X-Git-Url: https://codewiz.org/gitweb?a=blobdiff_plain;f=bertos%2Fkern%2Fsignal.c;h=4333fa7937fe251a91ee2a0c5bdb1e4120d35006;hb=274910d36c7db1d556864384534f43c69d1bae19;hp=5abd7062679aeacd91f085741c18db31e71d29f8;hpb=791e167e053bdd9250d34a9a5ccae6ccde4d6679;p=bertos.git

diff --git a/bertos/kern/signal.c b/bertos/kern/signal.c
index 5abd7062..4333fa79 100644
--- a/bertos/kern/signal.c
+++ b/bertos/kern/signal.c
@@ -26,9 +26,8 @@
  * invalidate any other reasons why the executable file might be covered by
  * the GNU General Public License.
  *
- * Copyright 2004 Develer S.r.l. (http://www.develer.com/)
- * Copyright 1999, 2000, 2001 Bernardo Innocenti <bernie@develer.com>
- *
+ * Copyright 2004, 2008 Develer S.r.l. (http://www.develer.com/)
+ * Copyright 1999, 2000, 2001 Bernie Innocenti <bernie@codewiz.org>
  * -->
  *
  * \brief IPC signals implementation.
@@ -96,8 +95,7 @@
  *    timer_delayTickes().
  *
  * \version $Id$
- *
- * \author Bernardo Innocenti <bernie@develer.com>
+ * \author Bernie Innocenti <bernie@codewiz.org>
  */
 
 #include "signal.h"
@@ -106,7 +104,7 @@
 #include <drv/timer.h>
 #include <kern/proc.h>
 #include <kern/proc_p.h>
- 
+
 
 #if CONFIG_KERN_SIGNALS
 
@@ -137,18 +135,45 @@ sigmask_t sig_wait(sigmask_t sigs)
 	sigmask_t result;
 	cpuflags_t flags;
 
+	/*
+	 * This is subtle: there's a race condition where a concurrent
+	 * process or an interrupt calls sig_signal() to set a bit in
+	 * out sig_recv just after we have checked for it, but before
+	 * we've set sig_wait to tell them we want to be awaken.
+	 *
+	 * In this case, we'd deadlock with the signal bit already
+	 * set and the process never being reinserted into the ready
+	 * list.
+	 */
 	IRQ_SAVE_DISABLE(flags);
 
 	/* Loop until we get at least one of the signals */
 	while (!(result = CurrentProcess->sig_recv & sigs))
 	{
-		/* go to sleep and proc_schedule() another process */
+		/*
+		 * Tell "them" that we want to be awaken when any of these
+		 * signals arrives.
+		 */
 		CurrentProcess->sig_wait = sigs;
-		proc_schedule();
 
-		/* When we come back here, a signal must be arrived */
+		/*
+		 * Go to sleep and proc_schedule() another process.
+		 *
+		 * We re-enable IRQs because proc_schedule() does not
+		 * guarantee to save and restore the interrupt mask.
+		 */
+		IRQ_RESTORE(flags);
+		proc_schedule();
+		IRQ_SAVE_DISABLE(flags);
+
+		/*
+		 * When we come back here, the wait mask must have been
+		 * cleared by someone through sig_signal(), and at least
+		 * one of the signals we were expecting must have been
+		 * delivered to us.
+		 */
 		ASSERT(!CurrentProcess->sig_wait);
-		ASSERT(CurrentProcess->sig_recv);
+		ASSERT(CurrentProcess->sig_recv & sigs);
 	}
 
 	/* Signals found: clear them and return */
@@ -198,6 +223,8 @@ sigmask_t sig_waitTimeout(sigmask_t sigs, ticks_t timeout)
 void sig_signal(Process *proc, sigmask_t sigs)
 {
 	cpuflags_t flags;
+
+	/* See comment in sig_wait() for why this protection is necessary */
 	IRQ_SAVE_DISABLE(flags);
 
 	/* Set the signals */
@@ -215,4 +242,3 @@ void sig_signal(Process *proc, sigmask_t sigs)
 }
 
 #endif /* CONFIG_KERN_SIGNALS */
-