On 26.11.19 10:29, Michal Hocko wrote: > On Mon 25-11-19 10:54:53, Roman Gushchin wrote: >> Christian reported a warning like the following obtained during running some >> KVM-related tests on s390: >> >> WARNING: CPU: 8 PID: 208 at lib/percpu-refcount.c:108 percpu_ref_exit+0x50/0x58 >> Modules linked in: kvm(-) xt_CHECKSUM xt_MASQUERADE bonding xt_tcpudp ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 xt_conntrack ip6table_na> >> CPU: 8 PID: 208 Comm: kworker/8:1 Not tainted 5.2.0+ #66 >> Hardware name: IBM 2964 NC9 712 (LPAR) >> Workqueue: events sysfs_slab_remove_workfn >> Krnl PSW : 0704e00180000000 0000001529746850 (percpu_ref_exit+0x50/0x58) >> R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3 >> Krnl GPRS: 00000000ffff8808 0000001529746740 000003f4e30e8e18 0036008100000000 >> 0000001f00000000 0035008100000000 0000001fb3573ab8 0000000000000000 >> 0000001fbdb6de00 0000000000000000 0000001529f01328 0000001fb3573b00 >> 0000001fbb27e000 0000001fbdb69300 000003e009263d00 000003e009263cd0 >> Krnl Code: 0000001529746842: f0a0000407fe srp 4(11,%r0),2046,0 >> 0000001529746848: 47000700 bc 0,1792 >> #000000152974684c: a7f40001 brc 15,152974684e >> >0000001529746850: a7f4fff2 brc 15,1529746834 >> 0000001529746854: 0707 bcr 0,%r7 >> 0000001529746856: 0707 bcr 0,%r7 >> 0000001529746858: eb8ff0580024 stmg %r8,%r15,88(%r15) >> 000000152974685e: a738ffff lhi %r3,-1 >> Call Trace: >> ([<000003e009263d00>] 0x3e009263d00) >> [<00000015293252ea>] slab_kmem_cache_release+0x3a/0x70 >> [<0000001529b04882>] kobject_put+0xaa/0xe8 >> [<000000152918cf28>] process_one_work+0x1e8/0x428 >> [<000000152918d1b0>] worker_thread+0x48/0x460 >> [<00000015291942c6>] kthread+0x126/0x160 >> [<0000001529b22344>] ret_from_fork+0x28/0x30 >> [<0000001529b2234c>] kernel_thread_starter+0x0/0x10 >> Last Breaking-Event-Address: >> [<000000152974684c>] percpu_ref_exit+0x4c/0x58 >> ---[ end trace b035e7da5788eb09 ]--- >> >> The problem occurs because kmem_cache_destroy() is called immediately >> after deleting of a memcg, so it races with the memcg kmem_cache >> deactivation. >> >> flush_memcg_workqueue() at the beginning of kmem_cache_destroy() >> is supposed to guarantee that all deactivation processes are finished, >> but failed to do so. It waits for an rcu grace period, after which all >> children kmem_caches should be deactivated. During the deactivation >> percpu_ref_kill() is called for non root kmem_cache refcounters, >> but it requires yet another rcu grace period to finish the transition >> to the atomic (dead) state. >> >> So in a rare case when not all children kmem_caches are destroyed >> at the moment when the root kmem_cache is about to be gone, we need >> to wait another rcu grace period before destroying the root >> kmem_cache. > > Could you explain how rare this really is please? I still have to wrap > my head around the overall logic here. It looks quite fragile to me TBH. > I am worried that is relies on implementation detail of the PCP ref > counters too much. I can actually reproduce this very reliably by doing an # virsh destroy <lastguest>; rmmod kvm
