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Sender: Minchan Kim <minchan.kim@gmail.com>
Date:   Mon, 4 Apr 2022 10:41:25 -0700
From:   Minchan Kim <minchan@kernel.org>
To:     Jirka Hladky <jhladky@redhat.com>, tj@kernel.org,
        gregkh@linuxfoundation.org
Cc:     tj@kernel.org, linux-kernel <linux-kernel@vger.kernel.org>,
        regressions@lists.linux.dev,
        Thorsten Leemhuis <regressions@leemhuis.info>,
        Justin Forbes <jforbes@fedoraproject.org>
Subject: Re: PANIC: "Oops: 0000 [#1] PREEMPT SMP PTI" starting from 5.17 on
 dual socket Intel Xeon Gold servers
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On Fri, Apr 01, 2022 at 02:04:03PM +0200, Jirka Hladky wrote:
> > Could you decode exact source code line from the oops?
> 
> Yes - please see below [1].

Thanks.

> 
> > I think it's fine to attach in the reply because kernel test bot
> 
> OK. The reproducer is attached. Please unpack it and follow the
> instructions in the README file. [2]

Unfortunately, I failed to run the script in my machine.

> 
> Thanks a lot for looking into it!
> Jirka
> 
> [1]
> =============================================
> Source code line numbers for the Oops message
> =============================================
> 
> 1) RIP: 0010:kernfs_remove+0x8/0x50:
> (gdb) l *kernfs_remove+0x8
> 0xffffffff81418588 is in kernfs_remove (fs/kernfs/kernfs-internal.h:48).
> 43       * Return the kernfs_root @kn belongs to.
> 44       */
> 45      static inline struct kernfs_root *kernfs_root(struct kernfs_node *kn)
> 46      {
> 47              /* if parent exists, it's always a dir; otherwise, @sd
> is a dir */
> 48              if (kn->parent)
> 49                      kn = kn->parent;
> 50              return kn->dir.root;
> 51      }
> 
> And here are source code lines from the 5 first functions in call trace:
> [ 8563.366280] Call Trace:
> [ 8563.366280]  <TASK>
> [ 8563.366280]  rdt_kill_sb+0x29d/0x350
> [ 8563.366280]  deactivate_locked_super+0x36/0xa0
> [ 8563.366280]  cleanup_mnt+0x131/0x190
> [ 8563.366280]  task_work_run+0x5c/0x90
> [ 8563.366280]  exit_to_user_mode_prepare+0x229/0x230
> [ 8563.366280]  syscall_exit_to_user_mode+0x18/0x40
> [ 8563.366280]  do_syscall_64+0x48/0x90
> [ 8563.366280]  entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> 2)(gdb) l *rdt_kill_sb+0x29d
> 0xffffffff810506bd is in rdt_kill_sb
> (arch/x86/kernel/cpu/resctrl/rdtgroup.c:2442).
> 2437            /* Notify online CPUs to update per cpu storage and
> PQR_ASSOC MSR */
> 2438            update_closid_rmid(cpu_online_mask, &rdtgroup_default);
> 2439
> 2440            kernfs_remove(kn_info);
> 2441            kernfs_remove(kn_mongrp);
> 2442            kernfs_remove(kn_mondata);
> 2443    }
> 
> 3)(gdb) l *deactivate_locked_super+0x36
> 0xffffffff813650f6 is in deactivate_locked_super (fs/super.c:342).
> 337                     /*
> 338                      * Since list_lru_destroy() may sleep, we
> cannot call it from
> 339                      * put_super(), where we hold the sb_lock.
> Therefore we destroy
> 340                      * the lru lists right now.
> 341                      */
> 342                     list_lru_destroy(&s->s_dentry_lru);
> 343                     list_lru_destroy(&s->s_inode_lru);
> 344
> 345                     put_filesystem(fs);
> 346                     put_super(s);
> 
> 4) (gdb) l *cleanup_mnt+0x131
> 0xffffffff813890a1 is in cleanup_mnt (fs/namespace.c:137).
> 132             return 0;
> 133     }
> 134
> 135     static void mnt_free_id(struct mount *mnt)
> 136     {
> 137             ida_free(&mnt_id_ida, mnt->mnt_id);
> 138     }
> 
> 5) (gdb) l *task_work_run+0x5c
> 0xffffffff8110620c is in task_work_run (./include/linux/sched.h:2017).
> 2012
> 2013    DECLARE_STATIC_CALL(cond_resched, __cond_resched);
> 2014
> 2015    static __always_inline int _cond_resched(void)
> 2016    {
> 2017            return static_call_mod(cond_resched)();
> 2018    }
> 
> 6) (gdb) l *exit_to_user_mode_prepare+0x229
> 0xffffffff81176d19 is in exit_to_user_mode_prepare
> (./include/linux/tracehook.h:189).
> 184              * This barrier pairs with
> task_work_add()->set_notify_resume() after
> 185              * hlist_add_head(task->task_works);
> 186              */
> 187             smp_mb__after_atomic();
> 188             if (unlikely(current->task_works))
> 189                     task_work_run();
> 190
> 191     #ifdef CONFIG_KEYS_REQUEST_CACHE
> 192             if (unlikely(current->cached_requested_key)) {
> 193                     key_put(current->cached_requested_key);
> 
> [2]
> =============================================
> Reproducer - README
> =============================================
> 
> 1) HW
> This issue seems to be platform specific. I was not able to reproduce
> it on AMD Zen and also not on Intel Ice Lake platform.
> I see the issue on dual socket Intel Skylake systems. Reproduced on a
> Supermicro Super Server/X11DDW-L with 2x Xeon Gold 6126 CPU.

Based on your report, kernel was crashed due to kn_mondata was NULL

  rdt_kill_sb
    rmdir_all_sub
      ..
      kernfs_remove(kn_mondata);
        struct kernfs_root *root = kernfs_root(kn); <-- crashed


Before the my patch[1], it worked like this.

  rdt_kill_sb
    rmdir_all_sub
      ..
      kernfs_remove(kn_mondata);
        down_write(&kernfs_rwsem);
          if (!kn)
            return;
        up_write(&kernfs_rwsem);

IOW, before, kernfs_remove worked with NULL argument via just bailing
but with the my patch[1], it doesn't work any longer.

It makes me have questions for kernfs maintainers:

Should kernfs_remove API support NULL parameter? If so, can we support
it atomically without old global kernfs_rwsem?

[1] 393c3714081a, kernfs: switch global kernfs_rwsem lock to per-fs lock