3 files changed, 44 insertions, 3 deletions

diff --git a/arch/um/include/sysdep-i386/barrier.h b/arch/um/include/sysdep-i386/barrier.h
new file mode 100644
index 000000000000..b58d52c5b2f4
--- /dev/null
+++ b/arch/um/include/sysdep-i386/barrier.h
@@ -0,0 +1,9 @@
+#ifndef __SYSDEP_I386_BARRIER_H
+#define __SYSDEP_I386_BARRIER_H
+
+/* Copied from include/asm-i386 for use by userspace.  i386 has the option
+ * of using mfence, but I'm just using this, which works everywhere, for now.
+ */
+#define mb() asm volatile("lock; addl $0,0(%esp)")
+
+#endif
diff --git a/arch/um/include/sysdep-x86_64/barrier.h b/arch/um/include/sysdep-x86_64/barrier.h
new file mode 100644
index 000000000000..7b610befdc8f
--- /dev/null
+++ b/arch/um/include/sysdep-x86_64/barrier.h
@@ -0,0 +1,7 @@
+#ifndef __SYSDEP_X86_64_BARRIER_H
+#define __SYSDEP_X86_64_BARRIER_H
+
+/* Copied from include/asm-x86_64 for use by userspace. */
+#define mb()    asm volatile("mfence":::"memory")
+
+#endif
diff --git a/arch/um/os-Linux/signal.c b/arch/um/os-Linux/signal.c
index 6b81739279d1..b897e8592d77 100644
--- a/arch/um/os-Linux/signal.c
+++ b/arch/um/os-Linux/signal.c
@@ -15,6 +15,7 @@
 #include "user.h"
 #include "signal_kern.h"
 #include "sysdep/sigcontext.h"
+#include "sysdep/barrier.h"
 #include "sigcontext.h"
 #include "mode.h"
 #include "os.h"
@@ -34,8 +35,12 @@
 #define SIGALRM_BIT 2
 #define SIGALRM_MASK (1 << SIGALRM_BIT)
 
-static int signals_enabled = 1;
-static int pending = 0;
+/* These are used by both the signal handlers and
+ * block/unblock_signals.  I don't want modifications cached in a
+ * register - they must go straight to memory.
+ */
+static volatile int signals_enabled = 1;
+static volatile int pending = 0;
 
 void sig_handler(int sig, struct sigcontext *sc)
 {
@@ -152,6 +157,12 @@ int change_sig(int signal, int on)
 void block_signals(void)
 {
         signals_enabled = 0;
+        /* This must return with signals disabled, so this barrier
+         * ensures that writes are flushed out before the return.
+         * This might matter if gcc figures out how to inline this and
+         * decides to shuffle this code into the caller.
+         */
+        mb();
 }
 
 void unblock_signals(void)
@@ -171,9 +182,23 @@ void unblock_signals(void)
                  */
                 signals_enabled = 1;
 
+                /* Setting signals_enabled and reading pending must
+                 * happen in this order.
+                 */
+                mb();
+
                 save_pending = pending;
-                if(save_pending == 0)
+                if(save_pending == 0){
+                        /* This must return with signals enabled, so
+                         * this barrier ensures that writes are
+                         * flushed out before the return.  This might
+                         * matter if gcc figures out how to inline
+                         * this (unlikely, given its size) and decides
+                         * to shuffle this code into the caller.
+                         */
+                        mb();
                         return;
+                }
 
                 pending = 0;