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Subject[PATCH v3 10/13] futex: Inform FUTEX_LOCK callers on how the lock is acquired
Date
As there are three different ways for a TP futex waiter in the kernel
to acquire the lock. It may be useful to pass this information out so
that user space has a better view of what is happening in the kernel.
With this change, different non-negative values will now be returned
depending on how the lock is acquired in the kernel.

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
---
kernel/futex.c | 30 ++++++++++++++++++++++--------
1 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index d260410..1219f32 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3337,7 +3337,17 @@ void exit_robust_list(struct task_struct *curr)
#define TP_SPIN_THRESHOLD (1 << 14)
#define TP_SLEEP_DECREMENT (TP_SPIN_THRESHOLD/64)

-/**
+/*
+ * futex_lock() returned values to identify how the lock is acquired:
+ * 0 - steals the lock
+ * 1 - top waiter (mutex owner) acquires the lock
+ * 2 - handed off the lock
+ */
+#define TP_LOCK_STOLEN 0
+#define TP_LOCK_ACQUIRED 1
+#define TP_LOCK_HANDOFF 2
+
+ /**
* lookup_futex_state - Looking up the futex state structure.
* @hb: hash bucket
* @key: futex key
@@ -3420,9 +3430,11 @@ static inline int put_futex_state_unlocked(struct futex_state *state)
* preserve the flag bits
* endif
*
- * Return: 1 if lock acquired;
+ * Return: TP_LOCK_ACQUIRED if lock acquired;
+ * TP_LOCK_HANDOFF if lock was handed off;
* 0 if lock acquisition failed;
* -EFAULT if an error happened.
+ * *puval will contain the latest futex value when trylock fails.
*/
static inline int futex_trylock(u32 __user *uaddr, u32 vpid, u32 *puval,
const bool waiter)
@@ -3435,7 +3447,7 @@ static inline int futex_trylock(u32 __user *uaddr, u32 vpid, u32 *puval,
*puval = uval;

if (waiter && (uval & FUTEX_TID_MASK) == vpid)
- return 1;
+ return TP_LOCK_HANDOFF;

if (uval & FUTEX_TID_MASK)
return 0; /* Trylock fails */
@@ -3446,7 +3458,7 @@ static inline int futex_trylock(u32 __user *uaddr, u32 vpid, u32 *puval,
if (unlikely(cmpxchg_futex_value(puval, uaddr, uval, vpid|flags)))
return -EFAULT;

- return *puval == uval;
+ return (*puval == uval) ? TP_LOCK_ACQUIRED : 0;
}

/**
@@ -3659,8 +3671,10 @@ efault:
* This function is not inlined so that it can show up separately in perf
* profile for performance analysis purpose.
*
- * Return: 0 - lock acquired
- * < 0 - an error happens
+ * Return: TP_LOCK_ACQUIRED - lock acquired normally
+ * TP_LOCK_HANDOFF - lock handed off directly from unlocker
+ * TP_LOCK_STOLEN - lock stolen
+ * < 0 - an error happens
*/
static noinline int futex_lock(u32 __user *uaddr, unsigned int flags)
{
@@ -3676,7 +3690,7 @@ static noinline int futex_lock(u32 __user *uaddr, unsigned int flags)
*/
ret = futex_trylock(uaddr, vpid, &uval, false);
if (ret)
- goto out;
+ return (ret < 0) ? ret : TP_LOCK_STOLEN;

/*
* Detect deadlocks.
@@ -3750,7 +3764,7 @@ out_put_state_key:
put_futex_key(&key);

out:
- return (ret < 0) ? ret : 0;
+ return ret;
}

/*
--
1.7.1
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