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Subject[PATCH] futex: assume all mappings are private on !MMU systems
From: Thomas Gleixner <tglx@linutronix.de>

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.

Reported-by: Rich Felker <dalias@libc.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[bigeasy@l.de: description]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
kernel/futex.c | 23 ++++++++++++++++++++++-
1 file changed, 22 insertions(+), 1 deletion(-)

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);
--
2.8.1
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