On 06/24/2013 03:13 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.
*/
In mutex_spin_on_owner, after rcu_read_lock, the owner_running()
function de-references the owner pointer.
Only after establishing the following preconditions:
1. snapshot of owner is non-NULL
2. mutex->owner == snapshot owner
3. memory holding mutex has not been freed (that's what the
rcu_read_lock() is for)
Only then is the owner dereferenced and only through the snapshot
(not the now-possibly-rewritten sem->owner).
I'm using similar logic in rw-sem.
With crucial details absent.
Regards,
Peter Hurley
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