Hello Darren, On 12/15/2015 10:18 PM, Darren Hart wrote: > On Tue, Dec 15, 2015 at 02:43:50PM +0100, Michael Kerrisk (man-pages) wrote: [...] >> When executing a futex operation that requests to block a thread, >> the kernel will block only if the futex word has the value that >> the calling thread supplied (as one of the arguments of the >> futex() call) as the expected value of the futex word. The load‐ >> ing of the futex word's value, the comparison of that value with >> the expected value, and the actual blocking will happen atomi‐ >> >> FIXME: for next line, it would be good to have an explanation of >> "totally ordered" somewhere around here. >> >> cally and totally ordered with respect to concurrently executing > > Totally ordered with respect futex operations refers to semantics of the > ACQUIRE/RELEASE operations and how they impact ordering of memory reads and > writes. The kernel futex operations are protected by spinlocks, which ensure > that that all operations are serialized with respect to one another. > > This is a lot to attempt to define in this document. Perhaps a reference to > linux/Documentation/memory-barriers.txt as a footnote would be sufficient? Or > perhaps for this manual, "serialized" would be sufficient, with a footnote > regarding "totally ordered" and a pointer to the memory-barrier documentation? I think I'll just settle for writing serialized in the man page, and be done with it :-). >> futex operations on the same futex word. Thus, the futex word is >> used to connect the synchronization in user space with the imple‐ >> mentation of blocking by the kernel. Analogously to an atomic >> compare-and-exchange operation that potentially changes shared >> memory, blocking via a futex is an atomic compare-and-block oper‐ >> ation. > > ... > >> Futex operations >> The futex_op argument consists of two parts: a command that spec‐ >> ifies the operation to be performed, bit-wise ORed with zero or >> or more options that modify the behaviour of the operation. The >> options that may be included in futex_op are as follows: > > ... > >> >> FUTEX_CLOCK_REALTIME (since Linux 2.6.28) >> This option bit can be employed only with the >> FUTEX_WAIT_BITSET and FUTEX_WAIT_REQUEUE_PI operations. > > That caught me by surprise, but it's true. We reject FUTEX_WAIT | > FUTEX_CLOCK_REALTIME, even though FUTEX_WAIT treated as FUTEX_WAIT_BITSET with > val3=FUTEX_BITSET_MATCH_ANY. You uncover all sorts of interesting stuff when you document APIs ;-). > > Thomas, this looks like an oversight to me - do you recall if we intentionally > disallow FUTEX_CLOCK_REALTIME with FUTEX_WAIT? > >> If this option is set, the kernel treats timeout as an >> absolute time based on CLOCK_REALTIME. >> >> If this option is not set, the kernel treats timeout as >> relative time, measured against the CLOCK_MONOTONIC clock. > > ... > >> Priority-inheritance futexes > > ... > >> * If the lock is owned and there are threads contending for the >> lock, then the FUTEX_WAITERS bit shall be set in the futex >> word's value; in other words, this value is: >> >> FUTEX_WAITERS | TID >> >> >> (Note that is invalid for a PI futex word to have no owner and > > ^ it > >> FUTEX_WAITERS set.) > ... > >> FUTEX_TRYLOCK_PI (since Linux 2.6.18) >> This operation tries to acquire the futex at uaddr. It is >> invoked when a user-space atomic acquire did not succeed >> because the futex word was not 0. >> >> >> FIXME(Next sentence) The wording "The trylock in kernel" below >> needs clarification. Suggestions? >> >> The trylock in kernel might succeed because the futex word > > The lock acquisition might succeed in the kernel because the futex word Already did some rewording here which I think makes things better. >> contains stale state (FUTEX_WAITERS and/or >> FUTEX_OWNER_DIED). This can happen when the owner of the >> futex died. User space cannot handle this condition in a >> race-free manner, but the kernel can fix this up and >> acquire the futex. >> >> The uaddr2, val, timeout, and val3 arguments are ignored. > > ... > >> EXAMPLE >> >> FIXME I think it would be helpful here to say a few more words about >> the difference(s) between FUTEX_LOCK_PI and FUTEX_TRYLOCK_PI. >> Can someone propose something? > > Hrm. It seems pretty straightforward to me. I guess I'm too close to it. What > about it seems unclear and needs clarification? On reflection, I agree that the difference is perhaps well-enough explained. Thanks for the comments, Darren. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- 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
