Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers

From: Andy Lutomirski
Date: Wed Mar 09 2016 - 15:57:52 EST

On Wed, Mar 9, 2016 at 11:47 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Mar 9, 2016 at 3:34 AM, Szabolcs Nagy <nsz@xxxxxxxxxx> wrote:
>>> Could someone remind me why cancellation points matter to user-space?
>> because of standards.
> So quite frankly, if we have to do kernel support for this, then let's
> do it right, instead of just perpetuating a hack that was done in user
> space in a new way.
> We already have support for cancelling blocking system calls early: we
> do it for fatal signals (exactly because we know that it's ok to
> return -EINTR without failing POSIX semantics - the dying thread will
> never actually *see* the -EINTR because it's dying).
> I suspect that what you guys want is the same semantics as a fatal
> signal (return early with -EINTR), but without the actual fatality
> (you want to do cleanup in the cancelled thread).

How safe would this be in a multithreaded process? For example, if
open() gets canceled in the "killable" sense, is it guaranteed that no
file descriptor will be allocated?

> I suspect that we could fairly easily give those kinds of semantics.
> We could add a new flag to the sigaction (sa_flags) that says "this
> signal interrupts even uninterruptible system calls".
> Would that be good for you?
> And if not, can you explain the exact semantics you need? IThere might
> be some reason why you cannot reserve a particular signal for this,
> for example, but I'd like to know more precisely..
> Because this "let's compare addresses" seems just excessively hacky.
> It's a clever little hack when you're doing user space and don't want
> to rely on kernel changes, but now that Andy is actuallty trying to
> push kernel changes it turns into just disgusting.

Let me try to summarize my understanding of the semantics.

Thread A sends thread B a signal. Thread B wants to ignore the signal
and defer handling unless it's either in a particular syscall and
returns -EINTR or unless the thread is about to do the syscall.

This would all be trivial if there were a way to set up a signal that
is *only* delivered in response to a syscall, no? SA_ONLY_IN_SYSCALL,

Frankly, I'm a bir surprised that musl didn't take the approach of
"pthread cancellation is not such a great idea -- let's just not
support it".

> Linus

Andy Lutomirski
AMA Capital Management, LLC