linux-kernel - Re: [Linaro-mm-sig] Re: [PATCH] dma-fence: allow dma fence to have their own lock

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Date:   Wed, 1 Jun 2022 14:45:42 +0200
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Christian König <christian.koenig@....com>
Cc:     Sumit Semwal <sumit.semwal@...aro.org>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Tomasz Figa <tfiga@...omium.org>,
        Ricardo Ribalda <ribalda@...omium.org>,
        Christoph Hellwig <hch@...radead.org>,
        linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [Linaro-mm-sig] Re: [PATCH] dma-fence: allow dma fence to have
 their own lock

Am 31.05.22 um 04:51 schrieb Sergey Senozhatsky:
> On (22/05/30 16:55), Christian König wrote:
>> Am 30.05.22 um 16:22 schrieb Sergey Senozhatsky:
>>> [SNIP]
>>> So the `lock` should have at least same lifespan as the DMA fence
>>> that borrows it, which is impossible to guarantee in our case.
>> Nope, that's not correct. The lock should have at least same lifespan as the
>> context of the DMA fence.
> How does one know when it's safe to release the context? DMA fence
> objects are still transparently refcount-ed and "live their own lives",
> how does one synchronize lifespans?

Well, you don't.

If you have a dynamic context structure you need to reference count that 
as well. In other words every time you create a fence in your context 
you need to increment the reference count and every time a fence is 
release you decrement it.

If you have a static context structure like most drivers have then you 
must make sure that all fences at least signal before you unload your 
driver. We still somewhat have a race when you try to unload a driver 
and the fence_ops structure suddenly disappear, but we currently live 
with that.

Apart from that you are right, fences can live forever and we need to 
deal with that.

Regards,
Christian.

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