linux-kernel - Re: [PATCH v5 07/14] futex: Move the retry

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Message-ID: <87ttb3743d.ffs@tglx>
Date: Mon, 16 Dec 2024 21:41:42 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 linux-kernel@...r.kernel.org
Cc: André Almeida <andrealmeid@...lia.com>, Darren Hart
 <dvhart@...radead.org>, Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar
 <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, Peter Zijlstra
 <peterz@...radead.org>, Valentin Schneider <vschneid@...hat.com>, Waiman
 Long <longman@...hat.com>, Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>
Subject: Re: [PATCH v5 07/14] futex: Move the retry_private label.

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