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Subject[tip:locking/urgent] futex: Assume all mappings are private on !MMU systems
Commit-ID:  784bdf3bb694b256fcd6120b93e8947a84249a3a
Gitweb: http://git.kernel.org/tip/784bdf3bb694b256fcd6120b93e8947a84249a3a
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Fri, 29 Jul 2016 16:32:30 +0200
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Fri, 29 Jul 2016 18:44:14 +0200

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>

---
kernel/futex.c | 23 ++++++++++++++++++++++-
1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 33664f7..46cb3a3 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);
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