linux-kernel - Re: [BUG] lockdep splat with kernfs lockdep annotations and slab mutex from drm patch??

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Message-ID: <20190614153837.GE538958@devbig004.ftw2.facebook.com>
Date:   Fri, 14 Jun 2019 08:38:37 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Subject: Re: [BUG] lockdep splat with kernfs lockdep annotations and slab
 mutex from drm patch??

Hello,

On Fri, Jun 14, 2019 at 04:08:33PM +0100, Chris Wilson wrote:
> #ifdef CONFIG_MEMCG
>         if (slab_state >= FULL && err >= 0 && is_root_cache(s)) {
>                 struct kmem_cache *c;
> 
>                 mutex_lock(&slab_mutex);
> 
> so it happens to hit the error + FULL case with the additional slabcaches?
> 
> Anyway, according to lockdep, it is dangerous to use the slab_mutex inside
> slab_attr_store().

Didn't really look into the code but it looks like slab_mutex is held
while trying to remove sysfs files.  sysfs file removal flushes
on-going accesses, so if a file operation then tries to grab a mutex
which is held during removal, it leads to a deadlock.

Thanks.

-- 
tejun