Fast sigblock (AKA rtld speedup)

[David Xu]

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.

> 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.