Make all the filetable member functions lock free.

..allocfd might still cause bottlenecks as threads race to allocate FD slots..

Make all the filetable member functions lock free.
d99a5ad0 · Silas Boyd-Wickizer · 85235464 · d99a5ad0
--- a/include/filetable.hh
+++ b/include/filetable.hh
@@ -5,27 +5,24 @@ public:
  filetable() : ref_(1) {
    for(int fd = 0; fd < NOFILE; fd++)
      ofile_[fd] = nullptr;
-    initlock(&lock_, "filetable", 0);
  }

  filetable(const filetable &f) : ref_(1) {
    for(int fd = 0; fd < NOFILE; fd++) {
      if (f.ofile_[fd])
-        ofile_[fd] = f.ofile_[fd]->dup();
+        ofile_[fd].store(f.ofile_[fd].load()->dup());
      else
        ofile_[fd] = nullptr;
    }
-    initlock(&lock_, "filetable", 0);
  }
  
  ~filetable() {
    for(int fd = 0; fd < NOFILE; fd++){
      if (ofile_[fd]){
-        ofile_[fd]->dec();
-        ofile_[fd] = 0;
+        ofile_[fd].load()->dec();
+        ofile_[fd] = nullptr;
      }
    }
-    destroylock(&lock_);
  }

  bool getfile(int fd, sref<file> *sf) {
@@ -40,15 +37,9 @@ public:
  }

  int allocfd(struct file *f) {
-    acquire(&lock_);
-    for (int fd = 0; fd < NOFILE; fd++) {
-      if (ofile_[fd] == nullptr){
-        ofile_[fd] = f;
-        release(&lock_);
+    for (int fd = 0; fd < NOFILE; fd++)
+      if (ofile_[fd] == nullptr && cmpxch(&ofile_[fd], (file*)nullptr, f))
        return fd;
-      }
-    }
-    release(&lock_);    
    cprintf("filetable::allocfd: failed\n");
    return -1;
  }
@@ -56,15 +47,13 @@ public:
  void close(int fd) {
    // XXX(sbw) if f->ref_ > 1 the kernel will not actually close 
    // the file when this function returns (i.e. sys_close can return 
-    // while the file/pipe/socket is still open).  Maybe we should clear 
-    // ofile_[fd], wait until f.ref_ == 1, f->dec(), and then return.
-    
-    acquire(&lock_);
-    struct file *f = ofile_[fd];
-    ofile_[fd] = nullptr;
-    release(&lock_);
-    if (f)
+    // while the file/pipe/socket is still open).
+
+    file* f = ofile_[fd].exchange(nullptr);
+    if (f != nullptr)
      f->dec();
+    else
+      cprintf("filetable::close: bad fd %u\n", fd);
  }

  void decref() {
@@ -79,7 +68,6 @@ public:
  NEW_DELETE_OPS(filetable)

 private:
-  struct file *ofile_[NOFILE];
+  std::atomic<file*> ofile_[NOFILE];
  std::atomic<u64> ref_;
-  struct spinlock lock_;
 };