MW3PR11MB4555.namprd11.prod.outlook.com>
     [not found]     ` <20200630173435.GK25301@ziepe.ca>
2020-06-30 18:46       ` Xiong, Jianxin
2020-06-30 19:17         ` Jason Gunthorpe
2020-06-30 20:08           ` Xiong, Jianxin
2020-07-02 12:27             ` Jason Gunthorpe
2020-07-01  9:03         ` Christian König
2020-07-01 12:07           ` Daniel Vetter
2020-07-01 12:14             ` Daniel Vetter
2020-07-01 12:39           ` Jason Gunthorpe
2020-07-01 12:55             ` Christian König
2020-07-01 15:42               ` Daniel Vetter
2020-07-01 17:15                 ` Jason Gunthorpe
2020-07-02 13:10                   ` Daniel Vetter
2020-07-02 13:29                     ` Jason Gunthorpe
2020-07-02 14:50                       ` Christian König
2020-07-02 18:15                         ` Daniel Vetter
2020-07-03 12:03                           ` Jason Gunthorpe [this message]
2020-07-03 12:52                             ` Daniel Vetter
2020-07-03 13:14                               ` Jason Gunthorpe
2020-07-03 13:21                                 ` Christian König
2020-07-07 21:58                                   ` Xiong, Jianxin
2020-07-08  9:38                                     ` Christian König
2020-07-08  9:49                                       ` Daniel Vetter
2020-07-08 14:20                                         ` Christian König
2020-07-08 14:33                                           ` Alex Deucher
2020-06-30 18:56 ` Xiong, Jianxin
     [not found] ` <1593451903-30959-2-git-send-email-jianxin.xiong@intel.com>
2020-06-30 19:04   ` [RFC PATCH v2 1/3] RDMA/umem: Support importing dma-buf as user memory region Xiong, Jianxin
     [not found] ` <1593451903-30959-3-git-send-email-jianxin.xiong@intel.com>
2020-06-30 19:04   ` [RFC PATCH v2 2/3] RDMA/core: Expand the driver method 'reg_user_mr' to support dma-buf Xiong, Jianxin
     [not found] ` <1593451903-30959-4-git-send-email-jianxin.xiong@intel.com>
2020-06-30 19:05   ` [RFC PATCH v2 3/3] RDMA/uverbs: Add uverbs command for dma-buf based MR registration Xiong, Jianxin

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