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futex: Document pi_state refcounting in requeue code

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Documentation of the pi_state refcounting in the requeue code is non
existent. Add it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Darren Hart <darren@dvhart.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Bhuvanesh_Surachari@mentor.com
Cc: Andy Lowe <Andy_Lowe@mentor.com>
Link: http://lkml.kernel.org/r/20151219200607.335938312@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Thomas Gleixner 2015-12-19 20:07:39 +00:00
parent 29e9ee5d48
commit ecb38b78f6
1 changed files with 39 additions and 12 deletions
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51
kernel/futex.c
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@ -1709,27 +1709,31 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
* exist yet, look it up one more time to ensure we have a
* reference to it. If the lock was taken, ret contains the
* vpid of the top waiter task.
* If the lock was not taken, we have pi_state and an initial
* refcount on it. In case of an error we have nothing.
*/
if (ret > 0) {
WARN_ON(pi_state);
drop_count++;
task_count++;
/*
* If we acquired the lock, then the user
* space value of uaddr2 should be vpid. It
* cannot be changed by the top waiter as it
* is blocked on hb2 lock if it tries to do
* so. If something fiddled with it behind our
* back the pi state lookup might unearth
* it. So we rather use the known value than
* rereading and handing potential crap to
* lookup_pi_state.
* If we acquired the lock, then the user space value
* of uaddr2 should be vpid. It cannot be changed by
* the top waiter as it is blocked on hb2 lock if it
* tries to do so. If something fiddled with it behind
* our back the pi state lookup might unearth it. So
* we rather use the known value than rereading and
* handing potential crap to lookup_pi_state.
*
* If that call succeeds then we have pi_state and an
* initial refcount on it.
*/
ret = lookup_pi_state(ret, hb2, &key2, &pi_state);
}
switch (ret) {
case 0:
/* We hold a reference on the pi state. */
break;
case -EFAULT:
put_pi_state(pi_state);
@ -1804,19 +1808,37 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
* of requeue_pi if we couldn't acquire the lock atomically.
*/
if (requeue_pi) {
/* Prepare the waiter to take the rt_mutex. */
/*
* Prepare the waiter to take the rt_mutex. Take a
* refcount on the pi_state and store the pointer in
* the futex_q object of the waiter.
*/
atomic_inc(&pi_state->refcount);
this->pi_state = pi_state;
ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
this->rt_waiter,
this->task);
if (ret == 1) {
/* We got the lock. */
/*
* We got the lock. We do neither drop the
* refcount on pi_state nor clear
* this->pi_state because the waiter needs the
* pi_state for cleaning up the user space
* value. It will drop the refcount after
* doing so.
*/
requeue_pi_wake_futex(this, &key2, hb2);
drop_count++;
continue;
} else if (ret) {
/* -EDEADLK */
/*
* rt_mutex_start_proxy_lock() detected a
* potential deadlock when we tried to queue
* that waiter. Drop the pi_state reference
* which we took above and remove the pointer
* to the state from the waiters futex_q
* object.
*/
this->pi_state = NULL;
put_pi_state(pi_state);
goto out_unlock;
@ -1827,6 +1849,11 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
}
out_unlock:
/*
* We took an extra initial reference to the pi_state either
* in futex_proxy_trylock_atomic() or in lookup_pi_state(). We
* need to drop it here again.
*/
put_pi_state(pi_state);
double_unlock_hb(hb1, hb2);
wake_up_q(&wake_q);