Fix bio race that results in deadlock

a16be914 · Silas Boyd-Wickizer · 42508b84 · a16be914
--- a/kernel/bio.cc
+++ b/kernel/bio.cc
@@ -94,7 +94,10 @@ bread(u32 dev, u64 sector, int writer)
  if(!(b->flags & B_VALID))
    iderw(b);
  if (writer && !origwriter) {
+    // Lock to seriaize check-and-sleep in bget with clearing B_BUSY
+    acquire(&b->lock);
    b->flags &= ~B_BUSY;
+    release(&b->lock);
    cv_wakeup(&b->cv);
  }
  return b;
@@ -118,7 +121,10 @@ brelse(struct buf *b, int writer)
  if (writer) {
    if((b->flags & B_BUSY) == 0)
      panic("brelse");
+    // Lock to seriaize check-and-sleep in bget with clearing B_BUSY
+    acquire(&b->lock);
    b->flags &= ~B_BUSY;
+    release(&b->lock);
    cv_wakeup(&b->cv);
  }
 }