On 1/17/15, 1:16 AM, "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> wrote: >Hello Darren, > >On 01/17/2015 02:33 AM, Darren Hart wrote: >> Corrected Davidlohr's email address. > >Thanks! > >> On 1/15/15, 7:12 AM, "Michael Kerrisk (man-pages)" >> <mtk.manpages@xxxxxxxxx> wrote: >> >>> Hello Darren, >>> >>> I give you the same apology as to Thomas for the >>> long-delayed response to your mail. >>> >>> And I repeat my note to Thomas: >>> In the next day or two, I hope to send out the new version >>> of the futex(2) page for review. The new draft is a bit >>> bigger (okay -- 4 x bigger) than the current page. And there >>> are a quite number of FIXMEs that I've placed in the page >>> for various points--some minor, but a few major--that need >>> to be checked or fixed. Would you have some time to review >>> that page? >> >> I'll make the time for that. I've wanted to see this for a while, so >>thank >> you for working on it! > >Great! > >>> In the meantime, I have a couple of questions, which, if >>> you could answer them, I would work some changes into the >>> page before sending. >>> >>> 1. In various places, distinction is made between non-PI >>> futexs and PI futexes. But what determines that distinction? >>> From the kernel's perspective, hat make a futex one type >>> or another? I presume it is to do with the types of blocking >>> waiters on the futex, but it would be good to have a formal >>> definition. >> >> You're right in that a uaddr is a uaddr is a uaddr. Also "there is no >>such >> thing as a futex", it doesn't exist as any kind of identifiable object, >>so >> these discussions can get rather confusing :-) > >So, I want to make sure that I am clear on what you mean you say this. >You say "there is no such thing as a futex" because from the kernel's >perspective there is no visible entity in the uncontended case >(where everything can be dealt with in user space). And from user-space, >in the uncontended case all we're doing is memory operations. Right? > >On the other hand, from a kernel perspective, we could say that a >futex "exists" in the contended phases, since the kernel has allocated >state associated with the uaddr. Right? Sorry, this was more anecdotal, and probably more of a distraction than constructive. I just meant that unlike other things which you can point to a specific struct for (task, rt_mutex, etc.), a "futex" has it's state distributed across the backing store (uaddr), the queue (futex_q), the pi_state, the rt_mutex, etc, and these span kernel space and userspace. Your description above is correct. > >> A "futex" becomes a PI futex when it is "created" via a PI futex op >>code. > >Precisely which PI op codes? Is it: FUTEX_LOCK_PI, FUTEX_TRYLOCK_PI, and >FUTEX_CMP_REQUEUE_PI, and not FUTEX_WAIT_REQUEUE_PI or FUTEX_UNLOCK_PI? Based on your wording below about taking a user POV on this, I'm going to say "yes" here. These opcodes paired with the PI futex value policy (described below) defines a "futex" as PI aware. These were created very specifically in support of PI pthread_mutexes, so it makes a lot more sense to talk about a PI aware pthread_mutex, than a PI aware futex, since there is a lot of policy and scaffolding that has to be built up around it to use it properly (this is what a PI pthread_mutex is). >> At that point, the syscall will ensure a pi_state is populated for the >> futex_q entry. See futex_lock_pi() for example. Before the locks are >> taken, there is a call to refill_pi_state_cache() which preps a pi_state >> for assignment later in futex_lock_pi_atomic(). This pi_state provides >>the >> necessary linkage to perform the priority boosting in the event of a >> priority inversion. This is handled externally from the futexes via the >> rt_mutex construct. >> >> Clear as mud? > >Not quite that bad, but... The thing is, still, the man page has text >such as the following (based on your wording): > > FUTEX_CMP_REQUEUE_PI (since Linux 2.6.31) > This operation is a PI-aware variant of FUTEX_CMP_REQUEUE. > It requeues waiters that are blocked via > FUTEX_WAIT_REQUEUE_PI on uaddr from a non-PI source futex > (uaddr) to a PI target futex (uaddr2). > >And elsewhere you said > > EINVAL is returned if the non-pi to pi or > op pairing semantics are violated. > >When someone in user-land (e.g., me) reads pieces like that, they then >want to find somewhere in the man page a description of what makes a >futex a *PI futex* and probably some statements of the distinction >between PI and non-PI futexes. And those statements should be from a >perspective that is somewhat comprehensible to user-space. I'm not >yet confident that I can do that. Do you care to take a shot at it? Hrm, tricky indeed. From userspace, what makes a "futex" PI is the policy agreement between kernel and userspace (which is the value of the futex: 0, TID, TID|WAITERS, and never just WAITERS, and the use of PI aware futex op codes when making the futex syscalls. For a longer discussion of this policy, see Documentation/pi-futex.txt. Also note that this policy can be combined with that for robust futexes, adding the OWNERDIED component. -- Darren Hart Intel Open Source Technology Center -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
