write-behind: fix data corruption

There was a bug in write-behind that allowed a previous completed write
to overwrite the overlapping region of data from a future write.

Suppose we want to send three writes (W1, W2 and W3). W1 and W2 are
sequential, and W3 writes at the same offset of W2:

    W2.offset = W3.offset = W1.offset + W1.size

Both W1 and W2 are sent in parallel. W3 is only sent after W2 completes.
So W3 should *always* overwrite the overlapping part of W2.

Suppose write-behind processes the requests from 2 concurrent threads:

    Thread 1                    Thread 2

    <received W1>
                                <received W2>
    wb_enqueue_tempted(W1)
    /* W1 is assigned gen X */
                                wb_enqueue_tempted(W2)
                                /* W2 is assigned gen X */

                                wb_process_queue()
                                  __wb_preprocess_winds()
                                    /* W1 and W2 are sequential and all
                                     * other requisites are met to merge
                                     * both requests. */
                                    __wb_collapse_small_writes(W1, W2)
                                    __wb_fulfill_request(W2)

                                  __wb_pick_unwinds() -> W2
                                  /* In this case, since the request is
                                   * already fulfilled, wb_inode->gen
                                   * is not updated. */

                                wb_do_unwinds()
                                  STACK_UNWIND(W2)

                                /* The application has received the
                                 * result of W2, so it can send W3. */
                                <received W3>

                                wb_enqueue_tempted(W3)
                                /* W3 is assigned gen X */

                                wb_process_queue()
                                  /* Here we have W1 (which contains
                                   * the conflicting W2) and W3 with
                                   * same gen, so they are interpreted
                                   * as concurrent writes that do not
                                   * conflict. */
                                  __wb_pick_winds() -> W3

                                wb_do_winds()
                                  STACK_WIND(W3)

    wb_process_queue()
      /* Eventually W1 will be
       * ready to be sent */
      __wb_pick_winds() -> W1
      __wb_pick_unwinds() -> W1
        /* Here wb_inode->gen is
         * incremented. */

    wb_do_unwinds()
      STACK_UNWIND(W1)

    wb_do_winds()
      STACK_WIND(W1)

So, as we can see, W3 is sent before W1, which shouldn't happen.

The problem is that wb_inode->gen is only incremented for requests that
have not been fulfilled but, after a merge, the request is marked as
fulfilled even though it has not been sent to the brick. This allows
that future requests are assigned to the same generation, which could
be internally reordered.

Solution:

Increment wb_inode->gen before any unwind, even if it's for a fulfilled
request.

Special thanks to Stefan Ring for writing a reproducer that has been
crucial to identify the issue.

Change-Id: Id4ab0f294a09aca9a863ecaeef8856474662ab45
Signed-off-by: Xavi Hernandez <xhernandez@redhat.com>
Fixes: #884

1 files changed, 2 insertions, 2 deletions

diff --git a/xlators/performance/write-behind/src/write-behind.c b/xlators/performance/write-behind/src/write-behind.c
index ab6b76cace8..e1d0e9aaf00 100644
--- a/xlators/performance/write-behind/src/write-behind.c
+++ b/xlators/performance/write-behind/src/write-behind.c
@@ -1283,14 +1283,14 @@ __wb_pick_unwinds(wb_inode_t *wb_inode, list_head_t *lies)
 
         wb_inode->window_current += req->orig_size;
 
+        wb_inode->gen++;
+
         if (!req->ordering.fulfilled) {
             /* burden increased */
             list_add_tail(&req->lie, &wb_inode->liability);
 
             req->ordering.lied = 1;
 
-            wb_inode->gen++;
-
             uuid_utoa_r(req->gfid, gfid);
             gf_msg_debug(wb_inode->this->name, 0,
                          "(unique=%" PRIu64