linux-kernel - Re: [tip: locking/urgent] futex: Allow to resize the private local hash

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Message-ID: <20250611152019.GA2273450@noisy.programming.kicks-ass.net>
Date: Wed, 11 Jun 2025 17:20:19 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
	"Lai, Yi" <yi1.lai@...ux.intel.com>, x86@...nel.org
Subject: Re: [tip: locking/urgent] futex: Allow to resize the private local
 hash

On Wed, Jun 11, 2025 at 05:11:13PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 11, 2025 at 04:43:02PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2025-06-11 14:39:16 [-0000], tip-bot2 for Sebastian Andrzej Siewior wrote:
> > > The following commit has been merged into the locking/urgent branch of tip:
> > > 
> > > Commit-ID:     703b5f31aee5bda47868c09a3522a78823c1bb77
> > > Gitweb:        https://git.kernel.org/tip/703b5f31aee5bda47868c09a3522a78823c1bb77
> > > Author:        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > > AuthorDate:    Mon, 02 Jun 2025 13:00:27 +02:00
> > > Committer:     Peter Zijlstra <peterz@...radead.org>
> > > CommitterDate: Wed, 11 Jun 2025 16:26:44 +02:00
> > > 
> > > futex: Allow to resize the private local hash
> > > 
> > > Once the global hash is requested there is no way back to switch back to
> > > the per-task private hash. This is checked at the begin of the function.
> > > 
> > > It is possible that two threads simultaneously request the global hash
> > > and both pass the initial check and block later on the
> > > mm::futex_hash_lock. In this case the first thread performs the switch
> > > to the global hash. The second thread will also attempt to switch to the
> > > global hash and while doing so, accessing the nonexisting slot 1 of the
> > > struct futex_private_hash.
> > > This has been reported by Yi Lai.
> > > 
> > > Verify under mm_struct::futex_phash that the global hash is not in use.
> > 
> > Could you please replace it with
> > 	https://lore.kernel.org/all/20250610104400.1077266-5-bigeasy@linutronix.de/
> > 
> > It also looks like the subject from commit bd54df5ea7cad ("futex: Allow
> > to resize the private local hash")
> 
> Now done so, unless I messed up again :/

ARGH, let me try that again :-(