From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id F2896ECAAD5 for ; Tue, 6 Sep 2022 11:21:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239371AbiIFLVv (ORCPT ); Tue, 6 Sep 2022 07:21:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36856 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239081AbiIFLVr (ORCPT ); Tue, 6 Sep 2022 07:21:47 -0400 Received: from mga09.intel.com (mga09.intel.com [134.134.136.24]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A8743491DD; Tue, 6 Sep 2022 04:21:46 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1662463306; x=1693999306; h=message-id:date:mime-version:subject:to:references:from: in-reply-to:content-transfer-encoding; bh=3ZFIWu1iVTrSZh9swgEip3UeXgSl7SQaM85kMXnUI04=; b=PEQF6nsdxKlfhRhl75RTpa2gVhAX0ynjXbhyxy9rbQTHrpmTaZf2bCrS Eya6AwoNFJbAhjhbFvDUf21IW5pjz1BPSd63p9+eBa1d3cJBFk3Ed6SeA DAov27PhQNR6M9mklD54ooDjIJQsYbEZah9sAfpfHehViFV1lX8oDjbQh uE7DyLM8o6EndAwF/6rlO2qZ6t0oqEFOMcWhF6HxnR4GXVPprx13Ma7K5 NaiedF1aKzIsdgtYxVPeoWNP54P+TdTkXLXyopcnD5nAtQM9//mHGFK5J uZBnggahPYWrBizKJvze7s5c8cvnnCznvicLrQrc1yAd6v1z3boOlfgWa w==; X-IronPort-AV: E=McAfee;i="6500,9779,10461"; a="297353054" X-IronPort-AV: E=Sophos;i="5.93,294,1654585200"; d="scan'208";a="297353054" Received: from orsmga008.jf.intel.com ([10.7.209.65]) by orsmga102.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Sep 2022 04:21:46 -0700 X-IronPort-AV: E=Sophos;i="5.93,294,1654585200"; d="scan'208";a="644132421" Received: from holmesda-mobl.ger.corp.intel.com (HELO [10.213.204.21]) ([10.213.204.21]) by orsmga008-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Sep 2022 04:21:43 -0700 Message-ID: <691e636f-07d6-f4d3-6d83-29a3834ac1a2@linux.intel.com> Date: Tue, 6 Sep 2022 12:21:41 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.13.0 Subject: Re: [PATCH v2 4/4] dma-buf: Check status of enable-signaling bit on debug Content-Language: en-US To: =?UTF-8?Q?Christian_K=c3=b6nig?= , Arvind Yadav , andrey.grodzovsky@amd.com, shashank.sharma@amd.com, amaranath.somalapuram@amd.com, Arunpravin.PaneerSelvam@amd.com, sumit.semwal@linaro.org, gustavo@padovan.org, airlied@linux.ie, daniel@ffwll.ch, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, linux-kernel@vger.kernel.org References: <20220905163502.4032-1-Arvind.Yadav@amd.com> <20220905163502.4032-5-Arvind.Yadav@amd.com> From: Tvrtko Ursulin Organization: Intel Corporation UK Plc In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/09/2022 11:43, Christian König wrote: > Am 06.09.22 um 12:20 schrieb Tvrtko Ursulin: >> >> On 06/09/2022 09:39, Christian König wrote: >>> Am 05.09.22 um 18:35 schrieb Arvind Yadav: >>>> The core DMA-buf framework needs to enable signaling >>>> before the fence is signaled. The core DMA-buf framework >>>> can forget to enable signaling before the fence is signaled. >>> >>> This sentence is a bit confusing. I'm not a native speaker of English >>> either, but I suggest something like: >>> >>> "Fence signaling must be enable to make sure that the >>> dma_fence_is_signaled() function ever returns true." >>> >>>> To avoid this scenario on the debug kernel, check the >>>> DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT status bit before checking >>>> the signaling bit status to confirm that enable_signaling >>>> is enabled. >>> >>> This describes the implementation, but we should rather describe the >>> background of the change. The implementation should be obvious. >>> Something like this maybe: >>> >>> " >>> Since drivers and implementations sometimes mess this up enforce >>> correct behavior when DEBUG_WW_MUTEX_SLOWPATH is used during debugging. >>> >>> This should make any implementations bugs resulting in not signaled >>> fences much more obvious. >>> " >> >> I think I follow the idea but am not sure coupling (well "coupling".. >> not really, but cross-contaminating in a way) dma-fence.c with a >> foreign and effectively unrelated concept of a ww mutex is the best way. >> >> Instead, how about a dma-buf specific debug kconfig option? > > Yeah, I was thinking about that as well. Cool, lets see about the specifics below and then decide. > The slowpath config option was just at hand because when you want to > test the slowpath you want to test this here as well. > >> >> Condition would then be, according to my understanding of the rules >> and expectations, along the lines of: >> >> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h >> index 775cdc0b4f24..147a9df2c9d0 100644 >> --- a/include/linux/dma-fence.h >> +++ b/include/linux/dma-fence.h >> @@ -428,6 +428,17 @@ dma_fence_is_signaled_locked(struct dma_fence >> *fence) >>  static inline bool >>  dma_fence_is_signaled(struct dma_fence *fence) >>  { >> +#ifdef CONFIG_DEBUG_DMAFENCE >> +       /* >> +        * Implementations not providing the enable_signaling callback >> are >> +        * required to always have signaling enabled or fences are not >> +        * guaranteed to ever signal. >> +        */ > > Well that comment is a bit misleading. The intention of the extra check > is to find bugs in the frontend and not the backend. By backend you mean drivers/dma-buf/dma-fence.c and by front end driver specific implementations? Or simply users for dma-fence? Could be that I got confused.. I was reading these two: * This callback is optional. If this callback is not present, then the * driver must always have signaling enabled. */ bool (*enable_signaling)(struct dma_fence *fence); And dma_fence_is_signaled: * Returns true if the fence was already signaled, false if not. Since this * function doesn't enable signaling, it is not guaranteed to ever return * true if dma_fence_add_callback(), dma_fence_wait() or * dma_fence_enable_sw_signaling() haven't been called before. Right, I think I did get confused, apologies. What I was thinking was probably two separate conditions: static inline bool dma_fence_is_signaled(struct dma_fence *fence) { +#ifdef CONFIG_DEBUG_DMAFENCE + if (WARN_ON_ONCE(!fence->ops->enable_signaling && + !test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags))) + return false; + + if (!test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags)) + return false; +#endif + if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) return true; Not sure "is signaled" is the best place for the first one or that it should definitely be added. Regards, Tvrtko > In other words somewhere in the drm_syncobj code we have a > dma_fence_is_signaled() call without matching > dma_fence_enable_sw_signaling(). > > Regards, > Christian. > >> + if (!fence->ops->enable_signaling && >> +           !test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags)) >> +               return false; >> +#endif >> + >>         if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) >>                 return true; >> >> Thoughts? >> >> Regards, >> >> Tvrtko >> >>> >>>> >>>> Signed-off-by: Arvind Yadav >>> >>> With the improved commit message this patch is Reviewed-by: Christian >>> König >>> >>> Regards, >>> Christian. >>> >>>> --- >>>> >>>> Changes in v1 : >>>> 1- Addressing Christian's comment to replace >>>> CONFIG_DEBUG_WW_MUTEX_SLOWPATH instead of CONFIG_DEBUG_FS. >>>> 2- As per Christian's comment moving this patch at last so >>>> The version of this patch is also changed and previously >>>> it was [PATCH 1/4] >>>> >>>> >>>> --- >>>>   include/linux/dma-fence.h | 5 +++++ >>>>   1 file changed, 5 insertions(+) >>>> >>>> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h >>>> index 775cdc0b4f24..ba1ddc14c5d4 100644 >>>> --- a/include/linux/dma-fence.h >>>> +++ b/include/linux/dma-fence.h >>>> @@ -428,6 +428,11 @@ dma_fence_is_signaled_locked(struct dma_fence >>>> *fence) >>>>   static inline bool >>>>   dma_fence_is_signaled(struct dma_fence *fence) >>>>   { >>>> +#ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH >>>> +    if (!test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags)) >>>> +        return false; >>>> +#endif >>>> + >>>>       if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) >>>>           return true; >>> >