From fc2897d2abe5a307e3b28943bcb4c1401432ea26 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 8 Mar 2024 13:26:26 -0800 Subject: [PATCH] rcu: Inform KCSAN of one-byte cmpxchg() in rcu_trc_cmpxchg_need_qs() Tasks Trace RCU needs a single-byte cmpxchg(), but no such thing exists. Therefore, rcu_trc_cmpxchg_need_qs() emulates one using field substitution and a four-byte cmpxchg(), such that the other three bytes are always atomically updated to their old values. This works, but results in false-positive KCSAN failures because as far as KCSAN knows, this cmpxchg() operation is updating all four bytes. This commit therefore encloses the cmpxchg() in a data_race() and adds a single-byte instrument_atomic_read_write(), thus telling KCSAN exactly what is going on so as to avoid the false positives. Signed-off-by: Paul E. McKenney Cc: Marco Elver Signed-off-by: Uladzislau Rezki (Sony) --- kernel/rcu/tasks.h | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index 147b5945d67a04..327fbfc999c87b 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -1457,6 +1457,7 @@ static void rcu_st_need_qs(struct task_struct *t, u8 v) /* * Do a cmpxchg() on ->trc_reader_special.b.need_qs, allowing for * the four-byte operand-size restriction of some platforms. + * * Returns the old value, which is often ignored. */ u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new) @@ -1468,7 +1469,14 @@ u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new) if (trs_old.b.need_qs != old) return trs_old.b.need_qs; trs_new.b.need_qs = new; - ret.s = cmpxchg(&t->trc_reader_special.s, trs_old.s, trs_new.s); + + // Although cmpxchg() appears to KCSAN to update all four bytes, + // only the .b.need_qs byte actually changes. + instrument_atomic_read_write(&t->trc_reader_special.b.need_qs, + sizeof(t->trc_reader_special.b.need_qs)); + // Avoid false-positive KCSAN failures. + ret.s = data_race(cmpxchg(&t->trc_reader_special.s, trs_old.s, trs_new.s)); + return ret.b.need_qs; } EXPORT_SYMBOL_GPL(rcu_trc_cmpxchg_need_qs);