Re: [PATCH] futex: fix shared futex operations on nommu

[Thomas Gleixner <tglx@linutronix.de>]

On Mon, 25 Apr 2016, Rich Felker wrote:

> The shared get_futex_key code does not work on nommu, but is not
> needed anyway because it's impossible for a given backing to have
> multiple distinct virtual addresses on nommu. Simply disable these
> code paths by refraining from setting FLAG_SHARED when CONFIG_MMU is
> not enabled.
> 
> Signed-off-by: Rich Felker <dalias@libc.org>
> ---
>  kernel/futex.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index a5d2e74..ed6f475 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -3131,8 +3131,10 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
>  	int cmd = op & FUTEX_CMD_MASK;
>  	unsigned int flags = 0;
>  
> +#ifdef CONFIG_MMU
>  	if (!(op & FUTEX_PRIVATE_FLAG))
>  		flags |= FLAGS_SHARED;
> +#endif

Well, that's lame. The proper solution is to disable the code which is
involved and let the compiler throw it away. Saves quite some text.

Thanks,

	tglx

8<-------------------

--- 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 fut
 	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 fu
 		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);