[Qemu-block] [PATCH] block: make BDRV_POLL_WHILE() re-entrancy safe

[Stefan Hajnoczi]

Nested BDRV_POLL_WHILE() calls can occur.  Currently
assert(!bs_->wakeup) will fail when this happens.

This patch converts bs->wakeup from bool to a counter.

Nesting works correctly because outer BDRV_POLL_WHILE() callers evaluate
the condition again after the inner caller completes (invoking the inner
caller counts as aio_poll() progress).

Reported-by: "fuweiwei (C)" <address@hidden>
Cc: Paolo Bonzini <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
---
Fu Weiwei: Please retest and let us know if this fixes the assertion
failure you hit.  Thanks!
---
 include/block/block.h     | 7 +++----
 include/block/block_int.h | 2 +-
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/include/block/block.h b/include/block/block.h
index fac401ba3e..990b97f0ad 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -385,9 +385,8 @@ void bdrv_drain_all(void);
          * other I/O threads' AioContexts (see for example \
          * block_job_defer_to_main_loop for how to do it). \
          */                                                \
-        assert(!bs_->wakeup);                              \
-        /* Set bs->wakeup before evaluating cond.  */      \
-        atomic_mb_set(&bs_->wakeup, true);                 \
+        /* Increment bs->wakeup before evaluating cond. */ \
+        atomic_inc(&bs_->wakeup);                          \
         while (busy_) {                                    \
             if ((cond)) {                                  \
                 waited_ = busy_ = true;                    \
@@ -399,7 +398,7 @@ void bdrv_drain_all(void);
                 waited_ |= busy_;                          \
             }                                              \
         }                                                  \
-        atomic_set(&bs_->wakeup, false);                   \
+        atomic_dec(&bs_->wakeup);                          \
     }                                                      \
     waited_; })
 
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 5ea63f8fa8..0f360c0ed5 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -712,7 +712,7 @@ struct BlockDriverState {
     /* Internal to BDRV_POLL_WHILE and bdrv_wakeup.  Accessed with atomic
      * ops.
      */
-    bool wakeup;
+    unsigned wakeup;
 
     /* counter for nested bdrv_io_plug.
      * Accessed with atomic ops.
-- 
2.14.3