After some tweaking this patch results in very little change in the
results - but does cut down the complexity of the solution quite a bit.
Two questions inline (not included in the new patch) and a new version
of the patch below.
Why remove this?
Removing the preempt_disable() makes it more likely for a spinner to get
preempted while spinning. Putting these back and adding need_resched()
in trylock_futex_adaptive() will make it much more likely for a task to
queue up on the futex before getting descheduled.
Note: this version has been modified by Darren Hart to:
o remove the timeout handling changes as the timer is not yet armed.
o "owner" conversion from ti to task
o trylock_futex_adaptive owner null check
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
---
include/linux/futex.h | 1 -
kernel/futex.c | 66 ++++++++++++++++--------------------------------
kernel/sched.c | 66 -------------------------------------------------
3 files changed, 22 insertions(+), 111 deletions(-)
diff --git a/include/linux/futex.h b/include/linux/futex.h
index 3ca93d1..ac374f2 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -182,7 +182,6 @@ union futex_key {
extern void exit_robust_list(struct task_struct *curr);
extern void exit_pi_state_list(struct task_struct *curr);
extern int futex_cmpxchg_enabled;
-extern struct thread_info *futex_owner(u32 __user *uaddr);
#else
static inline void exit_robust_list(struct task_struct *curr)
{
diff --git a/kernel/futex.c b/kernel/futex.c
index f9e56b9..37f763f 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -371,16 +371,15 @@ static int get_futex_value_locked(u32 *dest, u32 __user *from)
#ifdef CONFIG_SMP
/**
- * futex_owner() - Lookup the thread_info struct of ther futex owner
+ * futex_owner() - Lookup the task struct of ther futex owner
* @uaddr: The user address containing the TID of the owner
*
- * If a non-NULL ti is returned, the caller must call
- * put_task_struct(ti->task).
+ * If a non-NULL task is returned, the caller must call
+ * put_task_struct(task).
* */
-struct thread_info *futex_owner(u32 __user *uaddr)
+struct task_struct *futex_owner(u32 __user *uaddr)
{
- struct thread_info *ti = NULL;
- struct task_struct *p;
+ struct task_struct *p = NULL;
pid_t pid;
u32 uval;
@@ -389,20 +388,11 @@ struct thread_info *futex_owner(u32 __user *uaddr)
pid = uval & FUTEX_TID_MASK;
rcu_read_lock();
- p = find_task_by_vpid(pid);
- if (p) {
- const struct cred *cred, *pcred;
-
- cred = current_cred();
- pcred = __task_cred(p);
- if (cred->euid == pcred->euid || cred->euid == pcred->uid) {
- ti = task_thread_info(p);
- get_task_struct(ti->task);
- }
- }
+ if ((p = find_task_by_vpid(pid)))
+ get_task_struct(p);
rcu_read_unlock();
- return ti;
+ return p;
}
#endif
@@ -2408,7 +2398,7 @@ out:
*/
static int trylock_futex_adaptive(u32 __user *uaddr, ktime_t *timeout)
{
- struct thread_info *owner = NULL;
+ struct task_struct *owner = NULL;
pid_t curtid, ownertid = 0;
ktime_t now;
int ret = 0;
@@ -2425,48 +2415,38 @@ static int trylock_futex_adaptive(u32 __user *uaddr, ktime_t *timeout)
/*
* Try to acquire the lock.
*/
- if (curval == 0)
+ if (curval == 0) {
curval = cmpxchg_futex_value_locked(uaddr, 0, curtid);
- if (curval == 0) {
- ret = 1;
- break;
- }
+ if (curval == 0) {
+ ret = 1;
+ break;
+ }
- if (unlikely(curval == -EFAULT)) {
- ret = -EFAULT;
- break;
+ if (unlikely(curval == -EFAULT)) {
+ ret = -EFAULT;
+ break;
+ }
}
- /*
- * Futex locks manage the owner atomically. We are not in
- * danger of deadlock by preempting a pending owner. Abort the
- * loop if our time slice has expired. RT Threads can spin
- * until they preempt the owner, at which point they will break
- * out of the loop anyway.
- */
- if (need_resched())
- break;
-
if (timeout) {
now = ktime_get();
if (timeout->tv64 < now.tv64)
break;
}
- cpu_relax();
-
if ((curval & FUTEX_TID_MASK) != ownertid) {
if (owner)
- put_task_struct(owner->task);
+ put_task_struct(owner);
owner = futex_owner(uaddr);
}
- if (owner && !futex_spin_on_owner(uaddr, owner))
+
+ if (owner && !owner->oncpu)
break;
}
if (owner)
- put_task_struct(owner->task);
+ put_task_struct(owner);
return ret;
}
@@ -2510,9 +2490,7 @@ static int futex_lock(u32 __user *uaddr, int flags, int detect, ktime_t *time)
retry:
#ifdef CONFIG_SMP
if (flags & FLAGS_ADAPTIVE) {
- preempt_disable();
ret = trylock_futex_adaptive(uaddr, time);
- preempt_enable();
/* We got the lock. */
if (ret == 1) {
diff --git a/kernel/sched.c b/kernel/sched.c
index 9915bdf..84424e6 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3827,72 +3827,6 @@ out:
}
#endif
-#ifdef CONFIG_FUTEX
-#include <linux/futex.h>
-
-int futex_spin_on_owner(u32 __user *uaddr, struct thread_info *owner)
-{
- u32 ownertid, uval;
- unsigned int cpu;
- struct rq *rq;
-
- if (!sched_feat(OWNER_SPIN))
- return 0;
-
-#ifdef CONFIG_DEBUG_PAGEALLOC
- /*
- * Need to access the cpu field knowing that
- * DEBUG_PAGEALLOC could have unmapped it if
- * the mutex owner just released it and exited.
- */
- if (probe_kernel_address(&owner->cpu, cpu))
- goto out;
-#else
- cpu = owner->cpu;
-#endif
-
- /*
- * Even if the access succeeded (likely case),
- * the cpu field may no longer be valid.
- */
- if (cpu >= nr_cpumask_bits)
- goto out;
-
- /*
- * We need to validate that we can do a
- * get_cpu() and that we have the percpu area.
- */
- if (!cpu_online(cpu))
- goto out;
-
- rq = cpu_rq(cpu);
-
- if (get_user(ownertid, uaddr))
- return -EFAULT;
- ownertid &= FUTEX_TID_MASK;
-
- for (;;) {
- /*
- * Owner changed, break to re-assess state.
- */
- if (get_user(uval, uaddr))
- return -EFAULT;
- if ((uval & FUTEX_TID_MASK) != ownertid)
- break;
-
- /*
- * Is that owner really running on that cpu?
- */
- if (task_thread_info(rq->curr) != owner || need_resched())
- return 0;
-
- cpu_relax();
- }
-out:
- return 1;
-}
-#endif
-
#ifdef CONFIG_PREEMPT
/*
* this is the entry point to schedule() from in-kernel preemption
--
1.6.3.3
--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
--
