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From:   Mark Rutland <mark.rutland@arm.com>
To:     linux-kernel@vger.kernel.org
Cc:     benh@kernel.crashing.org, boqun.feng@gmail.com, bp@alien8.de,
        catalin.marinas@arm.com, dvyukov@google.com, elver@google.com,
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        mark.rutland@arm.com, mattst88@gmail.com, mingo@redhat.com,
        monstr@monstr.eu, mpe@ellerman.id.au, paulmck@kernel.org,
        paulus@samba.org, peterz@infradead.org, rth@twiddle.net,
        shorne@gmail.com, stefan.kristiansson@saunalahti.fi,
        tglx@linutronix.de, vincent.guittot@linaro.org, will@kernel.org
Subject: [RFC PATCH 02/10] entry: snapshot thread flags
Date:   Wed,  9 Jun 2021 13:19:53 +0100
Message-Id: <20210609122001.18277-3-mark.rutland@arm.com>
In-Reply-To: <20210609122001.18277-1-mark.rutland@arm.com>
References: <20210609122001.18277-1-mark.rutland@arm.com>
Precedence: bulk

Some thread flags can be set remotely, and so even when IRQs are
disabled, the flags can change under our feet. Generally this is
unlikely to cause a problem in practice, but it is somewhat unsound, and
KCSAN will legitimately warn that there is a data race.

To avoid such issues, we should snapshot the flags prior to using them.
Let's use the new helpers to do so in the common entry code.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
 include/linux/entry-kvm.h | 2 +-
 kernel/entry/common.c     | 4 ++--
 kernel/entry/kvm.c        | 4 ++--
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/include/linux/entry-kvm.h b/include/linux/entry-kvm.h
index 8b2b1d68b954..bc487dffb803 100644
--- a/include/linux/entry-kvm.h
+++ b/include/linux/entry-kvm.h
@@ -70,7 +70,7 @@ static inline void xfer_to_guest_mode_prepare(void)
  */
 static inline bool __xfer_to_guest_mode_work_pending(void)
 {
-	unsigned long ti_work = READ_ONCE(current_thread_info()->flags);
+	unsigned long ti_work = read_thread_flags();
 
 	return !!(ti_work & XFER_TO_GUEST_MODE_WORK);
 }
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index a0b3b04fb596..3147a1f2ed74 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -188,7 +188,7 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
 		/* Check if any of the above work has queued a deferred wakeup */
 		rcu_nocb_flush_deferred_wakeup();
 
-		ti_work = READ_ONCE(current_thread_info()->flags);
+		ti_work = read_thread_flags();
 	}
 
 	/* Return the latest work state for arch_exit_to_user_mode() */
@@ -197,7 +197,7 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
 
 static void exit_to_user_mode_prepare(struct pt_regs *regs)
 {
-	unsigned long ti_work = READ_ONCE(current_thread_info()->flags);
+	unsigned long ti_work = read_thread_flags();
 
 	lockdep_assert_irqs_disabled();
 
diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c
index 49972ee99aff..96d476e06c77 100644
--- a/kernel/entry/kvm.c
+++ b/kernel/entry/kvm.c
@@ -26,7 +26,7 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
 		if (ret)
 			return ret;
 
-		ti_work = READ_ONCE(current_thread_info()->flags);
+		ti_work = read_thread_flags();
 	} while (ti_work & XFER_TO_GUEST_MODE_WORK || need_resched());
 	return 0;
 }
@@ -43,7 +43,7 @@ int xfer_to_guest_mode_handle_work(struct kvm_vcpu *vcpu)
 	 * disabled in the inner loop before going into guest mode. No need
 	 * to disable interrupts here.
 	 */
-	ti_work = READ_ONCE(current_thread_info()->flags);
+	ti_work = read_thread_flags();
 	if (!(ti_work & XFER_TO_GUEST_MODE_WORK))
 		return 0;
 
-- 
2.11.0