On 04/10/2021 10:53, Christian König wrote:
Am 04.10.21 um 11:29 schrieb Tvrtko Ursulin:On 01/10/2021 11:05, 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 v3: restructor the code, move rcu_read_lock()/unlock() into the iterator, add dma_resv_iter_is_restarted() v4: fix NULL deref when no explicit fence exists, drop superflous rcu_read_lock()/unlock() calls. v5: fix typos in the documentation v6: fix coding error when excl fence is NULL v7: one more logic fix Signed-off-by: Christian König <christian.koenig@xxxxxxx> --- drivers/dma-buf/dma-resv.c | 100 +++++++++++++++++++++++++++++++++++++ include/linux/dma-resv.h | 95 +++++++++++++++++++++++++++++++++++ 2 files changed, 195 insertions(+) diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c index 84fbe60629e3..3cbcf66a137e 100644 --- a/drivers/dma-buf/dma-resv.c +++ b/drivers/dma-buf/dma-resv.c@@ -323,6 +323,106 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)} EXPORT_SYMBOL(dma_resv_add_excl_fence); +/** + * dma_resv_iter_restart_unlocked - restart the unlocked iterator + * @cursor: The dma_resv_iter object to restart + * + * Restart the unlocked iteration by initializing the cursor object. + */+static void dma_resv_iter_restart_unlocked(struct dma_resv_iter *cursor)+{ + cursor->seq = read_seqcount_begin(&cursor->obj->seq); + cursor->index = -1; + if (cursor->all_fences) + cursor->fences = dma_resv_shared_list(cursor->obj); + else + cursor->fences = NULL; + cursor->is_restarted = true; +} + +/** + * dma_resv_iter_walk_unlocked - walk over fences in a dma_resv obj + * @cursor: cursor to record the current position + *+ * 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 iteration is started over again.+ */ +static void dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor) +{ + struct dma_resv *obj = cursor->obj; + + do { + /* Drop the reference from the previous round */ + dma_fence_put(cursor->fence); + + if (cursor->index == -1) { + cursor->fence = dma_resv_excl_fence(obj); + cursor->index++; + if (!cursor->fence) + continue; + + } else if (!cursor->fences || + cursor->index >= cursor->fences->shared_count) { + cursor->fence = NULL; + break; + + } else { + struct dma_resv_list *fences = cursor->fences; + unsigned int idx = cursor->index++; + + cursor->fence = rcu_dereference(fences->shared[idx]); + } + cursor->fence = dma_fence_get_rcu(cursor->fence);Worth having an assert dma_fence_get_rcu does not fail here? Not sure that I have seen debug build only asserts though on the DRM core side.That won't work. It's perfectly valid for dma_fence_get_rcu() to return NULL when we are racing here. Keep in mind that we don't hold any locks.
Ah yes.. No need to change anything then, sorry for the confusion. I did not find any holes, the rest was just about how to maybe make the flow more obvious. Let me know if you want r-b now or later.
Regards, Tvrtko
What we could do is to return NULL and repeat with a new sequence immediately though.On the bike shedding front, would it be clearer if the continue condition on signaled fences was standalone, using the continue statement? I'd also possibly re-arrange the three if-else blocks so that the end of iteration is not sandwiched between blocks handling exclusive and shared, and flow tweaked a bit, like:struct dma_fence *fence = cursor->fence; int index = cursor->index; dma_fence_put(fence); fence = NULL; next: if (index == -1) { /* Try picking the exclusive fence. */ index++; fence = dma_resv_excl_fence(obj); if (!fence) goto next; } else if (cursor->fences && index < cursor->fences->shared_count) { /* Try picking next shared fence. */ struct dma_resv_list *fences = cursor->fences; fence = rcu_dereference(fences->shared[index++]); } if (fence) { if (dma_fence_is_signaled(fence)) goto next; /* Skip signaled. */ fence = dma_fence_get_rcu(fence); WARN_ON(!fence); } cursor->fence = fence; cursor->index = index;(I started with a loop here but ended with goto based flow since it ended up more succinct.)At least if I don't have a handling flaw in there it looks like easier to follow flow to me. Plus picking a not signaled fence works without a reference FWIW.I strongly don't think that this will work correctly. You need to grab a reference first when you want to call dma_fence_is_signaled(), that's why I used the testbit approach initially.How does it look to you?Mhm, let me try to reorder the loop once more. Thanks, Christian.Regards, Tvrtko+ if (!cursor->fence || !dma_fence_is_signaled(cursor->fence)) + break; + } while (true); +} + +/**+ * dma_resv_iter_first_unlocked - first fence in an unlocked dma_resv obj.+ * @cursor: the cursor with the current position + * + * Returns the first fence from an unlocked dma_resv obj. + */+struct dma_fence *dma_resv_iter_first_unlocked(struct dma_resv_iter *cursor)+{ + rcu_read_lock(); + do { + dma_resv_iter_restart_unlocked(cursor); + dma_resv_iter_walk_unlocked(cursor); + } while (read_seqcount_retry(&cursor->obj->seq, cursor->seq)); + rcu_read_unlock(); + + return cursor->fence; +} +EXPORT_SYMBOL(dma_resv_iter_first_unlocked); + +/**+ * dma_resv_iter_next_unlocked - next fence in an unlocked dma_resv obj.+ * @cursor: the cursor with the current position + * + * Returns the next fence from an unlocked dma_resv obj. + */+struct dma_fence *dma_resv_iter_next_unlocked(struct dma_resv_iter *cursor)+{ + bool restart; + + rcu_read_lock(); + cursor->is_restarted = false; + restart = read_seqcount_retry(&cursor->obj->seq, cursor->seq); + do { + if (restart) + dma_resv_iter_restart_unlocked(cursor); + dma_resv_iter_walk_unlocked(cursor); + restart = true; + } while (read_seqcount_retry(&cursor->obj->seq, cursor->seq)); + rcu_read_unlock(); + + return cursor->fence; +} +EXPORT_SYMBOL(dma_resv_iter_next_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..5d7d28cb9008 100644 --- a/include/linux/dma-resv.h +++ b/include/linux/dma-resv.h @@ -149,6 +149,101 @@ 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 */ + unsigned int index; + + /** @fences: the shared fences */ + struct dma_resv_list *fences; + + /** @is_restarted: true if this is the first returned fence */ + bool is_restarted; +}; ++struct dma_fence *dma_resv_iter_first_unlocked(struct dma_resv_iter *cursor); +struct dma_fence *dma_resv_iter_next_unlocked(struct dma_resv_iter *cursor);+ +/** + * 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 iterate over+ * @all_fences: If all fences should be returned or just the exclusive one+ */ +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. + */ +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_iter_is_restarted - test if this is the first fence after a restart+ * @cursor: the cursor with the current position + *+ * Return true if this is the first fence in an iteration after a restart.+ */+static inline bool dma_resv_iter_is_restarted(struct dma_resv_iter *cursor)+{ + return cursor->is_restarted; +} + +/** + * 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 and using RCU instead. The cursor needs to be initialized + * with dma_resv_iter_begin() and cleaned up with dma_resv_iter_end(). Inside + * the iterator a reference to the dma_fence is held and the RCU lock dropped.+ * When the dma_resv is modified the iteration starts over again. + */ +#define dma_resv_for_each_fence_unlocked(cursor, fence) \ + for (fence = dma_resv_iter_first_unlocked(cursor); \ + fence; fence = dma_resv_iter_next_unlocked(cursor)) + #define dma_resv_held(obj) lockdep_is_held(&(obj)->lock.base)#define dma_resv_assert_held(obj) lockdep_assert_held(&(obj)->lock.base)
