Two 'dynamic driver' features in FS:

.  When drivers disappear that have pending select()s, wake up
   those user processes with EAGAIN so that they can retry their
   select() and won't hang forever on it.
.  When drivers re-appear and are mapped into the dmap, run through
   the list of mounted filesystems and re-dev_open() every one (for
   partition tables and such). This can't happen before the driver
   has exec()ced itself, so processes that have fork()ed but not
   exec()ced yet are marked as DMAP_BABY in the dmap table if they
   are dmapped before they are execced. If that happens, the above
   procedure happens after the exec(). If the exec() happens before
   the dmapping, it (the dev_open()ing) happens right away.

servers/fs/pipe.c

@@ -11,6 +11,7 @@
  *   release:	  check to see if a suspended process can be released and do
  *                it
  *   revive:	  mark a suspended process as able to run again
+ *   unsuspend_by_proc: revive all processes blocking on a given process
  *   do_unpause:  a signal has been sent to a process; see if it suspended
  */
 
@@ -197,17 +198,22 @@ int task;			/* who is proc waiting for? (PIPE = pipe) */
 /*===========================================================================*
  *				unsuspend_by_proc			     *
  *===========================================================================*/
-void unsuspend_by_proc(int proc)
+PUBLIC void unsuspend_by_proc(int proc)
 {
   struct fproc *rp;
   int client = 0;
 
   /* Revive processes waiting for drivers (SUSPENDed) that have
-   * disappeared with return code EIO.
+   * disappeared with return code EAGAIN.
    */
   for (rp = &fproc[0]; rp < &fproc[NR_PROCS]; rp++, client++)
 	if(rp->fp_suspended == SUSPENDED && rp->fp_task == -proc)
-		revive(client, EIO);
+		revive(client, EAGAIN);
+
+  /* Revive processes waiting in drivers on select()s
+   * with EAGAIN too.
+   */
+  select_unsuspend_by_proc(proc);
 
   return;
 }