async_tx: remove walk of tx->parent chain in dma_wait_for_async_tx

We currently walk the parent chain when waiting for a given tx to
complete however this walk may race with the driver cleanup routine.
The routines in async_raid6_recov.c may fall back to the synchronous
path at any point so we need to be prepared to call async_tx_quiesce()
(which calls  dma_wait_for_async_tx).  To remove the ->parent walk we
guarantee that every time a dependency is attached ->issue_pending() is
invoked, then we can simply poll the initial descriptor until
completion.

This also allows for a lighter weight 'issue pending' implementation as
there is no longer a requirement to iterate through all the channels'
->issue_pending() routines as long as operations have been submitted in
an ordered chain.  async_tx_issue_pending() is added for this case.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/include/linux/async_tx.h b/include/linux/async_tx.h
index 3d21a251751..12a2efcbd56 100644
--- a/include/linux/async_tx.h
+++ b/include/linux/async_tx.h
@@ -83,6 +83,24 @@ struct async_submit_ctl {
 
 #ifdef CONFIG_DMA_ENGINE
 #define async_tx_issue_pending_all dma_issue_pending_all
+
+/**
+ * async_tx_issue_pending - send pending descriptor to the hardware channel
+ * @tx: descriptor handle to retrieve hardware context
+ *
+ * Note: any dependent operations will have already been issued by
+ * async_tx_channel_switch, or (in the case of no channel switch) will
+ * be already pending on this channel.
+ */
+static inline void async_tx_issue_pending(struct dma_async_tx_descriptor *tx)
+{
+        if (likely(tx)) {
+                struct dma_chan *chan = tx->chan;
+                struct dma_device *dma = chan->device;
+
+                dma->device_issue_pending(chan);
+        }
+}
 #ifdef CONFIG_ARCH_HAS_ASYNC_TX_FIND_CHANNEL
 #include <asm/async_tx.h>
 #else
@@ -98,6 +116,11 @@ static inline void async_tx_issue_pending_all(void)
         do { } while (0);
 }
 
+static inline void async_tx_issue_pending(struct dma_async_tx_descriptor *tx)
+{
+        do { } while (0);
+}
+
 static inline struct dma_chan *
 async_tx_find_channel(struct async_submit_ctl *submit,
                       enum dma_transaction_type tx_type, struct page **dst,