Re: [PATCH 2/2] rwsem: do optimistic spinning for writer lock acquisition
From: Peter Hurley
Date: Mon Jun 24 2013 - 16:48:59 EST
On 06/24/2013 04:17 PM, Tim Chen wrote:
On Mon, 2013-06-24 at 14:49 -0400, Peter Hurley wrote:
On 06/24/2013 01:11 PM, Tim Chen wrote:
On Sun, 2013-06-23 at 13:03 -0700, Davidlohr Bueso wrote:
On Sat, 2013-06-22 at 03:57 -0400, Peter Hurley wrote:
On 06/21/2013 07:51 PM, Tim Chen wrote:
+static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
+{
+ int retval = true;
+
+ /* Spin only if active writer running */
+ if (!sem->owner)
+ return false;
+
+ rcu_read_lock();
+ if (sem->owner)
+ retval = sem->owner->on_cpu;
^^^^^^^^^^^^^^^^^^
Why is this a safe dereference? Could not another cpu have just
dropped the sem (and thus set sem->owner to NULL and oops)?
The rcu read lock should protect against sem->owner being NULL.
It doesn't.
Here's the comment from mutex_spin_on_owner():
/*
* Look out! "owner" is an entirely speculative pointer
* access and not reliable.
*/
On second thought, I agree with you. I should change this to
something like
int retval = true;
task_struct *sem_owner;
/* Spin only if active writer running */
if (!sem->owner)
return false;
rcu_read_lock();
sem_owner = sem->owner;
if (sem_owner)
retval = sem_owner->on_cpu;
Our emails passed each other.
Also, I haven't given a lot of thought to if preemption must be disabled
before calling rwsem_can_spin_on_owner(). If so, wouldn't you just drop
rwsem_can_spin_on_owner() (because the conditions tested in the loop are
equivalent)?
Regards,
Peter Hurley
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