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Date: Wed, 5 Feb 2014 12:32:43 -0800 (PST)
From: David Rientjes <rientjes@google.com>
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To: Steven Rostedt <rostedt@goodmis.org>
cc: Vladimir Davydov <vdavydov@parallels.com>, akpm@linux-foundation.org,
        penberg@kernel.org, cl@linux.com, linux-kernel@vger.kernel.org,
        a.p.zijlstra@chello.nl
Subject: Re: [PATCH v3] slub: fix false-positive lockdep warning in
 free_partial()
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On Wed, 5 Feb 2014, Steven Rostedt wrote:

> > Commit c65c1877bd68 ("slub: use lockdep_assert_held") requires
> > remove_partial() to be called with n->list_lock held, but free_partial()
> > called from kmem_cache_close() on cache destruction does not follow this
> > rule, leading to a warning:
> > 
> >   WARNING: CPU: 0 PID: 2787 at mm/slub.c:1536 __kmem_cache_shutdown+0x1b2/0x1f0()
> >   Modules linked in:
> >   CPU: 0 PID: 2787 Comm: modprobe Tainted: G        W    3.14.0-rc1-mm1+ #1
> >   Hardware name:
> >    0000000000000600 ffff88003ae1dde8 ffffffff816d9583 0000000000000600
> >    0000000000000000 ffff88003ae1de28 ffffffff8107c107 0000000000000000
> >    ffff880037ab2b00 ffff88007c240d30 ffffea0001ee5280 ffffea0001ee52a0
> >   Call Trace:
> >    [<ffffffff816d9583>] dump_stack+0x51/0x6e
> >    [<ffffffff8107c107>] warn_slowpath_common+0x87/0xb0
> >    [<ffffffff8107c145>] warn_slowpath_null+0x15/0x20
> >    [<ffffffff811c7fe2>] __kmem_cache_shutdown+0x1b2/0x1f0
> >    [<ffffffff811908d3>] kmem_cache_destroy+0x43/0xf0
> >    [<ffffffffa013a123>] xfs_destroy_zones+0x103/0x110 [xfs]
> >    [<ffffffffa0192b54>] exit_xfs_fs+0x38/0x4e4 [xfs]
> >    [<ffffffff811036fa>] SyS_delete_module+0x19a/0x1f0
> >    [<ffffffff816dfcd8>] ? retint_swapgs+0x13/0x1b
> >    [<ffffffff810d2125>] ? trace_hardirqs_on_caller+0x105/0x1d0
> >    [<ffffffff81359efe>] ? trace_hardirqs_on_thunk+0x3a/0x3f
> >    [<ffffffff816e8539>] system_call_fastpath+0x16/0x1b
> > 
> > Although this cannot actually result in a race, because on cache
> > destruction there should not be any concurrent frees or allocations from
> > the cache, let's add spin_lock/unlock to free_partial() just to keep
> > lockdep happy.
> 
> Really? We are adding a spin lock for a case where it is not needed just to
> quiet lockdep?
> 
> Now if it really isn't needed, then why don't we do the following instead of
> adding the overhead of taking a lock?
> 

There's an extremely small overhead of taking this lock, the cache has 
been destroyed and is the process of being torn down, there will be 
absolutely no contention on n->list_lock.