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 X-Spam-Level: X-Spam-Status: No, score=-14.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,HK_RANDOM_FROM,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 28243C433EF for ; Mon, 20 Sep 2021 08:43:37 +0000 (UTC) Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id DEA2E60F12 for ; Mon, 20 Sep 2021 08:43:36 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org DEA2E60F12 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=lists.freedesktop.org Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 791C86E42E; Mon, 20 Sep 2021 08:43:36 +0000 (UTC) Received: from mga04.intel.com (mga04.intel.com [192.55.52.120]) by gabe.freedesktop.org (Postfix) with ESMTPS id CA7B66E42E; Mon, 20 Sep 2021 08:43:35 +0000 (UTC) X-IronPort-AV: E=McAfee;i="6200,9189,10112"; a="221205249" X-IronPort-AV: E=Sophos;i="5.85,307,1624345200"; d="scan'208";a="221205249" Received: from fmsmga007.fm.intel.com ([10.253.24.52]) by fmsmga104.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 20 Sep 2021 01:43:27 -0700 X-IronPort-AV: E=Sophos;i="5.85,307,1624345200"; d="scan'208";a="473479639" Received: from gbradyx-mobl2.ger.corp.intel.com (HELO [10.213.235.119]) ([10.213.235.119]) by fmsmga007-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 20 Sep 2021 01:43:26 -0700 To: Daniel Vetter , =?UTF-8?Q?Christian_K=c3=b6nig?= Cc: linaro-mm-sig@lists.linaro.org, dri-devel@lists.freedesktop.org, linux-media@vger.kernel.org, intel-gfx@lists.freedesktop.org References: <20210917123513.1106-1-christian.koenig@amd.com> <20210917123513.1106-2-christian.koenig@amd.com> From: Tvrtko Ursulin Organization: Intel Corporation UK Plc Message-ID: <8268d2e8-8a37-0ff1-7065-c8aaf5c8672b@linux.intel.com> Date: Mon, 20 Sep 2021 09:43:14 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Subject: Re: [Intel-gfx] [PATCH 01/26] dma-buf: add dma_resv_for_each_fence_unlocked v2 X-BeenThere: intel-gfx@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Intel graphics driver community testing & development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: intel-gfx-bounces@lists.freedesktop.org Sender: "Intel-gfx" On 17/09/2021 14:23, Daniel Vetter wrote: > On Fri, Sep 17, 2021 at 02:34:48PM +0200, Christian König wrote: >> Abstract the complexity of iterating over all the fences >> in a dma_resv object. >> >> The new loop handles the whole RCU and retry dance and >> returns only fences where we can be sure we grabbed the >> right one. >> >> v2: fix accessing the shared fences while they might be freed, >> improve kerneldoc, rename _cursor to _iter, add >> dma_resv_iter_is_exclusive, add dma_resv_iter_begin/end >> >> Signed-off-by: Christian König >> --- >> drivers/dma-buf/dma-resv.c | 61 +++++++++++++++++++++++++++ >> include/linux/dma-resv.h | 84 ++++++++++++++++++++++++++++++++++++++ >> 2 files changed, 145 insertions(+) >> >> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c >> index 84fbe60629e3..3e77cad2c9d4 100644 >> --- a/drivers/dma-buf/dma-resv.c >> +++ b/drivers/dma-buf/dma-resv.c >> @@ -323,6 +323,67 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence) >> } >> EXPORT_SYMBOL(dma_resv_add_excl_fence); >> >> +/** >> + * dma_resv_iter_walk_unlocked - walk over fences in a dma_resv obj >> + * @cursor: cursor to record the current position >> + * @first: if we should start over >> + * >> + * Return all the fences in the dma_resv object which are not yet signaled. >> + * The returned fence has an extra local reference so will stay alive. >> + * If a concurrent modify is detected the whole iterration is started over again. >> + */ >> +struct dma_fence *dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor, > > Bit ocd, but I'd still just call that iter_next. > >> + bool first) > > Hm I'd put all the init code into iter_begin ... @Christian: Could you engineer something in here which would, at least in debug builds, catch failures to call "iter begin" before using the iterator macro? > >> +{ >> + struct dma_resv *obj = cursor->obj; > > Aren't we missing rcu_read_lock() around the entire thing here? > >> + >> + first |= read_seqcount_retry(&obj->seq, cursor->seq); >> + do { >> + /* Drop the reference from the previous round */ >> + dma_fence_put(cursor->fence); >> + >> + cursor->is_first = first; >> + if (first) { >> + cursor->seq = read_seqcount_begin(&obj->seq); >> + cursor->index = -1; >> + cursor->fences = dma_resv_shared_list(obj); > > And then also call iter_begin from here. That way we guarantee that > read_seqcount_begin is always called before _retry(). It's not a problem > with the seqcount implementation (I think at least), but it definitely > looks funny. > > Calling iter_begin here also makes it clear that we're essentially > restarting. > >> + >> + cursor->fence = dma_resv_excl_fence(obj); >> + if (cursor->fence && >> + test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, > > Please use the right dma_fence wrapper here for this and don't look at the > bits/flags outside of dma_fence.[hc] code. I just realized that we don't > have the right amount of barriers in there for the fastpath, i.e. if we > have: > > x = 0; /* static initializer */ > > thread a > x = 1; > dma_fence_signal(fence); > > > thread b; > if (dma_fence_is_signalled(fence)) > printk("%i\n", x); > > Then you might actually be able to observe x == 0 in thread b. Which is > not what we want at all. @Daniel: What do you mean here - in terms of if 'x' is "external" (not part of dma-fence), then are you suggesting dma-fence code should serialise it by using barriers? That would sound incorrect to me, or in other words, I think it's fine if x == 0 is observed in your example thread B since that code is mixing external data with dma-fence. Hm also, there is that annoying bit where by using dma_fence_is_signaled any code becomes a fence signaling critical path, which I never bought into. There should be a way to test the signaled status without actually doing the signaling. Or I am misunderstanding something so badly that is really really has to be like this? > So no open-coding of dma_fence flag bits code outside of drm_fence.[hc] > please. And yes i915-gem code is unfortunately a disaster. Don't even miss an opportunity for some good trashing no? :D But yes, deconstructed dma_fence_signal I thought we were supposed to add to core. Or at least propose, don't exactly remember how that went. > >> + &cursor->fence->flags)) >> + cursor->fence = NULL; >> + } else { >> + cursor->fence = NULL; >> + } >> + >> + if (cursor->fence) { >> + cursor->fence = dma_fence_get_rcu(cursor->fence); >> + } else if (cursor->all_fences && cursor->fences) { >> + struct dma_resv_list *fences = cursor->fences; >> + >> + while (++cursor->index < fences->shared_count) { >> + cursor->fence = rcu_dereference( >> + fences->shared[cursor->index]); >> + if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, >> + &cursor->fence->flags)) >> + break; >> + } >> + if (cursor->index < fences->shared_count) >> + cursor->fence = >> + dma_fence_get_rcu(cursor->fence); >> + else >> + cursor->fence = NULL; >> + } > > The control flow here is very hairy, but I'm not sure how to best do this. > With my suggestion to move the read_seqcount_begin into iter_begin maybe > something like this: > > iter_next() > { > do { > dma_fence_put(cursor->fence) > cursor->fence = NULL; > > if (cursor->index == -1) { /* reset by iter_begin() > cursor->fence = get_exclusive(); > cusor->index++; > } else { > cursor->fence = shared_fences[++cursor->index] > } > > if (!dma_fence_is_signalled(cursor->fence)) > continue; /* just grab the next fence. */ > > cursor->fence = dma_fence_get_rcu(cursor->fence); > > if (!cursor->fence || read_seqcount_retry()) { > /* we lost the race, restart completely */ > iter_begin(); /* ->fence will be cleaned up at beginning of the loop */ > continue; > } > > return cursor->fence; > } while (true); > } > > Maybe I missed something, but that avoids the duplication of all the > tricky code, i.e. checking for signalling, rcu protected conditional > fence_get, and the retry is also nicely at the end. >> + >> + /* For the eventually next round */ >> + first = true; >> + } while (read_seqcount_retry(&obj->seq, cursor->seq)); >> + >> + return cursor->fence; >> +} >> +EXPORT_SYMBOL_GPL(dma_resv_iter_walk_unlocked); >> + >> /** >> * dma_resv_copy_fences - Copy all fences from src to dst. >> * @dst: the destination reservation object >> diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h >> index 9100dd3dc21f..693d16117153 100644 >> --- a/include/linux/dma-resv.h >> +++ b/include/linux/dma-resv.h >> @@ -149,6 +149,90 @@ struct dma_resv { >> struct dma_resv_list __rcu *fence; >> }; >> >> +/** >> + * struct dma_resv_iter - current position into the dma_resv fences >> + * >> + * Don't touch this directly in the driver, use the accessor function instead. >> + */ >> +struct dma_resv_iter { >> + /** @obj: The dma_resv object we iterate over */ >> + struct dma_resv *obj; >> + >> + /** @all_fences: If all fences should be returned */ >> + bool all_fences; >> + >> + /** @fence: the currently handled fence */ >> + struct dma_fence *fence; >> + >> + /** @seq: sequence number to check for modifications */ >> + unsigned int seq; >> + >> + /** @index: index into the shared fences */ > > If you go with my suggestion (assuming it works): Please add "-1 indicates > to pick the exclusive fence instead." > >> + unsigned int index; >> + >> + /** @fences: the shared fences */ >> + struct dma_resv_list *fences; >> + >> + /** @is_first: true if this is the first returned fence */ >> + bool is_first; > > I think if we just rely on -1 == exclusive fence/is_first we don't need > this one here? > >> +}; >> + >> +struct dma_fence *dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor, >> + bool first); >> + >> +/** >> + * dma_resv_iter_begin - initialize a dma_resv_iter object >> + * @cursor: The dma_resv_iter object to initialize >> + * @obj: The dma_resv object which we want to iterator over >> + * @all_fences: If all fences should be returned or just the exclusive one > > Please add: "Callers must clean up the iterator with dma_resv_iter_end()." > >> + */ >> +static inline void dma_resv_iter_begin(struct dma_resv_iter *cursor, >> + struct dma_resv *obj, >> + bool all_fences) >> +{ >> + cursor->obj = obj; >> + cursor->all_fences = all_fences; >> + cursor->fence = NULL; >> +} >> + >> +/** >> + * dma_resv_iter_end - cleanup a dma_resv_iter object >> + * @cursor: the dma_resv_iter object which should be cleaned up >> + * >> + * Make sure that the reference to the fence in the cursor is properly >> + * dropped. > > Please add: > > "This function must be called every time dma_resv_iter_begin() was called > to clean up any references." >> + */ >> +static inline void dma_resv_iter_end(struct dma_resv_iter *cursor) >> +{ >> + dma_fence_put(cursor->fence); >> +} >> + >> +/** >> + * dma_resv_iter_is_exclusive - test if the current fence is the exclusive one >> + * @cursor: the cursor of the current position >> + * >> + * Returns true if the currently returned fence is the exclusive one. >> + */ >> +static inline bool dma_resv_iter_is_exclusive(struct dma_resv_iter *cursor) >> +{ >> + return cursor->index == -1; >> +} >> + >> +/** >> + * dma_resv_for_each_fence_unlocked - unlocked fence iterator >> + * @cursor: a struct dma_resv_iter pointer >> + * @fence: the current fence >> + * >> + * Iterate over the fences in a struct dma_resv object without holding the >> + * dma_resv::lock. The RCU read side lock must be hold when using this, but can >> + * be dropped and re-taken as necessary inside the loop. The cursor needs to be >> + * initialized with dma_resv_iter_begin_unlocked() and cleaned up with > > We don't have an _unlocked version? @Christian: I'd also mention that the fence reference is held during the walk so someone is less likely to grab extra ones. > >> + * dma_resv_iter_end_unlocked(). >> + */ >> +#define dma_resv_for_each_fence_unlocked(cursor, fence) \ >> + for (fence = dma_resv_iter_walk_unlocked(cursor, true); \ >> + fence; fence = dma_resv_iter_walk_unlocked(cursor, false)) >> + >> #define dma_resv_held(obj) lockdep_is_held(&(obj)->lock.base) >> #define dma_resv_assert_held(obj) lockdep_assert_held(&(obj)->lock.base) >> >> -- >> 2.25.1 >> > Regards, Tvrtko