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+I/O Barriers
+============
+Tejun Heo <htejun@gmail.com>, July 22 2005
+
+I/O barrier requests are used to guarantee ordering around the barrier
+requests.  Unless you're crazy enough to use disk drives for
+implementing synchronization constructs (wow, sounds interesting...),
+the ordering is meaningful only for write requests for things like
+journal checkpoints.  All requests queued before a barrier request
+must be finished (made it to the physical medium) before the barrier
+request is started, and all requests queued after the barrier request
+must be started only after the barrier request is finished (again,
+made it to the physical medium).
+
+In other words, I/O barrier requests have the following two properties.
+
+1. Request ordering
+
+Requests cannot pass the barrier request.  Preceding requests are
+processed before the barrier and following requests after.
+
+Depending on what features a drive supports, this can be done in one
+of the following three ways.
+
+i. For devices which have queue depth greater than 1 (TCQ devices) and
+support ordered tags, block layer can just issue the barrier as an
+ordered request and the lower level driver, controller and drive
+itself are responsible for making sure that the ordering contraint is
+met.  Most modern SCSI controllers/drives should support this.
+
+NOTE: SCSI ordered tag isn't currently used due to limitation in the
+      SCSI midlayer, see the following random notes section.
+
+ii. For devices which have queue depth greater than 1 but don't
+support ordered tags, block layer ensures that the requests preceding
+a barrier request finishes before issuing the barrier request.  Also,
+it defers requests following the barrier until the barrier request is
+finished.  Older SCSI controllers/drives and SATA drives fall in this
+category.
+
+iii. Devices which have queue depth of 1.  This is a degenerate case
+of ii.  Just keeping issue order suffices.  Ancient SCSI
+controllers/drives and IDE drives are in this category.
+
+2. Forced flushing to physcial medium
+
+Again, if you're not gonna do synchronization with disk drives (dang,
+it sounds even more appealing now!), the reason you use I/O barriers
+is mainly to protect filesystem integrity when power failure or some
+other events abruptly stop the drive from operating and possibly make
+the drive lose data in its cache.  So, I/O barriers need to guarantee
+that requests actually get written to non-volatile medium in order.
+
+There are four cases,
+
+i. No write-back cache.  Keeping requests ordered is enough.
+
+ii. Write-back cache but no flush operation.  There's no way to
+gurantee physical-medium commit order.  This kind of devices can't to
+I/O barriers.
+
+iii. Write-back cache and flush operation but no FUA (forced unit
+access).  We need two cache flushes - before and after the barrier
+request.
+
+iv. Write-back cache, flush operation and FUA.  We still need one
+flush to make sure requests preceding a barrier are written to medium,
+but post-barrier flush can be avoided by using FUA write on the
+barrier itself.
+
+
+How to support barrier requests in drivers
+------------------------------------------
+
+All barrier handling is done inside block layer proper.  All low level
+drivers have to are implementing its prepare_flush_fn and using one
+the following two functions to indicate what barrier type it supports
+and how to prepare flush requests.  Note that the term 'ordered' is
+used to indicate the whole sequence of performing barrier requests
+including draining and flushing.
+
+typedef void (prepare_flush_fn)(request_queue_t *q, struct request *rq);
+
+int blk_queue_ordered(request_queue_t *q, unsigned ordered,
+                      prepare_flush_fn *prepare_flush_fn,
+                      unsigned gfp_mask);
+
+int blk_queue_ordered_locked(request_queue_t *q, unsigned ordered,
+                             prepare_flush_fn *prepare_flush_fn,
+                             unsigned gfp_mask);
+
+The only difference between the two functions is whether or not the
+caller is holding q->queue_lock on entry.  The latter expects the
+caller is holding the lock.
+
+@q                      : the queue in question
+@ordered                : the ordered mode the driver/device supports
+@prepare_flush_fn       : this function should prepare @rq such that it
+                          flushes cache to physical medium when executed
+@gfp_mask               : gfp_mask used when allocating data structures
+                          for ordered processing
+
+For example, SCSI disk driver's prepare_flush_fn looks like the
+following.
+
+static void sd_prepare_flush(request_queue_t *q, struct request *rq)
+{
+        memset(rq->cmd, 0, sizeof(rq->cmd));
+        rq->flags |= REQ_BLOCK_PC;
+        rq->timeout = SD_TIMEOUT;
+        rq->cmd[0] = SYNCHRONIZE_CACHE;
+}
+
+The following seven ordered modes are supported.  The following table
+shows which mode should be used depending on what features a
+device/driver supports.  In the leftmost column of table,
+QUEUE_ORDERED_ prefix is omitted from the mode names to save space.
+
+The table is followed by description of each mode.  Note that in the
+descriptions of QUEUE_ORDERED_DRAIN*, '=>' is used whereas '->' is
+used for QUEUE_ORDERED_TAG* descriptions.  '=>' indicates that the
+preceding step must be complete before proceeding to the next step.
+'->' indicates that the next step can start as soon as the previous
+step is issued.
+
+            write-back cache    ordered tag     flush           FUA
+-----------------------------------------------------------------------
+NONE            yes/no          N/A             no              N/A
+DRAIN           no              no              N/A             N/A
+DRAIN_FLUSH     yes             no              yes             no
+DRAIN_FUA       yes             no              yes             yes
+TAG             no              yes             N/A             N/A
+TAG_FLUSH       yes             yes             yes             no
+TAG_FUA         yes             yes             yes             yes
+
+
+QUEUE_ORDERED_NONE
+        I/O barriers are not needed and/or supported.
+
+        Sequence: N/A
+
+QUEUE_ORDERED_DRAIN
+        Requests are ordered by draining the request queue and cache
+        flushing isn't needed.
+
+        Sequence: drain => barrier
+
+QUEUE_ORDERED_DRAIN_FLUSH
+        Requests are ordered by draining the request queue and both
+        pre-barrier and post-barrier cache flushings are needed.
+
+        Sequence: drain => preflush => barrier => postflush
+
+QUEUE_ORDERED_DRAIN_FUA
+        Requests are ordered by draining the request queue and
+        pre-barrier cache flushing is needed.  By using FUA on barrier
+        request, post-barrier flushing can be skipped.
+
+        Sequence: drain => preflush => barrier
+
+QUEUE_ORDERED_TAG
+        Requests are ordered by ordered tag and cache flushing isn't
+        needed.
+
+        Sequence: barrier
+
+QUEUE_ORDERED_TAG_FLUSH
+        Requests are ordered by ordered tag and both pre-barrier and
+        post-barrier cache flushings are needed.
+
+        Sequence: preflush -> barrier -> postflush
+
+QUEUE_ORDERED_TAG_FUA
+        Requests are ordered by ordered tag and pre-barrier cache
+        flushing is needed.  By using FUA on barrier request,
+        post-barrier flushing can be skipped.
+
+        Sequence: preflush -> barrier
+
+
+Random notes/caveats
+--------------------
+
+* SCSI layer currently can't use TAG ordering even if the drive,
+controller and driver support it.  The problem is that SCSI midlayer
+request dispatch function is not atomic.  It releases queue lock and
+switch to SCSI host lock during issue and it's possible and likely to
+happen in time that requests change their relative positions.  Once
+this problem is solved, TAG ordering can be enabled.
+
+* Currently, no matter which ordered mode is used, there can be only
+one barrier request in progress.  All I/O barriers are held off by
+block layer until the previous I/O barrier is complete.  This doesn't
+make any difference for DRAIN ordered devices, but, for TAG ordered
+devices with very high command latency, passing multiple I/O barriers
+to low level *might* be helpful if they are very frequent.  Well, this
+certainly is a non-issue.  I'm writing this just to make clear that no
+two I/O barrier is ever passed to low-level driver.
+
+* Completion order.  Requests in ordered sequence are issued in order
+but not required to finish in order.  Barrier implementation can
+handle out-of-order completion of ordered sequence.  IOW, the requests
+MUST be processed in order but the hardware/software completion paths
+are allowed to reorder completion notifications - eg. current SCSI
+midlayer doesn't preserve completion order during error handling.
+
+* Requeueing order.  Low-level drivers are free to requeue any request
+after they removed it from the request queue with
+blkdev_dequeue_request().  As barrier sequence should be kept in order
+when requeued, generic elevator code takes care of putting requests in
+order around barrier.  See blk_ordered_req_seq() and
+ELEVATOR_INSERT_REQUEUE handling in __elv_add_request() for details.
+
+Note that block drivers must not requeue preceding requests while
+completing latter requests in an ordered sequence.  Currently, no
+error checking is done against this.
+
+* Error handling.  Currently, block layer will report error to upper
+layer if any of requests in an ordered sequence fails.  Unfortunately,
+this doesn't seem to be enough.  Look at the following request flow.
+QUEUE_ORDERED_TAG_FLUSH is in use.
+
+ [0] [1] [2] [3] [pre] [barrier] [post] < [4] [5] [6] ... >
+                                          still in elevator
+
+Let's say request [2], [3] are write requests to update file system
+metadata (journal or whatever) and [barrier] is used to mark that
+those updates are valid.  Consider the following sequence.
+
+ i.     Requests [0] ~ [post] leaves the request queue and enters
+        low-level driver.
+ ii.    After a while, unfortunately, something goes wrong and the
+        drive fails [2].  Note that any of [0], [1] and [3] could have
+        completed by this time, but [pre] couldn't have been finished
+        as the drive must process it in order and it failed before
+        processing that command.
+ iii.   Error handling kicks in and determines that the error is
+        unrecoverable and fails [2], and resumes operation.
+ iv.    [pre] [barrier] [post] gets processed.
+ v.     *BOOM* power fails
+
+The problem here is that the barrier request is *supposed* to indicate
+that filesystem update requests [2] and [3] made it safely to the
+physical medium and, if the machine crashes after the barrier is
+written, filesystem recovery code can depend on that.  Sadly, that
+isn't true in this case anymore.  IOW, the success of a I/O barrier
+should also be dependent on success of some of the preceding requests,
+where only upper layer (filesystem) knows what 'some' is.
+
+This can be solved by implementing a way to tell the block layer which
+requests affect the success of the following barrier request and
+making lower lever drivers to resume operation on error only after
+block layer tells it to do so.
+
+As the probability of this happening is very low and the drive should
+be faulty, implementing the fix is probably an overkill.  But, still,
+it's there.
+
+* In previous drafts of barrier implementation, there was fallback
+mechanism such that, if FUA or ordered TAG fails, less fancy ordered
+mode can be selected and the failed barrier request is retried
+automatically.  The rationale for this feature was that as FUA is
+pretty new in ATA world and ordered tag was never used widely, there
+could be devices which report to support those features but choke when
+actually given such requests.
+
+ This was removed for two reasons 1. it's an overkill 2. it's
+impossible to implement properly when TAG ordering is used as low
+level drivers resume after an error automatically.  If it's ever
+needed adding it back and modifying low level drivers accordingly
+shouldn't be difficult.