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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        rcu@vger.kernel.org, linux-rt-users@vger.kernel.org
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Steven Price <steven.price@arm.com>,
        Ard Biesheuvel <ardb@kernel.org>
Subject: [PATCH v2 2/4] sched: Introduce is_pcpu_safe()
Date:   Sat,  7 Aug 2021 01:58:05 +0100
Message-Id: <20210807005807.1083943-3-valentin.schneider@arm.com>
In-Reply-To: <20210807005807.1083943-1-valentin.schneider@arm.com>
References: <20210807005807.1083943-1-valentin.schneider@arm.com>
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Some areas use preempt_disable() + preempt_enable() to safely access
per-CPU data. The PREEMPT_RT folks have shown this can also be done by
keeping preemption enabled and instead disabling migration (and acquiring a
sleepable lock, if relevant).

Introduce a helper which checks whether the current task can safely access
per-CPU data, IOW if the task's context guarantees the accesses will target
a single CPU. This accounts for preemption, CPU affinity, and migrate
disable - note that the CPU affinity check also mandates the presence of
PF_NO_SETAFFINITY, as otherwise userspace could concurrently render the
upcoming per-CPU access(es) unsafe.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 include/linux/sched.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index debc960f41e3..b77d65f677f6 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1715,6 +1715,16 @@ static inline bool is_percpu_thread(void)
 #endif
 }
 
+/* Is the current task guaranteed not to be migrated elsewhere? */
+static inline bool is_pcpu_safe(void)
+{
+#ifdef CONFIG_SMP
+	return !preemptible() || is_percpu_thread() || current->migration_disabled;
+#else
+	return true;
+#endif
+}
+
 /* Per-process atomic flags. */
 #define PFA_NO_NEW_PRIVS		0	/* May not gain new privileges. */
 #define PFA_SPREAD_PAGE			1	/* Spread page cache over cpuset */
-- 
2.25.1