Commit-ID: 784bdf3bb694b256fcd6120b93e8947a84249a3a Gitweb: http://git.kernel.org/tip/784bdf3bb694b256fcd6120b93e8947a84249a3a Author: Thomas Gleixner <t...@linutronix.de> AuthorDate: Fri, 29 Jul 2016 16:32:30 +0200 Committer: Thomas Gleixner <t...@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 <dal...@libc.org> Signed-off-by: Thomas Gleixner <t...@linutronix.de> Signed-off-by: Sebastian Andrzej Siewior <bige...@linutronix.de> Cc: Andrew Morton <a...@linux-foundation.org> Link: http://lkml.kernel.org/r/20160729143230.ga21...@linutronix.de Signed-off-by: Thomas Gleixner <t...@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);
