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rcu: Inform KCSAN of one-byte cmpxchg() in rcu_trc_cmpxchg_need_qs()

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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@kernel.org>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
This commit is contained in:
Paul E. McKenney 2024-03-08 13:26:26 -08:00 committed by Uladzislau Rezki (Sony)
parent ae2b217ab5
commit fc2897d2ab
1 changed files with 9 additions and 1 deletions
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10
kernel/rcu/tasks.h
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@ -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);