linux-kernel - Is perf_sigtrap synchronous?

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Message-ID: <m1im43qrug.fsf_-_@fess.ebiederm.org>
Date:   Fri, 30 Apr 2021 18:23:51 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Marco Elver <elver@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Florian Weimer <fweimer@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Collingbourne <pcc@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        sparclinux <sparclinux@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Is perf_sigtrap synchronous?


I am looking at perf_sigtrap and I am confused by the code.


	/*
	 * We'd expect this to only occur if the irq_work is delayed and either
	 * ctx->task or current has changed in the meantime. This can be the
	 * case on architectures that do not implement arch_irq_work_raise().
	 */
	if (WARN_ON_ONCE(event->ctx->task != current))
		return;

	/*
	 * perf_pending_event() can race with the task exiting.
	 */
	if (current->flags & PF_EXITING)
		return;


It performs tests that absolutely can never fail if we are talking about
a synchronous exception.  The code force_sig family of functions only
make sense to use with and are only safe to use with synchronous
exceptions.

Are the tests in perf_sigtrap necessary or is perf_sigtrap not reporting
a synchronous event?

Eric