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

From: Oleg Nesterov <oleg@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Dmitry Vyukov <dvyukov@google.com>,
	John Stultz <jstultz@google.com>, Marco Elver <elver@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	kasan-dev@googlegroups.com, Edward Liaw <edliaw@google.com>,
	Carlos Llamas <cmllamas@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH v6 1/2] posix-timers: Prefer delivery of signals to the current thread
Date: Thu, 4 Apr 2024 17:23:57 +0200	[thread overview]
Message-ID: <20240404152356.GE7153@redhat.com> (raw)
In-Reply-To: <87v84x9nad.ffs@tglx>

On 04/04, Thomas Gleixner wrote:
>
> On Thu, Apr 04 2024 at 15:43, Oleg Nesterov wrote:
>
> > 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.
>
> bcb7ee79029dca only directs the wakeup to current, but the signal is
> still queued in the process wide shared pending list. So the thread
> which sees sigpending() first will grab and deliver it to itself.

This is what I tried to say above.

> What we can actually test is the avoidance of waking up the main thread
> by doing the following in the main thread:

Hmm... I think it can be even simpler,

> I'm testing a modification which implements something like the above and
> the success condition is that the main thread does not return early from
> nanosleep() and has no signal accounted. It survived 2000 iterations by
> now.

Yes, but please see a trivial test-case I sent you few minutes ago.

Oleg.