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.
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