librbd: reset needs_refresh flag before re-reading header

This way we can't miss an update if we get a notify during ictx_refresh.
Specifically, a race like this:

Thread 1               Thread 2              Process 2

ictx_refresh()
read_header()
                                             snap_create()
                       notify()
                       need_refresh = true
process header...
need_refresh = false

If this happened, we would not re-read the header with the new
snapshot, so the snapshot would not happen at the intended point
in time, but only after we re-read the header again.

Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>

1 files changed, 7 insertions, 4 deletions

diff --git a/src/librbd.cc b/src/librbd.cc
index 22789e0dead..ec538151454 100644
--- a/src/librbd.cc
+++ b/src/librbd.cc
@@ -1192,6 +1192,10 @@ int ictx_refresh(ImageCtx *ictx)
 
   ldout(cct, 20) << "ictx_refresh " << ictx << dendl;
 
+  ictx->refresh_lock.Lock();
+  ictx->needs_refresh = false;
+  ictx->refresh_lock.Unlock();
+
   int r = read_header(ictx->md_ctx, ictx->md_oid(), &(ictx->header), NULL);
   if (r < 0) {
     lderr(cct) << "Error reading header: " << cpp_strerror(-r) << dendl;
@@ -1239,6 +1243,9 @@ int ictx_refresh(ImageCtx *ictx)
 
   if (!ictx->snapc.is_valid()) {
     lderr(cct) << "image snap context is invalid!" << dendl;
+    ictx->refresh_lock.Lock();
+    ictx->needs_refresh = true;
+    ictx->refresh_lock.Unlock();
     return -EIO;
   }
 
@@ -1251,10 +1258,6 @@ int ictx_refresh(ImageCtx *ictx)
 
   ictx->data_ctx.selfmanaged_snap_set_write_ctx(ictx->snapc.seq, ictx->snaps);
 
-  ictx->refresh_lock.Lock();
-  ictx->needs_refresh = false;
-  ictx->refresh_lock.Unlock();
-
   return 0;
 }