Fast sigblock (AKA rtld speedup)

[Konstantin Belousov]

On Sat, Jan 12, 2013 at 08:27:06AM +0800, David Xu wrote:
> On 2013/01/12 07:29, Jilles Tjoelker wrote:
> > On Fri, Jan 11, 2013 at 10:49:38PM +0200, Konstantin Belousov wrote:
> >> http://people.freebsd.org/~kib/misc/rtld-sigblock.3.patch
> > The new fields td_sigblock_ptr and td_sigblock_val are in the part that
> > is zeroed for new threads, while the code in rtld appears to expect them
> > to be copied (on fork, vfork and pthread_create). The fields are
> > correctly zeroed on exec.
> >
> > Sharing the magic variable between threads means that one thread holding
> > an rtld lock will block signals for all other threads as well. This is
> > different from how the normal signal mask works but I don't know whether
> > it may break things. However, the patch depends on it in some way
> > because sigtd() does not know about fast sigblock and will therefore
> > happily select a thread that is fast-sigblocking to handle a signal
> > directed at the process.
> >
> > Although libthr's postpone approach is somewhat ugly, it does not depend
> > on any non-standard kernel features and does not delay the default
> > action. Would it be possible to move that code to libc to make things
> > easier for rtld? It looks like this requires teaching libc about various
> > threading concepts, though.
> Long time ago, if i remembered correctly, kib said that he wanted to
> merge libthr code into libc, I don't know its state.
Yes, this is correct, and I still want this.

> 
> > Something feels ugly about not allowing applications to use this,
> > although I don't know how it would work. On the other hand, the strange
> > signal state created by this and implicit assumptions that the duration
> > will be short may be a reason to restrict its use to a relatively small
> > piece of code.
> >
> True, it seems it is for short duration.
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