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 <paulmck@xxxxxxxxxx> Cc: Marco Elver <elver@xxxxxxxxxx> --- Is this really the right way to do this? diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index d5319bbe8c982..e83adcdb49b5f 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -1460,6 +1460,7 @@ 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) @@ -1471,7 +1472,13 @@ 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)); + 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);
