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SubjectRe: futex(3) man page, final draft for pre-release review
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On Tue, 2015-12-15 at 14:43 +0100, Michael Kerrisk (man-pages) wrote:
> Hello all,
>
> After much too long a time, the revised futex man page *will*
> go out in the next man pages release (it has been merged
> into master).
>
> There are various places where the page could still be improved,
> but it is much better (and more than 5 times longer) than the
> existing page.

This looks good to me; I just saw minor things (see below). Thank you
for all the work you put into this (and to everybody who contributed)!

> 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
> 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.

Maybe -- should we just say that it refers to the mathematical notion of
a total order (or, technically, a strict total order in this case)?
Though I would hope that everyone using futexes is roughly aware of the
differences between partial and total orders.

> FUTEX_TRYLOCK_PI (since Linux 2.6.18)
> This operation tries to acquire the futex at uaddr. It is

s/futex/lock/ to make it consistent with FUTEX_LOCK.

> 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
> 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.

What about "The acquisition of the lock might suceed if performed by the
kernel in cases when the futex word contains stale state...".

> FUTEX_WAIT_REQUEUE_PI (since Linux 2.6.31)
> Wait on a non-PI futex at uaddr and potentially be
> requeued (via a FUTEX_CMP_REQUEUE_PI operation in another
> task) onto a PI futex at uaddr2. The wait operation on
> uaddr is the same as for FUTEX_WAIT.
>
> The waiter can be removed from the wait on uaddr without
> requeueing on uaddr2 via a FUTEX_WAKE operation in another
> task. In this case, the FUTEX_WAIT_REQUEUE_PI operation
> returns with the error EWOULDBLOCK.

This should be EAGAIN, I suppose, or the enumeration of errors should
include EWOULDBLOCK.

Torvald



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