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Subject[tip:locking/core] futex: Allow for compiling out PI support
Commit-ID:  bc2eecd7ecce40af43b6eb3d256b6076257df846
Gitweb: http://git.kernel.org/tip/bc2eecd7ecce40af43b6eb3d256b6076257df846
Author: Nicolas Pitre <nicolas.pitre@linaro.org>
AuthorDate: Tue, 1 Aug 2017 00:31:32 -0400
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Tue, 1 Aug 2017 14:36:35 +0200

futex: Allow for compiling out PI support

This makes it possible to preserve basic futex support and compile out the
PI support when RT mutexes are not available.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Darren Hart <dvhart@infradead.org>
Link: http://lkml.kernel.org/r/alpine.LFD.2.20.1708010024190.5981@knanqh.ubzr

---
include/linux/futex.h | 7 ++++++-
init/Kconfig | 7 ++++++-
kernel/futex.c | 22 ++++++++++++++++++++++
kernel/locking/rtmutex_common.h | 29 +++++++++++++++++++++++++++++
4 files changed, 63 insertions(+), 2 deletions(-)

diff --git a/include/linux/futex.h b/include/linux/futex.h
index 7c5b694..f36bfd2 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -54,7 +54,6 @@ union futex_key {

#ifdef CONFIG_FUTEX
extern void exit_robust_list(struct task_struct *curr);
-extern void exit_pi_state_list(struct task_struct *curr);
#ifdef CONFIG_HAVE_FUTEX_CMPXCHG
#define futex_cmpxchg_enabled 1
#else
@@ -64,8 +63,14 @@ extern int futex_cmpxchg_enabled;
static inline void exit_robust_list(struct task_struct *curr)
{
}
+#endif
+
+#ifdef CONFIG_FUTEX_PI
+extern void exit_pi_state_list(struct task_struct *curr);
+#else
static inline void exit_pi_state_list(struct task_struct *curr)
{
}
#endif
+
#endif
diff --git a/init/Kconfig b/init/Kconfig
index 8514b25..5f0ef85 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1275,12 +1275,17 @@ config BASE_FULL
config FUTEX
bool "Enable futex support" if EXPERT
default y
- select RT_MUTEXES
+ imply RT_MUTEXES
help
Disabling this option will cause the kernel to be built without
support for "fast userspace mutexes". The resulting kernel may not
run glibc-based applications correctly.

+config FUTEX_PI
+ bool
+ depends on FUTEX && RT_MUTEXES
+ default y
+
config HAVE_FUTEX_CMPXCHG
bool
depends on FUTEX
diff --git a/kernel/futex.c b/kernel/futex.c
index 16dbe4c..ad0af4d 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -875,6 +875,8 @@ static struct task_struct *futex_find_get_task(pid_t pid)
return p;
}

+#ifdef CONFIG_FUTEX_PI
+
/*
* This task is holding PI mutexes at exit time => bad.
* Kernel cleans up PI-state, but userspace is likely hosed.
@@ -932,6 +934,8 @@ void exit_pi_state_list(struct task_struct *curr)
raw_spin_unlock_irq(&curr->pi_lock);
}

+#endif
+
/*
* We need to check the following states:
*
@@ -1799,6 +1803,15 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
struct futex_q *this, *next;
DEFINE_WAKE_Q(wake_q);

+ /*
+ * When PI not supported: return -ENOSYS if requeue_pi is true,
+ * consequently the compiler knows requeue_pi is always false past
+ * this point which will optimize away all the conditional code
+ * further down.
+ */
+ if (!IS_ENABLED(CONFIG_FUTEX_PI) && requeue_pi)
+ return -ENOSYS;
+
if (requeue_pi) {
/*
* Requeue PI only works on two distinct uaddrs. This
@@ -2594,6 +2607,9 @@ static int futex_lock_pi(u32 __user *uaddr, unsigned int flags,
struct futex_q q = futex_q_init;
int res, ret;

+ if (!IS_ENABLED(CONFIG_FUTEX_PI))
+ return -ENOSYS;
+
if (refill_pi_state_cache())
return -ENOMEM;

@@ -2773,6 +2789,9 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
struct futex_q *top_waiter;
int ret;

+ if (!IS_ENABLED(CONFIG_FUTEX_PI))
+ return -ENOSYS;
+
retry:
if (get_user(uval, uaddr))
return -EFAULT;
@@ -2983,6 +3002,9 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
struct futex_q q = futex_q_init;
int res, ret;

+ if (!IS_ENABLED(CONFIG_FUTEX_PI))
+ return -ENOSYS;
+
if (uaddr == uaddr2)
return -EINVAL;

diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
index 72ad45a..8d039b9 100644
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -40,6 +40,9 @@ struct rt_mutex_waiter {
/*
* Various helpers to access the waiters-tree:
*/
+
+#ifdef CONFIG_RT_MUTEXES
+
static inline int rt_mutex_has_waiters(struct rt_mutex *lock)
{
return !RB_EMPTY_ROOT(&lock->waiters);
@@ -69,6 +72,32 @@ task_top_pi_waiter(struct task_struct *p)
pi_tree_entry);
}

+#else
+
+static inline int rt_mutex_has_waiters(struct rt_mutex *lock)
+{
+ return false;
+}
+
+static inline struct rt_mutex_waiter *
+rt_mutex_top_waiter(struct rt_mutex *lock)
+{
+ return NULL;
+}
+
+static inline int task_has_pi_waiters(struct task_struct *p)
+{
+ return false;
+}
+
+static inline struct rt_mutex_waiter *
+task_top_pi_waiter(struct task_struct *p)
+{
+ return NULL;
+}
+
+#endif
+
/*
* lock->owner state tracking:
*/
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