Messages in this thread | | | Date | Thu, 6 Apr 2017 14:28:32 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH -v6 05/13] futex: Change locking rules |
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On Wed, Apr 05, 2017 at 02:18:43PM -0700, Darren Hart wrote: > On Wed, Mar 22, 2017 at 11:35:52AM +0100, Peter Zijlstra wrote:
> > + * > > + * Serialization and lifetime rules: > > + * > > + * hb->lock: > > + * > > + * hb -> futex_q, relation > > + * futex_q -> pi_state, relation > > + * > > + * (cannot be raw because hb can contain arbitrary amount > > + * of futex_q's) > > + * > > + * pi_mutex->wait_lock: > > + * > > + * {uval, pi_state} > > + * > > + * (and pi_mutex 'obviously') > > + * > > + * p->pi_lock: > > This documentation uses a mix of types and common variable names. I'd recommend > some declarations just below "Serialization and lifetime rules:" to help make > this explicit, e.g.: > > struct futex_pi_state *pi_state; > struct futex_hash_bucket *hb; > struct rt_mutex *pi_mutex; > struct futex_q *q; > task_struct *p;
Yeah, not convinced it helps much. If you're stuck at that level, the rest of futex is going to make your head explode.
> > @@ -980,10 +1013,12 @@ void exit_pi_state_list(struct task_stru > > * the pi_state against the user space value. If correct, attach to > > * it. > > */ > > +static int attach_to_pi_state(u32 __user *uaddr, u32 uval, > > + struct futex_pi_state *pi_state, > > struct futex_pi_state **ps) > > { > > pid_t pid = uval & FUTEX_TID_MASK; > > + int ret, uval2; > > The uval should be an unsigned type: > > u32 uval2;
Right you are.
> > > > /* > > * Userspace might have messed up non-PI and PI futexes [3] > > @@ -991,9 +1026,34 @@ static int attach_to_pi_state(u32 uval, > > if (unlikely(!pi_state)) > > return -EINVAL; > > > > + /* > > + * We get here with hb->lock held, and having found a > > + * futex_top_waiter(). This means that futex_lock_pi() of said futex_q > > + * has dropped the hb->lock in between queue_me() and unqueue_me_pi(), > > This context got here like this: > > futex_lock_pi > hb lock > futex_lock_pi_atomic > top waiter > attach_to_pi_state() > > The queue_me and unqueue_me_pi both come after this in futex_lock_pi. > Also, the hb lock is dropped in queue_me, not between queue_me and > unqueue_me_pi. > > Are you saying that in order to be here, there are at least two tasks contending > for the lock, and one that has come before us has proceeded as far as queue_me() > but has not yet entered unqueue_me_pi(), therefor we know there is a waiter and > it has a pi_state? If so, I think we can make this much clearer by at least > noting the two tasks in play.
The point is that this other task must have a reference, and since we now hold hb->lock, it cannot go away.
> > ... > > > @@ -1336,6 +1418,7 @@ static int wake_futex_pi(u32 __user *uad > > > > if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)) { > > ret = -EFAULT; > > + > > Stray whitespace addition? Not explicitly against coding-style, but I don't > normally see a new line before the closing brace leading to an else...
I found it more readable that way. Sod checkpatch and co ;-)
> > } else if (curval != uval) { > > /* > > * If a unconditional UNLOCK_PI operation (user space did not
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