|We are calling them lightweight for 3 reasons:
| - in the user-space fastpath a PI-enabled futex involves no kernel work
| (or any other PI complexity) at all. No registration, no extra kernel
| calls - just pure fast atomic ops in userspace.
| - even in the slowpath, the system call and scheduling pattern is very
| similar to normal futexes.
| - the in-kernel PI implementation is streamlined around the mutex
| abstraction, with strict rules that keep the implementation
| relatively simple: only a single owner may own a lock (i.e. no
| read-write lock support), only the owner may unlock a lock, no
| recursive locking, etc.
|Priority Inheritance - why?
|The short reply: user-space PI helps achieving/improving determinism for
|user-space applications. In the best-case, it can help achieve
|determinism and well-bound latencies. Even in the worst-case, PI will
|improve the statistical distribution of locking related application
|The longer reply
|Firstly, sharing locks between multiple tasks is a common programming
|technique that often cannot be replaced with lockless algorithms. As we
|can see it in the kernel [which is a quite complex program in itself],
|lockless structures are rather the exception than the norm - the current
|ratio of lockless vs. locky code for shared data structures is somewhere
|between 1:10 and 1:100. Lockless is hard, and the complexity of lockless
|algorithms often endangers to ability to do robust reviews of said code.
|I.e. critical RT apps often choose lock structures to protect critical
|data structures, instead of lockless algorithms. Furthermore, there are
|cases (like shared hardware, or other resource limits) where lockless
|access is mathematically impossible.
|Media players (such as Jack) are an example of reasonable application
|design with multiple tasks (with multiple priority levels) sharing
|short-held locks: for example, a highprio audio playback thread is
|combined with medium-prio construct-audio-data threads and low-prio
|display-colory-stuff threads. Add video and decoding to the mix and
|we've got even more priority levels.
|So once we accept that synchronization objects (locks) are an
|unavoidable fact of life, and once we accept that multi-task userspace
|apps have a very fair expectation of being able to use locks, we've got
|to think about how to offer the option of a deterministic locking
|implementation to user-space.
|Most of the technical counter-arguments against doing priority
|inheritance only apply to kernel-space locks. But user-space locks are
|different, there we cannot disable interrupts or make the task
|non-preemptible in a critical section, so the 'use spinlocks' argument
|does not apply (user-space spinlocks have the same priority inversion
|problems as other user-space locking constructs). Fact is, pretty much
|the only technique that currently enables good determinism for userspace
|locks (such as futex-based pthread mutexes) is priority inheritance:
|Currently (without PI), if a high-prio and a low-prio task shares a lock
|[this is a quite common scenario for most non-trivial RT applications],
|even if all critical sections are coded carefully to be deterministic
|(i.e. all critical sections are short in duration and only execute a
|limited number of instructions), the kernel cannot guarantee any
|deterministic execution of the high-prio task: any medium-priority task
|could preempt the low-prio task while it holds the shared lock and
|executes the critical section, and could delay it indefinitely.
|As mentioned before, the userspace fastpath of PI-enabled pthread
|mutexes involves no kernel work at all - they behave quite similarly to
|normal futex-based locks: a 0 value means unlocked, and a value==TID
|means locked. (This is the same method as used by list-based robust
|futexes.) Userspace uses atomic ops to lock/unlock these mutexes without
|entering the kernel.
|To handle the slowpath, we have added two new futex ops:
| - FUTEX_LOCK_PI
| - FUTEX_UNLOCK_PI
|If the lock-acquire fastpath fails, [i.e. an atomic transition from 0 to
|TID fails], then FUTEX_LOCK_PI is called. The kernel does all the
|remaining work: if there is no futex-queue attached to the futex address
|yet then the code looks up the task that owns the futex [it has put its
|own TID into the futex value], and attaches a 'PI state' structure to
|the futex-queue. The pi_state includes an rt-mutex, which is a PI-aware,
|kernel-based synchronization object. The 'other' task is made the owner
|of the rt-mutex, and the FUTEX_WAITERS bit is atomically set in the
|futex value. Then this task tries to lock the rt-mutex, on which it
|blocks. Once it returns, it has the mutex acquired, and it sets the
|futex value to its own TID and returns. Userspace has no other work to
|perform - it now owns the lock, and futex value contains
|If the unlock side fastpath succeeds, [i.e. userspace manages to do a
|TID -> 0 atomic transition of the futex value], then no kernel work is
|If the unlock fastpath fails (because the FUTEX_WAITERS bit is set),
|then FUTEX_UNLOCK_PI is called, and the kernel unlocks the futex on the
|behalf of userspace - and it also unlocks the attached
|pi_state->rt_mutex and thus wakes up any potential waiters.
|Note that under this approach, contrary to previous PI-futex approaches,
|there is no prior 'registration' of a PI-futex. [which is not quite
|possible anyway, due to existing ABI properties of pthread mutexes.]
|Also, under this scheme, 'robustness' and 'PI' are two orthogonal
|properties of futexes, and all four combinations are possible: futex,
|robust-futex, PI-futex, robust+PI-futex.
|More details about priority inheritance can be found in