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Subject[PATCH 1/3] xen/blkback: Support 'feature-barrier' aka old-style BARRIER requests.
Date
We emulate the barrier requests by draining the outstanding bio's
and then sending the WRITE_FLUSH command. To drain the I/Os
we use the refcnt that is used during disconnect to wait for all
the I/Os before disconnecting from the frontend. We latch on its
value and if it reaches either the threshold for disconnect or when
there are no more outstanding I/Os, then we have drained all I/Os.

Suggested-by: Christopher Hellwig <hch@infradead.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
drivers/block/xen-blkback/blkback.c | 37 +++++++++++++++++++++++++++++++++-
drivers/block/xen-blkback/common.h | 5 ++++
drivers/block/xen-blkback/xenbus.c | 18 +++++++++++++++++
3 files changed, 58 insertions(+), 2 deletions(-)
diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index e0dab61..184b133 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -452,6 +452,23 @@ static void xen_blk_discard(struct xen_blkif *blkif, struct blkif_request *req)
make_response(blkif, req->id, req->operation, status);
}

+static void xen_blk_drain_io(struct xen_blkif *blkif)
+{
+ atomic_set(&blkif->drain, 1);
+ do {
+ wait_for_completion_interruptible_timeout(
+ &blkif->drain_complete, HZ);
+
+ if (!atomic_read(&blkif->drain))
+ break;
+ /* The initial value is one, and one refcnt taken at the
+ * start of the xen_blkif_schedule thread. */
+ if (atomic_read(&blkif->refcnt) <= 2)
+ break;
+ } while (!kthread_should_stop());
+ atomic_set(&blkif->drain, 0);
+}
+
/*
* Completion callback on the bio's. Called as bh->b_end_io()
*/
@@ -464,6 +481,11 @@ static void __end_block_io_op(struct pending_req *pending_req, int error)
pr_debug(DRV_PFX "flush diskcache op failed, not supported\n");
xen_blkbk_flush_diskcache(XBT_NIL, pending_req->blkif->be, 0);
pending_req->status = BLKIF_RSP_EOPNOTSUPP;
+ } else if ((pending_req->operation == BLKIF_OP_WRITE_BARRIER) &&
+ (error == -EOPNOTSUPP)) {
+ pr_debug(DRV_PFX "write barrier op failed, not supported\n");
+ xen_blkbk_barrier(XBT_NIL, pending_req->blkif->be, 0);
+ pending_req->status = BLKIF_RSP_EOPNOTSUPP;
} else if (error) {
pr_debug(DRV_PFX "Buffer not up-to-date at end of operation,"
" error=%d\n", error);
@@ -481,6 +503,10 @@ static void __end_block_io_op(struct pending_req *pending_req, int error)
pending_req->operation, pending_req->status);
xen_blkif_put(pending_req->blkif);
free_req(pending_req);
+ if (atomic_read(&pending_req->blkif->refcnt) <= 2) {
+ if (atomic_read(&pending_req->blkif->drain))
+ complete(&pending_req->blkif->drain_complete);
+ }
}
}

@@ -574,7 +600,6 @@ do_block_io_op(struct xen_blkif *blkif)

return more_to_do;
}
-
/*
* Transmutation of the 'struct blkif_request' to a proper 'struct bio'
* and call the 'submit_bio' to pass it to the underlying storage.
@@ -591,6 +616,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
int i, nbio = 0;
int operation;
struct blk_plug plug;
+ bool drain = false;

switch (req->operation) {
case BLKIF_OP_READ:
@@ -601,6 +627,8 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
blkif->st_wr_req++;
operation = WRITE_ODIRECT;
break;
+ case BLKIF_OP_WRITE_BARRIER:
+ drain = true;
case BLKIF_OP_FLUSH_DISKCACHE:
blkif->st_f_req++;
operation = WRITE_FLUSH;
@@ -609,7 +637,6 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
blkif->st_ds_req++;
operation = REQ_DISCARD;
break;
- case BLKIF_OP_WRITE_BARRIER:
default:
operation = 0; /* make gcc happy */
goto fail_response;
@@ -668,6 +695,12 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
}
}

+ /* Wait on all outstanding I/O's and once that has been completed
+ * issue the WRITE_FLUSH.
+ */
+ if (drain)
+ xen_blk_drain_io(pending_req->blkif);
+
/*
* If we have failed at this point, we need to undo the M2P override,
* set gnttab_set_unmap_op on all of the grant references and perform
diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index 1b1bc44..e638457 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -181,6 +181,9 @@ struct xen_blkif {
atomic_t refcnt;

wait_queue_head_t wq;
+ /* for barrier (drain) requests */
+ struct completion drain_complete;
+ atomic_t drain;
/* One thread per one blkif. */
struct task_struct *xenblkd;
unsigned int waiting_reqs;
@@ -229,6 +232,8 @@ int xen_blkif_schedule(void *arg);
int xen_blkbk_flush_diskcache(struct xenbus_transaction xbt,
struct backend_info *be, int state);

+int xen_blkbk_barrier(struct xenbus_transaction xbt,
+ struct backend_info *be, int state);
struct xenbus_device *xen_blkbk_xenbus(struct backend_info *be);

static inline void blkif_get_x86_32_req(struct blkif_request *dst,
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 1c44b32..a6d4303 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -114,6 +114,8 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
spin_lock_init(&blkif->blk_ring_lock);
atomic_set(&blkif->refcnt, 1);
init_waitqueue_head(&blkif->wq);
+ init_completion(&blkif->drain_complete);
+ atomic_set(&blkif->drain, 0);
blkif->st_print = jiffies;
init_waitqueue_head(&blkif->waiting_to_free);

@@ -474,6 +476,19 @@ kfree:
out:
return err;
}
+int xen_blkbk_barrier(struct xenbus_transaction xbt,
+ struct backend_info *be, int state)
+{
+ struct xenbus_device *dev = be->dev;
+ int err;
+
+ err = xenbus_printf(xbt, dev->nodename, "feature-barrier",
+ "%d", state);
+ if (err)
+ xenbus_dev_fatal(dev, err, "writing feature-barrier");
+
+ return err;
+}

/*
* Entry point to this code when a new device is created. Allocate the basic
@@ -708,6 +723,9 @@ again:

err = xen_blkbk_discard(xbt, be);

+ /* If we can't advertise it is OK. */
+ err = xen_blkbk_barrier(xbt, be, be->blkif->vbd.flush_support);
+
err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu",
(unsigned long long)vbd_sz(&be->blkif->vbd));
if (err) {
--
1.7.5.4


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