Re: [PATCH v3 5/8] block/write-threshold: don't use aio context lock

[Paolo Bonzini]

On 06/05/21 11:06, Vladimir Sementsov-Ogievskiy wrote:
  void bdrv_write_threshold_check_write(BlockDriverState *bs, int64_t offset,
                                        int64_t bytes)
  {
      int64_t end = offset + bytes;
-    uint64_t wtr = bs->write_threshold_offset;
+    uint64_t wtr;
  
-    if (wtr > 0 && end > wtr) {
-        qapi_event_send_block_write_threshold(bs->node_name, end - wtr, wtr);
+retry:
+    wtr = bdrv_write_threshold_get(bs);
+    if (wtr == 0 || wtr >= end) {
+        return;
+    }
  
-        /* autodisable to avoid flooding the monitor */
-        bdrv_write_threshold_set(bs, 0);
+    /* autodisable to avoid flooding the monitor */
+    if (qatomic_cmpxchg(&bs->write_threshold_offset, wtr, 0) != wtr) {
+        /* bs->write_threshold_offset changed in parallel */
+        goto retry;
      }
+
+    /* We have cleared bs->write_threshold_offset, so let's send event */
+    qapi_event_send_block_write_threshold(bs->node_name, end - wtr, wtr);
  }

This has the problem that 64-bit atomics are not always possible on 
32-bit builds.  We can use a spinlock (and probably just drop this patch 
for now).

Paolo