Re: [PATCH] futex: Fix missed wakeup during private hash resize

On 8/4/2026 5:16 PM, Peter Zijlstra wrote:
> On Mon, Aug 03, 2026 at 06:43:13PM +0800, Yao Kai wrote:
>> A task performing a custom private hash resize can remain blocked in
>> uninterruptible sleep indefinitely.  The hung-task detector reports:
>>
>>    INFO: task futex-resizer:314 blocked for more than 10 seconds.
>>    task:futex-resizer state:D stack:14824 pid:314 tgid:312 ppid:311
>>
>>    Call Trace:
>>     __schedule+0x521/0xf30
>>     schedule+0x22/0xa0
>>     futex_hash_allocate+0x3db/0x490
>>     __do_sys_prctl+0x6f5/0xbd0
>>     do_syscall_64+0xf9/0x530
>>     entry_SYSCALL_64_after_hwframe+0x77/0x7f
>>
>>    Kernel panic - not syncing: hung_task: blocked tasks
>>
>> futex_pivot_pending() allows the resize request to continue when
>> either no replacement hash is pending (hash_new == NULL) or the current
>> hash reference count has reached zero.
>>
>> After the final-reference wake, another futex task can complete the
>> pivot between the two observations:
>>
>>    T1                                  T2
>>
>>    futex_hash_allocate()
>>      wait_var_event(mm, ...)
>>        futex_pivot_pending(mm)
>>          hash_new != NULL
>>                                        futex_hash()
>>                                          futex_ref_get(old) -> false
>>                                          futex_pivot_hash(mm)
>>                                            hash_new = NULL
>>                                            __futex_pivot_hash(mm, new)
>>                                              rcu_assign_pointer(hash, new)
>>          fph = rcu_dereference(hash) /* new */
>>          futex_ref_is_dead(fph) -> false
>>        schedule()
>>
>> The pivot changes the state from hash_new != NULL with a dead current
>> hash to hash_new == NULL with a live current hash.  The resize task can
>> observe hash_new in the pre-pivot state and hash in the post-pivot state,
>> causing futex_pivot_pending() to return false even though the pivot has
>> completed.  Since a successful pivot does not notify waiters, the task
>> can go to sleep after the only preceding wakeup has already been
>> consumed.
>>
>> Wake waiters after every successful pivot.  A full memory barrier before
>> wake_up_var() pairs with set_current_state() in wait_var_event() and
>> orders the completed pivot before the lockless waitqueue_active() check
>> in wake_up_var().  The waiter therefore either observes hash_new == NULL
>> before sleeping or is made runnable.
> 
> Hmm, but isn't the problem a lack of serialization on futex_mm_phash
> access?
> 
> That is, all of this futex_mm_phash::hash_new and futex_mm_phash::hash
> swizzling happens while holding futex_mm_phash::lock, except for
> futex_pivot_pending(), that is looking at these values without holding
> the lock, resulting in it observing that inconsistent state per the
> above.
> 
> Taking a mutex in a wait loop is sorta yuck, but it should work. If the
> mutex is contended, it sleeps and the wait-loop 'spuriously' doesn't. If
> the mutex is uncontended, it doesn't sleep, but the wait-loop will.
> 
> Does this work for you?
> 
> ---
> 
> diff --git a/kernel/futex/core.c b/kernel/futex/core.c
> index f74ede3df161..72d4698e35fb 100644
> --- a/kernel/futex/core.c
> +++ b/kernel/futex/core.c
> @@ -1778,14 +1778,15 @@ void futex_hash_free(struct mm_struct *mm)
>   
>   static bool futex_pivot_pending(struct mm_struct *mm)
>   {
> +	struct futex_mm_phash *mmph = &mm->futex.phash;
>   	struct futex_private_hash *fph;
>   
> -	guard(rcu)();
> +	guard(mutex)(&mmph->lock);
>   
> -	if (!mm->futex.phash.hash_new)
> +	if (!mmph->hash_new)
>   		return true;
>   
> -	fph = rcu_dereference(mm->futex.phash.hash);
> +	fph = rcu_dereference_raw(mmph->hash);
>   	return futex_ref_is_dead(fph);
>   }
>   

Thanks! I verified your patch with my reproducer, and it completely fixes
the issue.
I will send out a v2 patch adopting your approach shortly.

Thanks,
Yao Kai