Re: [PATCH] futex: avoid race between requeue and wake

[Linus Torvalds]

On Tue, Apr 8, 2014 at 2:02 PM, Jan Stancek <jstancek@xxxxxxxxxx> wrote:
>
> I ran reproducer with following change on s390x system, where this
> can be reproduced usually within seconds:
>
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 67dacaf..9150ffd 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1095,6 +1095,7 @@ static int unlock_futex_pi(u32 __user *uaddr, u32 uval)
>  static inline void
>  double_lock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
>  {
> +       hb_waiters_inc(hb2);
>         if (hb1 <= hb2) {
>                 spin_lock(&hb1->lock);
>                 if (hb1 < hb2)
> @@ -1111,6 +1112,7 @@ double_unlock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
>         spin_unlock(&hb1->lock);
>         if (hb1 != hb2)
>                 spin_unlock(&hb2->lock);
> +       hb_waiters_dec(hb2);
>  }
>
>  /*
>
> Reproducer is running without failures over an hour now and
> made ~1.4 million iterations.

Ok, that's encouraging. That is the smallest patch I could come up
with, but as mentioned, it's not optimal. We only need it for
futex_requeue(), but if we do it there we'd have to handle all the
different error cases (there's only one call to double_lock_hb(), but
due to the error cases there's four calls to double_unlock_hb().

I'm not sure how much we care. The simple patch basically adds two
(unnecessary) atomics to the futex_wake_op() path. I don't know how
critical that path is - not as critical as the regular "futex_wake()",
I'd expect, but I guess pthread_cond_signal() is the main user.

So I'll have to leave this decision to the futex people. But the
attached slightly more complex patch *may* be the better one.

May I bother you to test this one too? I really think that
futex_requeue() is the only user that should need this, so doing it
there rather than in double_[un]lock_hb() should be slightly more
optimal, but who knows what I've missed. We clearly *all* missed this
race back when the ordering rules were documented..

Still hoping for comments from PeterZ and Davidlohr.

                  Linus
 kernel/futex.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/futex.c b/kernel/futex.c
index 67dacaf93e56..6801b3751a95 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1452,6 +1452,7 @@ retry:
 	hb2 = hash_futex(&key2);
 
 retry_private:
+	hb_waiters_inc(hb2);
 	double_lock_hb(hb1, hb2);
 
 	if (likely(cmpval != NULL)) {
@@ -1461,6 +1462,7 @@ retry_private:
 
 		if (unlikely(ret)) {
 			double_unlock_hb(hb1, hb2);
+			hb_waiters_dec(hb2);
 
 			ret = get_user(curval, uaddr1);
 			if (ret)
@@ -1510,6 +1512,7 @@ retry_private:
 			break;
 		case -EFAULT:
 			double_unlock_hb(hb1, hb2);
+			hb_waiters_dec(hb2);
 			put_futex_key(&key2);
 			put_futex_key(&key1);
 			ret = fault_in_user_writeable(uaddr2);
@@ -1519,6 +1522,7 @@ retry_private:
 		case -EAGAIN:
 			/* The owner was exiting, try again. */
 			double_unlock_hb(hb1, hb2);
+			hb_waiters_dec(hb2);
 			put_futex_key(&key2);
 			put_futex_key(&key1);
 			cond_resched();
@@ -1594,6 +1598,7 @@ retry_private:
 
 out_unlock:
 	double_unlock_hb(hb1, hb2);
+	hb_waiters_dec(hb2);
 
 	/*
 	 * drop_futex_key_refs() must be called outside the spinlocks. During