On Mon, Feb 14, 2022 at 9:59 AM Ira Weiny <ira.we...@intel.com> wrote: > > On Fri, Feb 11, 2022 at 11:11:11PM -0800, Tong Zhang wrote: > > A bug can be triggered by following command > > > > $ modprobe nd_pmem && modprobe -r nd_pmem > > > > [ 10.060014] BUG dax_cache (Not tainted): Objects remaining in dax_cache > > on __kmem_cache_shutdown() > > [ 10.060938] Slab 0x0000000085b729ac objects=9 used=1 > > fp=0x000000004f5ae469 flags=0x200000000010200(slab|head|node) > > [ 10.062433] Call Trace: > > [ 10.062673] dump_stack_lvl+0x34/0x44 > > [ 10.062865] slab_err+0x90/0xd0 > > [ 10.063619] __kmem_cache_shutdown+0x13b/0x2f0 > > [ 10.063848] kmem_cache_destroy+0x4a/0x110 > > [ 10.064058] __x64_sys_delete_module+0x265/0x300 > > > > This is caused by dax_fs_exit() not flushing inodes before destroy cache. > > To fix this issue, call rcu_barrier() before destroy cache. > > I don't doubt that this fixes the bug. However, I can't help but think this > is > hiding a bug, or perhaps a missing step, in the kmem_cache layer? As far as I > can see dax does not call call_rcu() and only uses srcu not rcu? I was > tempted > to suggest srcu_barrier() but dax does not call call_srcu() either.
This rcu_barrier() is associated with the call_rcu() in destroy_inode(). While kern_unmount() does a full sycnrhonize_rcu() after clearing ->mnt_ns. Any pending destroy_inode() callbacks need to be flushed before the kmem_cache is destroyed. > So I'm not clear about what is really going on and why this fixes it. I know > that dax is not using srcu is a standard way so perhaps this helps in a way I > don't quite grok? If so perhaps a comment here would be in order? Looks like a common pattern I missed that all filesystem exit paths implement.