Re: first little problem with private futexes

[Eric Dumazet <dada1@cosmosbay.com>]

Ulrich Drepper a écrit :
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> Here's a first little issue with private futex I came across.  But a
> real bug but a hole.
> 
> When we use clone() with CLONE_CHILD_CLEARTID possible waiters are woken
> upon termination of the thread.  This operation uses FUTEX_WAKE so far.
>  But it in almost all cases local memory and I would even be in favor of
> setting this into stone.   It wouldn't break anything I know of.
> 
> The problem is we cannot just go over to using
> FUTEX_WAIT|FUTEX_PRIVATE_FLAG since this would break binaries using any
> glibc out there so far.
> 
> There are three ways out of this I can see:
> 
> 1.  do nothing, always use the shared futexes.  Not very attractive IMO

Why do you find this non attractive ?

How is it performance critical ?

If a program is stupid enough to create/destroy many threads per second, I 
doubt it relies on a faster thread termination :)

> 
> 2.  try private futexes first, then shared one.  This is even less
>     attractive since in the many cases there is no waiter and we cannot
>     determine whether the private futex notification succeeded and we're
>     doing the expensive work as well
> 
> 3.  tell the kernel whether we want the new or the old notification.
>     This can be done using a number of ways
> 
>     a) using some prctl().  Another unconditional syscall, not nice.
> 
>     b) using a new CLONE_* flag.  We have currently 5 bits left and can
>        recover two more (CLONE_DETACHED, CLONE_STOPPED).  And we can
>        invent ways to add more bits.
> 
> 
> I'm in favor of 3b but if somebody argues the costs are not justified
> because the effects of using the shared futex notification isn't high
> enough I can accept that, too.