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

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Message-ID: <20250618164756.9CeqXYlG@linutronix.de>
Date: Wed, 18 Jun 2025 18:47:56 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Calvin Owens <calvin@...nvd.org>
Cc: linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
	"Lai, Yi" <yi1.lai@...ux.intel.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org
Subject: Re: [tip: locking/urgent] futex: Allow to resize the private local
 hash

On 2025-06-17 19:15:37 [-0700], Calvin Owens wrote:
> It takes longer with LTO disabled, but I'm still seeing some crashes.
> 
> First this WARN:
> 
>     ------------[ cut here ]------------
>     WARNING: CPU: 2 PID: 1866190 at mm/slub.c:4753 free_large_kmalloc+0xa5/0xc0
>     CPU: 2 UID: 1000 PID: 1866190 Comm: python3 Not tainted 6.16.0-rc2-nolto-00024-g9afe652958c3 #1 PREEMPT 
…
>     RIP: 0010:free_large_kmalloc+0xa5/0xc0
…
>     Call Trace:
>      <TASK>
>      futex_hash_free+0x10/0x40
This points me to kernel/futex/core.c:1535, which is futex_phash_new.
Thanks for the provided vmlinux.
This is odd. The assignment happens only under &mm->futex_hash_lock and
it a bad pointer. The kvmalloc() pointer is stored there and only
remains there if a rehash did not happen before the task ended.

>      __mmput+0xb4/0xd0
>      exec_mmap+0x1e2/0x210
>      begin_new_exec+0x491/0x6c0
>      load_elf_binary+0x25d/0x1050
…
> ...and then it oopsed (same stack as my last mail) about twenty minutes
> later when I hit Ctrl+C to stop the build:
> 
…
> I enabled lockdep and I've got it running again.
> 
> I set up a little git repo with a copy of all the traces so far, and the
> kconfigs I'm running:
> 
>     https://github.com/jcalvinowens/lkml-debug-616
> 
> ...and I pushed the actual vmlinux binaries here:
> 
>     https://github.com/jcalvinowens/lkml-debug-616/releases/tag/20250617
> 
> There were some block warnings on another machine running the same
> workload, but of course they aren't necessarily related.

I have no explanation so far.

Sebastian