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charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <26f766c9-9a3c-3894-9256-e07090655dc2@gmail.com> Precedence: bulk List-ID: X-Mailing-List: linux-media@vger.kernel.org On Thu, Sep 16, 2021 at 10:50 AM Christian König wrote: > Am 14.09.21 um 19:04 schrieb Daniel Vetter: > > On Fri, Sep 10, 2021 at 10:26:42AM +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. > >> > >> Signed-off-by: Christian König > >> --- > >> drivers/dma-buf/dma-resv.c | 63 ++++++++++++++++++++++++++++++++++++++ > >> include/linux/dma-resv.h | 36 ++++++++++++++++++++++ > >> 2 files changed, 99 insertions(+) > >> > >> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c > >> index 84fbe60629e3..213a9b7251ca 100644 > >> --- a/drivers/dma-buf/dma-resv.c > >> +++ b/drivers/dma-buf/dma-resv.c > >> @@ -323,6 +323,69 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence) > >> } > >> EXPORT_SYMBOL(dma_resv_add_excl_fence); > >> > >> +/** > >> + * dma_resv_walk_unlocked - walk over fences in a dma_resv obj > >> + * @obj: the dma_resv object > >> + * @cursor: cursor to record the current position > >> + * @all_fences: true returns also the shared fences > >> + * @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 iterator is started over again. > >> + */ > >> +struct dma_fence *dma_resv_walk_unlocked(struct dma_resv *obj, > >> + struct dma_resv_cursor *cursor, > >> + bool all_fences, bool first) > >> +{ > >> + struct dma_fence *fence = NULL; > >> + > >> + do { > >> + /* Drop the reference from the previous round */ > >> + dma_fence_put(fence); > >> + > >> + cursor->is_first = first; > >> + if (first) { > >> + cursor->seq = read_seqcount_begin(&obj->seq); > >> + cursor->index = -1; > >> + cursor->fences = dma_resv_shared_list(obj); > >> + cursor->is_exclusive = true; > >> + > >> + fence = dma_resv_excl_fence(obj); > >> + if (fence && test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, > >> + &fence->flags)) > >> + fence = NULL; > >> + } else { > >> + fence = NULL; > >> + } > >> + > >> + if (fence) { > >> + fence = dma_fence_get_rcu(fence); > >> + } else if (all_fences && cursor->fences) { > >> + struct dma_resv_list *fences = cursor->fences; > >> + > >> + cursor->is_exclusive = false; > >> + while (++cursor->index < fences->shared_count) { > >> + fence = rcu_dereference(fences->shared[ > >> + cursor->index]); > >> + if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, > >> + &fence->flags)) > >> + break; > >> + } > >> + if (cursor->index < fences->shared_count) > >> + fence = dma_fence_get_rcu(fence); > >> + else > >> + fence = NULL; > >> + } > >> + > >> + /* For the eventually next round */ > >> + first = true; > >> + } while (read_seqcount_retry(&obj->seq, cursor->seq)); > >> + > >> + return fence; > >> +} > >> +EXPORT_SYMBOL_GPL(dma_resv_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..f5b91c292ee0 100644 > >> --- a/include/linux/dma-resv.h > >> +++ b/include/linux/dma-resv.h > >> @@ -149,6 +149,39 @@ struct dma_resv { > >> struct dma_resv_list __rcu *fence; > >> }; > >> > >> +/** > >> + * struct dma_resv_cursor - current position into the dma_resv fences > >> + * @seq: sequence number to check > >> + * @index: index into the shared fences > >> + * @shared: the shared fences > >> + * @is_first: true if this is the first returned fence > >> + * @is_exclusive: if the current fence is the exclusive one > >> + */ > >> +struct dma_resv_cursor { > >> + unsigned int seq; > >> + unsigned int index; > >> + struct dma_resv_list *fences; > >> + bool is_first; > >> + bool is_exclusive; > >> +}; > > A bit a bikeshed, but I think I'd be nice to align this with the other > > iterators we have, e.g. for the drm_connector list. > > > > So struct dma_resv_fence_iter, dma_resv_fence_iter_begin/next/end(). > > I've renamed the structure to dma_resv_iter. > > > Also I think the for_each macro must not include begin/end calls. If we > > include that then it saves 2 lines of code at the cost of a pile of > > awkward bugs because people break; out of the loop or return early (only > > continue is safe) and we leak a fence. Or worse. > > > > Explicit begin/end is much more robust at a very marginal cost imo. > > The key point is that this makes it quite a bunch more complicated to > implement. See those functions are easiest when you centralize them and > try to not spread the functionality into begin/end. > > The only thing I could see in the end function would be to drop the > reference for the dma_fence and that is not really something I would > like to do because we actually need to keep that reference in a bunch of > cases. Yeah but it's extremely fragile. See with drm_connector_iter we also have the need to grab a reference to that connector in a few place, and I do think that open-code that is much clearer instead of inheriting a reference that the for_each macro acquired for you, and which you cleverly leaked through a break; Compare for_each_fence(fence) { if (fence) { found_fence = fence; break; } } /* do some itneresting stuff with found_fence */ dma_fence_put(found_fence); /* wtf, where is this fence reference from */ Versus what I'm proposing: fence_iter_init(&fence_iter) for_each_fence(fence, &fence_iter) { if (fence) { found_fence = fence; dma_fence_get(found_fence); break; } } fence_iter_end(&fence_iter) /* do some itneresting stuff with found_fence */ dma_fence_put(found_fence); /* 100% clear which reference we're putting here */ One of these patterns is maintainable and clear, at the cost of 3 more lines. The other one is frankly just clever but fragile nonsense. So yeah I really think we need the iter_init/end/next triple of functions here. Too clever is no good at all. And yes that version means you have an additional kref_get/put in there for the found fence, but I really don't think that matters in any of these paths here. Cheers, Daniel > > Regards, > Christian. > > > > > Otherwise I think this fence iterator is a solid concept that yeah we > > should roll out everywhere. > > -Daniel > > > >> + > >> +/** > >> + * dma_resv_for_each_fence_unlocked - fence iterator > >> + * @obj: a dma_resv object pointer > >> + * @cursor: a struct dma_resv_cursor pointer > >> + * @all_fences: true if all fences should be returned > >> + * @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. @all_fences controls > >> + * if the shared fences are returned as well. > >> + */ > >> +#define dma_resv_for_each_fence_unlocked(obj, cursor, all_fences, fence) \ > >> + for (fence = dma_resv_walk_unlocked(obj, cursor, all_fences, true); \ > >> + fence; dma_fence_put(fence), \ > >> + fence = dma_resv_walk_unlocked(obj, cursor, all_fences, false)) > >> + > >> #define dma_resv_held(obj) lockdep_is_held(&(obj)->lock.base) > >> #define dma_resv_assert_held(obj) lockdep_assert_held(&(obj)->lock.base) > >> > >> @@ -366,6 +399,9 @@ void dma_resv_fini(struct dma_resv *obj); > >> int dma_resv_reserve_shared(struct dma_resv *obj, unsigned int num_fences); > >> void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence); > >> void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence); > >> +struct dma_fence *dma_resv_walk_unlocked(struct dma_resv *obj, > >> + struct dma_resv_cursor *cursor, > >> + bool first, bool all_fences); > >> int dma_resv_get_fences(struct dma_resv *obj, struct dma_fence **pfence_excl, > >> unsigned *pshared_count, struct dma_fence ***pshared); > >> int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src); > >> -- > >> 2.25.1 > >> > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch 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=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 3537DC433F5 for ; Thu, 16 Sep 2021 12:15:02 +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 9CB1E60F58 for ; 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Thu, 16 Sep 2021 05:14:57 -0700 (PDT) Date: Thu, 16 Sep 2021 14:14:55 +0200 From: Daniel Vetter To: Christian =?iso-8859-1?Q?K=F6nig?= Cc: "moderated list:DMA BUFFER SHARING FRAMEWORK" , dri-devel , "open list:DMA BUFFER SHARING FRAMEWORK" Subject: Re: [PATCH 01/14] dma-buf: add dma_resv_for_each_fence_unlocked Message-ID: References: <20210910082655.82168-1-christian.koenig@amd.com> <26f766c9-9a3c-3894-9256-e07090655dc2@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <26f766c9-9a3c-3894-9256-e07090655dc2@gmail.com> X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Thu, Sep 16, 2021 at 10:50 AM Christian König wrote: > Am 14.09.21 um 19:04 schrieb Daniel Vetter: > > On Fri, Sep 10, 2021 at 10:26:42AM +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. > >> > >> Signed-off-by: Christian König > >> --- > >> drivers/dma-buf/dma-resv.c | 63 ++++++++++++++++++++++++++++++++++++++ > >> include/linux/dma-resv.h | 36 ++++++++++++++++++++++ > >> 2 files changed, 99 insertions(+) > >> > >> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c > >> index 84fbe60629e3..213a9b7251ca 100644 > >> --- a/drivers/dma-buf/dma-resv.c > >> +++ b/drivers/dma-buf/dma-resv.c > >> @@ -323,6 +323,69 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence) > >> } > >> EXPORT_SYMBOL(dma_resv_add_excl_fence); > >> > >> +/** > >> + * dma_resv_walk_unlocked - walk over fences in a dma_resv obj > >> + * @obj: the dma_resv object > >> + * @cursor: cursor to record the current position > >> + * @all_fences: true returns also the shared fences > >> + * @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 iterator is started over again. > >> + */ > >> +struct dma_fence *dma_resv_walk_unlocked(struct dma_resv *obj, > >> + struct dma_resv_cursor *cursor, > >> + bool all_fences, bool first) > >> +{ > >> + struct dma_fence *fence = NULL; > >> + > >> + do { > >> + /* Drop the reference from the previous round */ > >> + dma_fence_put(fence); > >> + > >> + cursor->is_first = first; > >> + if (first) { > >> + cursor->seq = read_seqcount_begin(&obj->seq); > >> + cursor->index = -1; > >> + cursor->fences = dma_resv_shared_list(obj); > >> + cursor->is_exclusive = true; > >> + > >> + fence = dma_resv_excl_fence(obj); > >> + if (fence && test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, > >> + &fence->flags)) > >> + fence = NULL; > >> + } else { > >> + fence = NULL; > >> + } > >> + > >> + if (fence) { > >> + fence = dma_fence_get_rcu(fence); > >> + } else if (all_fences && cursor->fences) { > >> + struct dma_resv_list *fences = cursor->fences; > >> + > >> + cursor->is_exclusive = false; > >> + while (++cursor->index < fences->shared_count) { > >> + fence = rcu_dereference(fences->shared[ > >> + cursor->index]); > >> + if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, > >> + &fence->flags)) > >> + break; > >> + } > >> + if (cursor->index < fences->shared_count) > >> + fence = dma_fence_get_rcu(fence); > >> + else > >> + fence = NULL; > >> + } > >> + > >> + /* For the eventually next round */ > >> + first = true; > >> + } while (read_seqcount_retry(&obj->seq, cursor->seq)); > >> + > >> + return fence; > >> +} > >> +EXPORT_SYMBOL_GPL(dma_resv_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..f5b91c292ee0 100644 > >> --- a/include/linux/dma-resv.h > >> +++ b/include/linux/dma-resv.h > >> @@ -149,6 +149,39 @@ struct dma_resv { > >> struct dma_resv_list __rcu *fence; > >> }; > >> > >> +/** > >> + * struct dma_resv_cursor - current position into the dma_resv fences > >> + * @seq: sequence number to check > >> + * @index: index into the shared fences > >> + * @shared: the shared fences > >> + * @is_first: true if this is the first returned fence > >> + * @is_exclusive: if the current fence is the exclusive one > >> + */ > >> +struct dma_resv_cursor { > >> + unsigned int seq; > >> + unsigned int index; > >> + struct dma_resv_list *fences; > >> + bool is_first; > >> + bool is_exclusive; > >> +}; > > A bit a bikeshed, but I think I'd be nice to align this with the other > > iterators we have, e.g. for the drm_connector list. > > > > So struct dma_resv_fence_iter, dma_resv_fence_iter_begin/next/end(). > > I've renamed the structure to dma_resv_iter. > > > Also I think the for_each macro must not include begin/end calls. If we > > include that then it saves 2 lines of code at the cost of a pile of > > awkward bugs because people break; out of the loop or return early (only > > continue is safe) and we leak a fence. Or worse. > > > > Explicit begin/end is much more robust at a very marginal cost imo. > > The key point is that this makes it quite a bunch more complicated to > implement. See those functions are easiest when you centralize them and > try to not spread the functionality into begin/end. > > The only thing I could see in the end function would be to drop the > reference for the dma_fence and that is not really something I would > like to do because we actually need to keep that reference in a bunch of > cases. Yeah but it's extremely fragile. See with drm_connector_iter we also have the need to grab a reference to that connector in a few place, and I do think that open-code that is much clearer instead of inheriting a reference that the for_each macro acquired for you, and which you cleverly leaked through a break; Compare for_each_fence(fence) { if (fence) { found_fence = fence; break; } } /* do some itneresting stuff with found_fence */ dma_fence_put(found_fence); /* wtf, where is this fence reference from */ Versus what I'm proposing: fence_iter_init(&fence_iter) for_each_fence(fence, &fence_iter) { if (fence) { found_fence = fence; dma_fence_get(found_fence); break; } } fence_iter_end(&fence_iter) /* do some itneresting stuff with found_fence */ dma_fence_put(found_fence); /* 100% clear which reference we're putting here */ One of these patterns is maintainable and clear, at the cost of 3 more lines. The other one is frankly just clever but fragile nonsense. So yeah I really think we need the iter_init/end/next triple of functions here. Too clever is no good at all. And yes that version means you have an additional kref_get/put in there for the found fence, but I really don't think that matters in any of these paths here. Cheers, Daniel > > Regards, > Christian. > > > > > Otherwise I think this fence iterator is a solid concept that yeah we > > should roll out everywhere. > > -Daniel > > > >> + > >> +/** > >> + * dma_resv_for_each_fence_unlocked - fence iterator > >> + * @obj: a dma_resv object pointer > >> + * @cursor: a struct dma_resv_cursor pointer > >> + * @all_fences: true if all fences should be returned > >> + * @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. @all_fences controls > >> + * if the shared fences are returned as well. > >> + */ > >> +#define dma_resv_for_each_fence_unlocked(obj, cursor, all_fences, fence) \ > >> + for (fence = dma_resv_walk_unlocked(obj, cursor, all_fences, true); \ > >> + fence; dma_fence_put(fence), \ > >> + fence = dma_resv_walk_unlocked(obj, cursor, all_fences, false)) > >> + > >> #define dma_resv_held(obj) lockdep_is_held(&(obj)->lock.base) > >> #define dma_resv_assert_held(obj) lockdep_assert_held(&(obj)->lock.base) > >> > >> @@ -366,6 +399,9 @@ void dma_resv_fini(struct dma_resv *obj); > >> int dma_resv_reserve_shared(struct dma_resv *obj, unsigned int num_fences); > >> void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence); > >> void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence); > >> +struct dma_fence *dma_resv_walk_unlocked(struct dma_resv *obj, > >> + struct dma_resv_cursor *cursor, > >> + bool first, bool all_fences); > >> int dma_resv_get_fences(struct dma_resv *obj, struct dma_fence **pfence_excl, > >> unsigned *pshared_count, struct dma_fence ***pshared); > >> int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src); > >> -- > >> 2.25.1 > >> > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch