fix getblk to actually lock the block

no more cons_put system calls
usertests tests two processes writing files

1 files changed, 19 insertions, 12 deletions

diff --git a/bio.c b/bio.c
index 2b17a52..7184a28 100644
--- a/bio.c
+++ b/bio.c
@@ -17,24 +17,31 @@ binit(void)
 }
 
 struct buf *
-getblk()
+getblk(uint dev, uint sector)
 {
-  int i;
+  struct buf *b;
 
   acquire(&buf_table_lock);
 
-  // XXX need to lock the block even if not caching, to
-  // avoid read modify write problems.
-
   while(1){
-    for(i = 0; i < NBUF; i++){
-      if((buf[i].flags & B_BUSY) == 0){
-        buf[i].flags |= B_BUSY;
-        release(&buf_table_lock);
-        return buf + i;
+    for(b = buf; b < buf+NBUF; b++)
+      if((b->flags & B_BUSY) && b->dev == dev && b->sector)
+        break;
+
+    if(b < buf+NBUF){
+      sleep(buf, &buf_table_lock);
+    } else {
+      for(b = buf; b < buf+NBUF; b++){
+        if((b->flags & B_BUSY) == 0){
+          b->flags |= B_BUSY;
+          b->dev = dev;
+          b->sector = sector;
+          release(&buf_table_lock);
+          return b;
+        }
       }
+      panic("getblk: no buffers");
     }
-    sleep(buf, &buf_table_lock);
   }
 }
 
@@ -45,7 +52,7 @@ bread(uint dev, uint sector)
   struct buf *b;
   extern struct spinlock ide_lock;
 
-  b = getblk();
+  b = getblk(dev, sector);
 
   acquire(&ide_lock);
   c = ide_start_rw(dev & 0xff, sector, b->data, 1, 1);