linux-kernel - Re: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug

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Date:   Tue, 24 Sep 2019 14:54:04 -0400
From:   Qian Cai <cai@....pw>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock

On Tue, 2019-09-24 at 17:11 +0200, Michal Hocko wrote:
> On Tue 24-09-19 11:03:21, Qian Cai wrote:
> [...]
> > While at it, it might be a good time to rethink the whole locking over there, as
> > it right now read files under /sys/kernel/slab/ could trigger a possible
> > deadlock anyway.
> > 
> 
> [...]
> > [  442.452090][ T5224] -> #0 (mem_hotplug_lock.rw_sem){++++}:
> > [  442.459748][ T5224]        validate_chain+0xd10/0x2bcc
> > [  442.464883][ T5224]        __lock_acquire+0x7f4/0xb8c
> > [  442.469930][ T5224]        lock_acquire+0x31c/0x360
> > [  442.474803][ T5224]        get_online_mems+0x54/0x150
> > [  442.479850][ T5224]        show_slab_objects+0x94/0x3a8
> > [  442.485072][ T5224]        total_objects_show+0x28/0x34
> > [  442.490292][ T5224]        slab_attr_show+0x38/0x54
> > [  442.495166][ T5224]        sysfs_kf_seq_show+0x198/0x2d4
> > [  442.500473][ T5224]        kernfs_seq_show+0xa4/0xcc
> > [  442.505433][ T5224]        seq_read+0x30c/0x8a8
> > [  442.509958][ T5224]        kernfs_fop_read+0xa8/0x314
> > [  442.515007][ T5224]        __vfs_read+0x88/0x20c
> > [  442.519620][ T5224]        vfs_read+0xd8/0x10c
> > [  442.524060][ T5224]        ksys_read+0xb0/0x120
> > [  442.528586][ T5224]        __arm64_sys_read+0x54/0x88
> > [  442.533634][ T5224]        el0_svc_handler+0x170/0x240
> > [  442.538768][ T5224]        el0_svc+0x8/0xc
> 
> I believe the lock is not really needed here. We do not deallocated
> pgdat of a hotremoved node nor destroy the slab state because an
> existing slabs would prevent hotremove to continue in the first place.
> 
> There are likely details to be checked of course but the lock just seems
> bogus.

Check 03afc0e25f7f ("slab: get_online_mems for
kmem_cache_{create,destroy,shrink}"). It actually talk about the races during
memory as well cpu hotplug, so it might even that cpu_hotplug_lock removal is
problematic?