time/sched_clock: Broaden sched_clock()'s instrumentation coverage

Most of sched_clock()'s implementation is ineligible for instrumentation
due to relying on sched_clock_noinstr().

Split the implementation off into an __always_inline function
__sched_clock(), which is then used by the noinstr and instrumentable
version, to allow more of sched_clock() to be covered by various
instrumentation.

This will allow instrumentation with the various sanitizers (KASAN,
KCSAN, KMSAN, UBSAN). For KCSAN, we know that raw seqcount_latch usage
without annotations will result in false positive reports: tell it that
all of __sched_clock() is "atomic" for the latch reader; later changes
in this series will take care of the writers.

Co-developed-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Signed-off-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241104161910.780003-3-elver@google.com

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 85595fcf6aa2203b021fb12b4344048939cfa44c..29bdf309dae853f69f107ff3bb6052d1726a60f6 100644 (file)
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -80,7 +80,7 @@ notrace int sched_clock_read_retry(unsigned int seq)
        return raw_read_seqcount_latch_retry(&cd.seq, seq);
 }
 
-unsigned long long noinstr sched_clock_noinstr(void)
+static __always_inline unsigned long long __sched_clock(void)
 {
        struct clock_read_data *rd;
        unsigned int seq;
@@ -98,11 +98,23 @@ unsigned long long noinstr sched_clock_noinstr(void)
        return res;
 }
 
+unsigned long long noinstr sched_clock_noinstr(void)
+{
+       return __sched_clock();
+}
+
 unsigned long long notrace sched_clock(void)
 {
        unsigned long long ns;
        preempt_disable_notrace();
-       ns = sched_clock_noinstr();
+       /*
+        * All of __sched_clock() is a seqcount_latch reader critical section,
+        * but relies on the raw helpers which are uninstrumented. For KCSAN,
+        * mark all accesses in __sched_clock() as atomic.
+        */
+       kcsan_nestable_atomic_begin();
+       ns = __sched_clock();
+       kcsan_nestable_atomic_end();
        preempt_enable_notrace();
        return ns;
 }