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

From: Andy Lutomirski <luto@amacapital.net>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>, Andy Lutomirski <luto@kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Borislav Petkov <bp@alien8.de>,
	"musl@lists.openwall.com" <musl@lists.openwall.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers
Date: Wed, 9 Mar 2016 12:57:20 -0800	[thread overview]
Message-ID: <CALCETrXXx36buZyOhnYu-N3boRrCdK0a8p8yPHD+te1k3zYY=Q@mail.gmail.com> (raw)
In-Reply-To: <CA+55aFzOcxbhXCm01+NMgY9=THYgjojvDGeYsnxe-vWfiX4X0g@mail.gmail.com>

On Wed, Mar 9, 2016 at 11:47 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Wed, Mar 9, 2016 at 3:34 AM, Szabolcs Nagy <nsz@port70.net> 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,
perhaps?

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