Messages in this thread | | | Date | Sun, 8 Jun 2014 15:07:18 +0200 | From | Oleg Nesterov <> | Subject | safety of *mutex_unlock() (Was: [BUG] signal: sighand unprotected when accessed by /proc) |
| |
On 06/06, Paul E. McKenney wrote: > > On Tue, Jun 03, 2014 at 10:01:25PM +0200, Oleg Nesterov wrote: > > > > I'll try to recheck rt_mutex_unlock() tomorrow. _Perhaps_ rcu_read_unlock() > > should be shifted from lock_task_sighand() to unlock_task_sighand() to > > ensure that rt_mutex_unlock() does nothihg with this memory after it > > makes another lock/unlock possible. > > > > But if we need this (currently I do not think so), this doesn't depend on > > SLAB_DESTROY_BY_RCU. And, at first glance, in this case rcu_read_unlock_special() > > might be wrong too. > > OK, I will bite... What did I mess up in rcu_read_unlock_special()? > > This function does not report leaving the RCU read-side critical section > until after its call to rt_mutex_unlock() has returned, so any RCU > read-side critical sections in rt_mutex_unlock() will be respected.
Sorry for confusion.
I only meant that afaics rcu_read_unlock_special() equally depends on the fact that rt_mutex_unlock() does nothing with "struct rt_mutex" after it makes another rt_mutex_lock() + rt_mutex_unlock() possible, otherwise this code is wrong (and unlock_task_sighand() would be wrong too).
Just to simplify the discussion... suppose we add "atomic_t nr_slow_unlock" into "struct rt_mutex" and change rt_mutex_slowunlock() to do atomic_inc(&lock->nr_slow_unlock) after it drops ->wait_lock. Of course this would be ugly, just for illustration.
In this case atomic_inc() above can write to rcu_boost()'s stack after this functions returns to the caller. And unlock_task_sighand() would be wrong too, atomic_inc() could write to the memory which was already returned to system because "unlock" path runs outside of rcu-protected section.
But it seems to me that currently we are safe, rt_mutex_unlock() doesn't do something like this, a concurrent rt_mutex_lock() must always take wait_lock too.
And while this is off-topic and I can be easily wrong, it seems that the normal "struct mutex" is not safe in this respect. If nothing else, once __mutex_unlock_common_slowpath()->__mutex_slowpath_needs_to_unlock() sets lock->count = 1, a concurent mutex_lock() can take and then release this mutex before __mutex_unlock_common_slowpath() takes ->wait_lock.
So _perhaps_ we should not rely on this property of rt_mutex "too much".
Oleg.
| |