futex: Assume all mappings are private on !MMU systems

To quote Rick why there is no need for shared mapping on !MMU systems:

|With MMU, shared futex keys need to identify the physical backing for
|a memory address because it may be mapped at different addresses in
|different processes (or even multiple times in the same process).
|Without MMU this cannot happen. You only have physical addresses. So
|the "private futex" behavior of using the virtual address as the key
|is always correct (for both shared and private cases) on nommu
|systems.

This patch disables the FLAGS_SHARED in a way that allows the compiler to
remove that code.

[bigeasy: Added changelog ]
Reported-by: Rich Felker <dalias@libc.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20160729143230.GA21715@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 22 insertions, 1 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index 33664f70e2d2..46cb3a301bc1 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -179,7 +179,15 @@ int __read_mostly futex_cmpxchg_enabled;
  * Futex flags used to encode options to functions and preserve them across
  * restarts.
  */
-#define FLAGS_SHARED            0x01
+#ifdef CONFIG_MMU
+# define FLAGS_SHARED           0x01
+#else
+/*
+ * NOMMU does not have per process address space. Let the compiler optimize
+ * code away.
+ */
+# define FLAGS_SHARED           0x00
+#endif
 #define FLAGS_CLOCKRT           0x02
 #define FLAGS_HAS_TIMEOUT       0x04
 
@@ -405,6 +413,16 @@ static void get_futex_key_refs(union futex_key *key)
         if (!key->both.ptr)
                 return;
 
+        /*
+         * On MMU less systems futexes are always "private" as there is no per
+         * process address space. We need the smp wmb nevertheless - yes,
+         * arch/blackfin has MMU less SMP ...
+         */
+        if (!IS_ENABLED(CONFIG_MMU)) {
+                smp_mb(); /* explicit smp_mb(); (B) */
+                return;
+        }
+
         switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
         case FUT_OFF_INODE:
                 ihold(key->shared.inode); /* implies smp_mb(); (B) */
@@ -436,6 +454,9 @@ static void drop_futex_key_refs(union futex_key *key)
                 return;
         }
 
+        if (!IS_ENABLED(CONFIG_MMU))
+                return;
+
         switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
         case FUT_OFF_INODE:
                 iput(key->shared.inode);