linux-kernel - Re: [PATCH v6 1/2] posix-timers: Prefer delivery of signals to the current thread

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date: Thu, 4 Apr 2024 15:43:57 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, John Stultz <jstultz@...gle.com>,
	Marco Elver <elver@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
	kasan-dev@...glegroups.com, Edward Liaw <edliaw@...gle.com>,
	Carlos Llamas <cmllamas@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v6 1/2] posix-timers: Prefer delivery of signals to the
 current thread

Perhaps I am totally confused, but.

On 04/04, Dmitry Vyukov wrote:
>
> On Wed, 3 Apr 2024 at 17:43, Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > > Why distribution_thread() can't simply exit if got_signal != 0 ?
> > >
> > > See https://lore.kernel.org/all/20230128195641.GA14906@redhat.com/
> >
> > Indeed. It's too obvious :)
>
> This test models the intended use-case that was the motivation for the change:
> We want to sample execution of a running multi-threaded program, it
> has multiple active threads (that don't exit), since all threads are
> running and consuming CPU,

Yes,

> they all should get a signal eventually.

Well, yes and no.

No, in a sense that the motivation was not to ensure that all threads
get a signal, the motivation was to ensure that cpu_timer_fire() paths
will use the current task as the default target for signal_wake_up/etc.
This is just optimization.

But yes, all should get a signal eventually. And this will happen with
or without the commit bcb7ee79029dca ("posix-timers: Prefer delivery of
signals to the current thread"). Any thread can dequeue a shared signal,
say, on return from interrupt.

Just without that commit this "eventually" means A_LOT_OF_TIME statistically.

> If threads will exit once they get a signal,

just in case, the main thread should not exit ...

> then the test will pass
> even if signal delivery is biased towards a single running thread all
> the time (the previous kernel impl).

See above.

But yes, I agree, if thread exits once it get a signal, then A_LOT_OF_TIME
will be significantly decreased. But again, this is just statistical issue,
I do not see how can we test the commit bcb7ee79029dca reliably.

OTOH. If the threads do not exit after they get signal, then _in theory_
nothing can guarantee that this test-case will ever complete even with
that commit. It is possible that one of the threads will "never" have a
chance to run cpu_timer_fire().

In short, I leave this to you and Thomas. I have no idea how to write a
"good" test for that commit.

Well... perhaps the main thread should just sleep in pause(), and
distribution_handler() should check that gettid() != getpid() ?
Something like this maybe... We need to ensure that the main thread
enters pause before timer_settime().

Oleg.