Re: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock

[Michal Hocko]

On Tue 24-09-19 14:54:04, Qian Cai wrote:
> 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?

I have to refresh my memory there but the changlog claims:
"To avoid issues like that we should hold get/put_online_mems() during
the whole kmem cache creation/destruction/shrink paths" and
show_slab_objects doesn't fall into any of those categories.

Anyway this seems unrelated to the original thread so I would recommend
discussing in its own thread for clarity.

-- 
Michal Hocko
SUSE Labs