Messages in this thread | | | Date | Wed, 21 Feb 2018 18:11:47 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [QUESTION] srcu: Remove the SCAN2 state |
| |
On Thu, Feb 22, 2018 at 08:57:27AM +0900, Byungchul Park wrote: > Hello, > > I'm sorry for bothering you, and I seem to be obviously missing > something, but I'm really wondering why we check try_check_zero() > again in the state, SCAN1, for the previous srcu_idx. > > I mean, since we've already checked try_check_zero() in the previous > grace period and gotten 'true' as a return value, all readers who see > the flipped idx via srcu_flip() won't update the src_{lock,unlock}_count > for the previous idx until it gets flipped back again. > > Is there any reasons we check try_check_zero() again in the state, SCAN1? > Is there any problems if the following patch's applied?
Indeed there are! Removing the second scan exposes us to a nasty race condition where a reader is preempted (or interrupted or whatever) just after fetching its counter. A detailed explanation for an essentially equivalent race in userspace RCU may be found on the second column of page 7 of this PDF:
http://www.computer.org/cms/Computer.org/dl/trans/td/2012/02/extras/ttd2012020375s.pdf
But please let me know if I am missing the point of your patch below.
Thanx, Paul
> Thanks in advance, > Byungchul > > --- > kernel/rcu/srcutree.c | 16 +--------------- > 1 file changed, 1 insertion(+), 15 deletions(-) > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > index 39e50fe..215c44a 100644 > --- a/kernel/rcu/srcutree.c > +++ b/kernel/rcu/srcutree.c > @@ -1125,24 +1125,10 @@ static void srcu_advance_state(struct srcu_struct *sp) > mutex_unlock(&sp->srcu_gp_mutex); > return; /* Someone else started the grace period. */ > } > - } > - > - if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN1) { > - idx = 1 ^ (sp->srcu_idx & 1); > - if (!try_check_zero(sp, idx, 1)) { > - mutex_unlock(&sp->srcu_gp_mutex); > - return; /* readers present, retry later. */ > - } > srcu_flip(sp); > - rcu_seq_set_state(&sp->srcu_gp_seq, SRCU_STATE_SCAN2); > } > > - if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN2) { > - > - /* > - * SRCU read-side critical sections are normally short, > - * so check at least twice in quick succession after a flip. > - */ > + if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN1) { > idx = 1 ^ (sp->srcu_idx & 1); > if (!try_check_zero(sp, idx, 2)) { > mutex_unlock(&sp->srcu_gp_mutex); > -- > 1.9.1 >
| |