linux-kernel - Re: [PATCH V2 4/5] workqueue: Put PWQ allocation and WQ enlistment in the same lock C.S.

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Message-ID: <CAJhGHyC=5FC1uFt0xzMwk42m=zm-_d9-OxoC4BQmSREAbAQrog@mail.gmail.com>
Date: Tue, 9 Jul 2024 22:55:10 +0800
From: Lai Jiangshan <jiangshanlai@...il.com>
To: Tejun Heo <tj@...nel.org>
Cc: kernel test robot <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev, lkp@...el.com, 
	linux-kernel@...r.kernel.org, Lai Jiangshan <jiangshan.ljs@...group.com>
Subject: Re: [PATCH V2 4/5] workqueue: Put PWQ allocation and WQ enlistment in
 the same lock C.S.

Hello,

On Tue, Jul 9, 2024 at 2:16 AM Tejun Heo <tj@...nel.org> wrote:
>
> Hello,
>
> On Mon, Jul 08, 2024 at 03:54:19PM +0800, kernel test robot wrote:
> ...
> > [ 2.269463][ T1] cpus_read_lock (include/linux/percpu-rwsem.h:53 kernel/cpu.c:488)
> > [ 2.273463][ T1] alloc_workqueue (kernel/workqueue.c:5152 kernel/workqueue.c:5730)
> > [ 2.273463][ T1] padata_alloc (kernel/padata.c:1007 (discriminator 1))
> > [ 2.273463][ T1] pcrypt_init_padata (crypto/pcrypt.c:327 (discriminator 1))
> > [ 2.277463][ T1] pcrypt_init (crypto/pcrypt.c:353)
> > [ 2.277463][ T1] do_one_initcall (init/main.c:1267)
> > [ 2.281464][ T1] do_initcalls (init/main.c:1328 (discriminator 1) init/main.c:1345 (discriminator 1))
> > [ 2.281464][ T1] kernel_init_freeable (init/main.c:1364)
> > [ 2.285464][ T1] kernel_init (init/main.c:1469)
> > [ 2.285464][ T1] ret_from_fork (arch/x86/kernel/process.c:153)
> > [ 2.289466][ T1] ret_from_fork_asm (arch/x86/entry/entry_32.S:737)
> > [ 2.289466][ T1] entry_INT80_32 (arch/x86/entry/entry_32.S:944)

Thanks for the report!

>
> Ah, this is unfortunate, so pcrypt is allocating a workqueue while holding
> cpus_read_lock(), so workqueue code can't do it again as that can lead to
> deadlocks if down_write starts after the first down_read. Lai, it looks like
> we'd need to switch to workqueue specific locking so that we don't depend on
> cpus_read_lock from alloc path.
>

I'm working on it. A new wq_online_mask will be added. The new
wq_online_mask mirrors the cpu_online_mask except during hotplugging;
specifically, it differs between the hotplugging stages of
workqueue_offline_cpu() and workqueue_online_cpu(), during which the
transitioning CPU is not represented in the mask.

Thanks
Lai