Re: [PATCH 1/1] rtmutex: Handle when top lock owner changes

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 4 Jun 2014 17:32:37 +0200 (CEST)
Thomas Gleixner <tglx@linutronix.de> wrote:

> T3 releases L3
> T2 gets L3
> T2 drops L3 and L2
> T2 blocks on L4 held by T4
> T4 blocked on L5 held by T5
> 
> So we happily boost T4 and T5. Not what we really want to do.
> 
> Nasty, isn't it ?
> 

Actually, we may go up a chain, but we never do any unnecessary
boosting. That's because the boost is done with rt_mutex_adjust_prio()
which gets the prio from rt_mutex_getprio() which reads the
task->normal_prio and compares it to the task_top_pi_waiter(task)->prio,
which will always be correct as we have the necessary locks.

And we don't even need to worry about the chain we miss. That is, if
task A is blocked on a lock owned by D at the time, but as we go up the
chain, D releases the lock and B grabs it, B will still up its priority
based on the waiters of the lock (that is A), and if B blocks, it will
boost the tasks that own the lock it blocks on, where B is still
influenced by A.

The fact that we only update the prio based on the actual waiters and
don't carry a prio up the chain (which you designed, and I thought was
quite ingenious by the way), we may waste time going up a chain, but
the priority inheritance is still accurate.

-- Steve