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

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Message-ID: <20250617095037.sOnrJlPX@linutronix.de>
Date: Tue, 17 Jun 2025 11:50:37 +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 02:23:08 [-0700], Calvin Owens wrote:
> Ugh, I'm sorry, I was in too much of a hurry this morning... cargo is
> obviously not calling PR_FUTEX_HASH which is new in 6.16 :/
No worries.

> > This is with LTO enabled.
> 
> Full lto with llvm-20.1.7.
> 
…
> Nothing showed up in the logs but the RCU stalls on CPU16, always in
> queued_spin_lock_slowpath().
> 
> I'll run the build it was doing when it happened in a loop overnight and
> see if I can trigger it again.

Please check if you can reproduce it and if so if it also happens
without lto.
I have no idea why one spinlock_t remains locked. It is either locked or
some stray memory.

Oh. Lockdep adds quite some overhead but it should complain that a
spinlock_t is still locked while returning to userland.

> > > Thanks,
> > > Calvin
> > 
Sebastian