VFS: fix short select(2) timeouts

Some select queries require a response from device drivers.  If a
select call is nonblocking (with a zero timeout), the response to
the caller may have to be deferred until all involved drivers have
responded to the initial query.  This is handled just fine.

However, if the select call has a timeout that is so short that it
triggers before all the involved drivers have responded, the
resulting alarm would be discarded, possibly resulting in the call
blocking forever.  This fix changes the alarm handler such that if
the alarm triggers too early, the select call is further handled
as though it was nonblocking.

This fix resolves a test77 deadlock on really slow systems.

Change-Id: Ib487c8fe436802c3e11c57355ae0c8480721f06e

diff --git a/minix/servers/vfs/select.c b/minix/servers/vfs/select.c
index 5bf2a1171c553b3d01f9cb67c15f99eb08db007a..c042698ff1a3db52a7817f52b1113f8ca5f0cfec 100644 (file)
--- a/minix/servers/vfs/select.c
+++ b/minix/servers/vfs/select.c
@@ -755,8 +755,10 @@ void select_timeout_check(minix_timer_t *timer)
   if (se->requestor == NULL) return;
   if (se->expiry <= 0) return; /* Strange, did we even ask for a timeout? */
   se->expiry = 0;
-  if (is_deferred(se)) return; /* Wait for initial replies to CDEV_SELECT */
-  select_return(se);
+  if (!is_deferred(se))
+       select_return(se);
+  else
+       se->block = 0;  /* timer triggered "too soon", treat as nonblocking */
 }