linux-kernel - [RFC PATCH 01/10] thread_info: add helpers to snapshot thread flags

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Date:   Wed,  9 Jun 2021 13:19:52 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     linux-kernel@...r.kernel.org
Cc:     benh@...nel.crashing.org, boqun.feng@...il.com, bp@...en8.de,
        catalin.marinas@....com, dvyukov@...gle.com, elver@...gle.com,
        ink@...assic.park.msu.ru, jonas@...thpole.se,
        juri.lelli@...hat.com, linux@...linux.org.uk, luto@...nel.org,
        mark.rutland@....com, mattst88@...il.com, mingo@...hat.com,
        monstr@...str.eu, mpe@...erman.id.au, paulmck@...nel.org,
        paulus@...ba.org, peterz@...radead.org, rth@...ddle.net,
        shorne@...il.com, stefan.kristiansson@...nalahti.fi,
        tglx@...utronix.de, vincent.guittot@...aro.org, will@...nel.org
Subject: [RFC PATCH 01/10] thread_info: add helpers to snapshot thread flags

We have common helpers to manipulate individual thread flags, but where
code wants to check several flags at once, it must open code reading
current_thread_info()->flags and operating on a snapshot.

As some flags can be set remotely it's necessary to use READ_ONCE() to
get a consistent snapshot even when IRQs are disabled, but some code
forgets to do this. Generally this is unlike to cause a problem in
practice, but it is somewhat unsound, and KCSAN will legitimately warn
that there is a data race.

To make it easier to do the right thing, and to highlight that
concurrent modification is possible, let's add a new helpers to snapshot
the flags, which should be used in preference to plain reads.
Subsequent patches will move existing code to use the new helpers.

Signed-off-by: Mark Rutland <mark.rutland@....com>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Marco Elver <elver@...gle.com>
Cc: Paul E. McKenney <paulmck@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Will Deacon <will@...nel.org>
---
 include/linux/thread_info.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index 157762db9d4b..f3769842046d 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -117,6 +117,11 @@ static inline int test_ti_thread_flag(struct thread_info *ti, int flag)
 	return test_bit(flag, (unsigned long *)&ti->flags);
 }
 
+static inline unsigned long read_ti_thread_flags(struct thread_info *ti)
+{
+	return READ_ONCE(ti->flags);
+}
+
 #define set_thread_flag(flag) \
 	set_ti_thread_flag(current_thread_info(), flag)
 #define clear_thread_flag(flag) \
@@ -129,6 +134,11 @@ static inline int test_ti_thread_flag(struct thread_info *ti, int flag)
 	test_and_clear_ti_thread_flag(current_thread_info(), flag)
 #define test_thread_flag(flag) \
 	test_ti_thread_flag(current_thread_info(), flag)
+#define read_thread_flags() \
+	read_ti_thread_flags(current_thread_info())
+
+#define read_task_thread_flags(t) \
+	read_ti_thread_flags(task_thread_info(t))
 
 #ifdef CONFIG_GENERIC_ENTRY
 #define set_syscall_work(fl) \
-- 
2.11.0