Re:Basics of Futexes
by ww520 on Saturday July 14, 2018 @12:25PM
(#17530941)
You would release the lock first before lowering the priority.
As a separate issue, the priority inversion problem is addressed with the priority inheritance technique, which futex supports directly. Basically the priority of the thread holding the lock is boosted to the highest priority of all the threads sleeping at the lock, making sure it can run to completion. When the lock is released, the thread's priority is lowered back to its original level. All the xx_pi() APIs are the priority inheritance versions.
There are a number of ways to ensure fairness for lock acquisition; however, performance often suffers, so people just live with the problem and design the workload around it.
As a separate issue, the priority inversion problem is addressed with the priority inheritance technique, which futex supports directly. Basically the priority of the thread holding the lock is boosted to the highest priority of all the threads sleeping at the lock, making sure it can run to completion. When the lock is released, the thread's priority is lowered back to its original level. All the xx_pi() APIs are the priority inheritance versions.
There are a number of ways to ensure fairness for lock acquisition; however, performance often suffers, so people just live with the problem and design the workload around it.