Ulrich Drepper wrote:
On 5/2/07, Davi Arnaut <[email protected]> wrote:
It's quite easy to implement this scheme by write()ing the futexes all
at once but that would break the one futex per fd association. For
atomicity: if one of the futexes can't be queued, we would rollback
(unqueue) the others.
Sounds sane?
I don't know how you use "unqueue" in this context. If a queued futex
is one which is /locked/ by te call, then yes, this is the semantics
needed. Atomically locking a number of futexes means that if one of
the set cannot be locked all operations done to lock the others have
to be undone. It's an all-or-nothing situation.
The waits are queued, thus then can be "unqueued". It's quite simple to
extend futex_wait_queue() to support this, but again you are thinking of
locks while what I want is fast events.
Locking is not as easy as you might think, though. For non-PI futexes
there is deliberately no protocol in place describing what "locked"
means. The locking operation has to be customizable. This is what
the FUTEX_OP_* stuff is about.
Events are simple. A event is either signaled or not. A futex value 0 means
not signaled, 1+ signaled.
And you wrote that currently each futex needs its own file descriptor.
So this would have to be changed, too.
If it's really worth, I have no problem with it.
--
Davi Arnaut
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