Re: [PATCH v5 07/14] futex: Move the retry_private label.

[Thomas Gleixner]

On Mon, Dec 16 2024 at 00:00, Sebastian Andrzej Siewior wrote:
> The label futex_requeue in futex_requeue() and futex_wake_op() is jumped
> after the lock is dropped in a retry operation.

The label is jumped? 

> This assumes that the hb does not need to be hashed again. If hb is
> resized then the hb can change if the reference is dropped.

Again 'hb' and the confusion of hash bucket (hb) resize.

> Move the retry_private label before the hashing operation.

The overall explanation is not really comprehensible.

    futex: Re-evaluate the hash bucket after dropping the lock

     Sebastian Andrzej Siewior wrote:

     In futex_requeue() and futex_wake_op() the hash bucket lock is
     dropped in the failure paths for handling page faults and other
     error scenarios. After that the code jumps back to retry_private
     which relocks the hash bucket[s] under the assumption that the hash
     bucket pointer which was retrieved via futex_hash() is still valid.
     
     With resizable private hash buckets, that assumption is not longer
     true as the waiters can be moved to a larger hash in the meantime.

     Move the retry_private label above the hashing function to handle
     this correctly.

Or so.

Thanks,

        tglx