Re: Are jump labels broken on 6.11-rc1?

Peter Zijlstra
2024-07-31T10:56:07Z

On Tue, Jul 30, 2024 at 10:33:41PM -0700, Darrick J. Wong wrote:

> Sooooo... it turns out that somehow your patch got mismerged on the
> first go-round, and that worked. The second time, there was no
> mismerge, which mean that the wrong atomic_cmpxchg() callsite was
> tested.
>
> Looking back at the mismerge, it actually changed
> __static_key_slow_dec_cpuslocked, which had in 6.10:
>
> if (atomic_dec_and_test(&key->enabled))
> jump_label_update(key);
>
> Decrement, then return true if the value was set to zero. With the 6.11
> code, it looks like we want to exchange a 1 with a 0, and act only if
> the previous value had been 1.
>
> So perhaps we really want this change? I'll send it out to the fleet
> and we'll see what it reports tomorrow morning.

Bah yes, I missed we had it twice. Definitely both sites want this.

I'll tentatively merge the below patch in tip/locking/urgent. I can
rebase if there is need.

---
Subject: jump_label: Fix the fix, brown paper bags galore
From: Peter Zijlstra <peterz@infradead.org>
Date: Wed Jul 31 12:43:21 CEST 2024

Per the example of:

!atomic_cmpxchg(&key->enabled, 0, 1)

the inverse was written as:

atomic_cmpxchg(&key->enabled, 1, 0)

except of course, that while !old is only true for old == 0, old is
true for everything except old == 0.

Fix it to read:

atomic_cmpxchg(&key->enabled, 1, 0) == 1

such that only the 1->0 transition returns true and goes on to disable
the keys.

Fixes: 83ab38ef0a0b ("jump_label: Fix concurrency issues in static_key_slow_dec()")
Reported-by: Darrick J. Wong <djwong@kernel.org>
Tested-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/jump_label.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -236,7 +236,7 @@ void static_key_disable_cpuslocked(struc
}

jump_label_lock();
- if (atomic_cmpxchg(&key->enabled, 1, 0))
+ if (atomic_cmpxchg(&key->enabled, 1, 0) == 1)
jump_label_update(key);
jump_label_unlock();
}
@@ -289,7 +289,7 @@ static void __static_key_slow_dec_cpuslo
return;

guard(mutex)(&jump_label_mutex);
- if (atomic_cmpxchg(&key->enabled, 1, 0))
+ if (atomic_cmpxchg(&key->enabled, 1, 0) == 1)
jump_label_update(key);
else
WARN_ON_ONCE(!static_key_slow_try_dec(key));