Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 784bdf3b authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

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: default avatarRich Felker <dalias@libc.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarSebastian 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: default avatarThomas Gleixner <tglx@linutronix.de>
parent e6580525

kernel/futex.c

+22 −1
Original line number Original line Diff line number Diff line
@@ -179,7 +179,15 @@ int __read_mostly futex_cmpxchg_enabled; 
 * Futex flags used to encode options to functions and preserve them across

 * Futex flags used to encode options to functions and preserve them across

 * restarts.

 * restarts.

 */

 */

#ifdef CONFIG_MMU

# define FLAGS_SHARED		0x01

# 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_CLOCKRT		0x02

#define FLAGS_HAS_TIMEOUT	0x04

#define FLAGS_HAS_TIMEOUT	0x04
@@ -405,6 +413,16 @@ static void get_futex_key_refs(union futex_key *key) 
	if (!key->both.ptr)

	if (!key->both.ptr)

		return;

		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)) {

	switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {

	case FUT_OFF_INODE:

	case FUT_OFF_INODE:

		ihold(key->shared.inode); /* implies smp_mb(); (B) */

		ihold(key->shared.inode); /* implies smp_mb(); (B) */
@@ -436,6 +454,9 @@ static void drop_futex_key_refs(union futex_key *key) 
		return;

		return;

	}

	}





	if (!IS_ENABLED(CONFIG_MMU))

		return;



	switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {

	switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {

	case FUT_OFF_INODE:

	case FUT_OFF_INODE:

		iput(key->shared.inode);

		iput(key->shared.inode);