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SubjectRe: [PATCH/RFC] mutex: Fix optimistic spinning vs. BKL
On Thu, Apr 29, 2010 at 08:35:21AM +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2010-04-28 at 14:06 +0200, Arnd Bergmann wrote:
> >
> > This needs to use time_before() to avoid problems on jiffies
> > wraparound.
>
> Ah right, forgot about that, been a while I didn't use jiffies for
> anything :-)
>
> I'll respin later today.

Like thie perhaps? If this looks good it would be great to get this in .34

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 70abfd3..991d86f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -351,7 +351,8 @@ extern signed long schedule_timeout_killable(signed long timeout);
extern signed long schedule_timeout_uninterruptible(signed long timeout);
asmlinkage void __schedule(void);
asmlinkage void schedule(void);
-extern int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner);
+extern int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner,
+ unsigned long timeout);

struct nsproxy;
struct user_namespace;
diff --git a/kernel/mutex.c b/kernel/mutex.c
index 947b3ad..fcf2573 100644
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -145,6 +145,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
struct task_struct *task = current;
struct mutex_waiter waiter;
unsigned long flags;
+ unsigned long timeout;

preempt_disable();
mutex_acquire(&lock->dep_map, subclass, 0, ip);
@@ -168,15 +169,22 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
* to serialize everything.
*/

- for (;;) {
+ for (timeout = jiffies + 2; time_before(jiffies, timeout);) {
struct thread_info *owner;

/*
+ * If we own the BKL, then don't spin. The owner of the mutex
+ * might be waiting on us to release the BKL.
+ */
+ if (current->lock_depth >= 0)
+ break;
+
+ /*
* If there's an owner, wait for it to either
* release the lock or go to sleep.
*/
owner = ACCESS_ONCE(lock->owner);
- if (owner && !mutex_spin_on_owner(lock, owner))
+ if (owner && !mutex_spin_on_owner(lock, owner, timeout))
break;

if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
diff --git a/kernel/sched.c b/kernel/sched.c
index 663a1d0..ef58dc0 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -5523,7 +5523,8 @@ EXPORT_SYMBOL(schedule);
* Look out! "owner" is an entirely speculative pointer
* access and not reliable.
*/
-int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)
+int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner,
+ unsigned long timeout)
{
unsigned int cpu;
struct rq *rq;
@@ -5559,7 +5560,7 @@ int mutex_spin_on_owner(struct mutex *lock, struct thread_info *owner)

rq = cpu_rq(cpu);

- for (;;) {
+ while (time_before(jiffies, timeout)) {
/*
* Owner changed, break to re-assess state.
*/
Yours Tony


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