Re: [PATCH v4 00/10] tracing: introducing eventfs

Ajay Kaher
2023-07-19T18:37:18Z

> On 19-Jul-2023, at 7:53 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
>
> !! External Email
>
> On Wed, 19 Jul 2023 10:25:28 +0000
> Ajay Kaher <akaher@vmware.com> wrote:
>
>> - Is it also reproducible on v3?
>> - Is it manually reproducible or reproducible using any specific script?
>>
>> Let me know if I can help.
>
> Just tried it against v3, and it gave me the splat that I originally had
> and starting to fix, which now gives me another splat. I'll spend a couple
> more days on it and start sharing code and seeing if we can work together
> on this.
>
> Here's the reproducer (of both v3 splat and the bug I'm hitting now).
>
> ~# echo 'p:sock_getattr 0xffffffff9b55cef0 sk=%di' > /sys/kernel/tracing/kprobe_events
> ~# ls /sys/kernel/debug/tracing/events/kprobes/sock_getattr/
> ~# echo '-:sock_getattr 0xffffffff9b55cef0 sk=%di' > /sys/kernel/tracing/kprobe_events

I tried above steps on v4 but couldn’t reproduce:

root@photon-6 [ ~/sdb/linux ]# echo 'p:sock_getattr 0xffffffff9b55cef0 sk=%di' > /sys/kernel/tracing/kprobe_events
root@photon-6 [ ~/sdb/linux ]# ls /sys/kernel/debug/tracing/events/kprobes/sock_getattr/
enable filter format id trigger
root@photon-6 [ ~/sdb/linux ]# echo '-:sock_getattr 0xffffffff9b55cef0 sk=%di' > /sys/kernel/tracing/kprobe_events
-bash: echo: write error: No such file or directory

I have doubt on call_srcu(), it may first end the grace period for parent then for child. If this is true then free_list
will have unordered list and could cause problem.

>
> v3 gives me (and my updates too)
>
> ======================================================
> WARNING: possible circular locking dependency detected
> 6.5.0-rc1-test+ #576 Not tainted
> ------------------------------------------------------
> trace-cmd/840 is trying to acquire lock:
> ffff8881007e5de0 (&sb->s_type->i_mutex_key#5){++++}-{3:3}, at: dcache_dir_open_wrapper+0xc1/0x1b0
>
> but task is already holding lock:
> ffff888103ad7e70 (eventfs_rwsem/1){.+.+}-{3:3}, at: dcache_dir_open_wrapper+0x6f/0x1b0
>
> which lock already depends on the new lock.
>
>
> the existing dependency chain (in reverse order) is:
>
> -> #1 (eventfs_rwsem/1){.+.+}-{3:3}:
> down_read_nested+0x41/0x180
> eventfs_root_lookup+0x42/0x120
> __lookup_slow+0xff/0x1b0
> walk_component+0xdb/0x150
> path_lookupat+0x67/0x1a0
> filename_lookup+0xe4/0x1f0
> vfs_statx+0x9e/0x180
> vfs_fstatat+0x51/0x70
> __do_sys_newfstatat+0x3f/0x80
> do_syscall_64+0x3a/0xc0
> entry_SYSCALL_64_after_hwframe+0x6e/0xd8
>
> -> #0 (&sb->s_type->i_mutex_key#5){++++}-{3:3}:
> __lock_acquire+0x165d/0x2390
> lock_acquire+0xd4/0x2d0
> down_write+0x3b/0xd0
> dcache_dir_open_wrapper+0xc1/0x1b0
> do_dentry_open+0x20c/0x510
> path_openat+0x7ad/0xc60
> do_filp_open+0xaf/0x160
> do_sys_openat2+0xab/0xe0
> __x64_sys_openat+0x6a/0xa0
> do_syscall_64+0x3a/0xc0
> entry_SYSCALL_64_after_hwframe+0x6e/0xd8
>
> other info that might help us debug this:
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> rlock(eventfs_rwsem/1);
> lock(&sb->s_type->i_mutex_key#5);
> lock(eventfs_rwsem/1);
> lock(&sb->s_type->i_mutex_key#5);
>
> *** DEADLOCK ***
>
> 1 lock held by trace-cmd/840:
> #0: ffff888103ad7e70 (eventfs_rwsem/1){.+.+}-{3:3}, at: dcache_dir_open_wrapper+0x6f/0x1b0
>
> stack backtrace:
> CPU: 7 PID: 840 Comm: trace-cmd Not tainted 6.5.0-rc1-test+ #576
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> Call Trace:
> <TASK>
> dump_stack_lvl+0x57/0x90
> check_noncircular+0x14b/0x160
> __lock_acquire+0x165d/0x2390
> lock_acquire+0xd4/0x2d0
> ? dcache_dir_open_wrapper+0xc1/0x1b0
> down_write+0x3b/0xd0
> ? dcache_dir_open_wrapper+0xc1/0x1b0
> dcache_dir_open_wrapper+0xc1/0x1b0
> ? __pfx_dcache_dir_open_wrapper+0x10/0x10
> do_dentry_open+0x20c/0x510
> path_openat+0x7ad/0xc60
> do_filp_open+0xaf/0x160
> do_sys_openat2+0xab/0xe0
> __x64_sys_openat+0x6a/0xa0
> do_syscall_64+0x3a/0xc0
> entry_SYSCALL_64_after_hwframe+0x6e/0xd8
> RIP: 0033:0x7f1743267e41
> Code: 44 24 18 31 c0 41 83 e2 40 75 3e 89 f0 25 00 00 41 00 3d 00 00 41 00 74 30 89 f2 b8 01 01 00 00 48 89 fe bf 9c ff ff ff 0f 05 <48> 3d 00 f0 ff ff 77 3f 48 8b 54 24 18 64 48 2b 14 25 28 00 00 00
> RSP: 002b:00007ffec10ff5d0 EFLAGS: 00000287 ORIG_RAX: 0000000000000101
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f1743267e41
> RDX: 0000000000090800 RSI: 00007ffec10ffdb0 RDI: 00000000ffffff9c
> RBP: 00007ffec10ffda0 R08: 00007ffec11003e0 R09: 0000000000000040
> R10: 0000000000000000 R11: 0000000000000287 R12: 00007ffec11003e0
> R13: 0000000000000040 R14: 0000000000000000 R15: 00007ffec110034b
> </TASK>
>

This is expected from v3 (just ignore as of now), if eventfs_set_ef_status_free crash not
reproduced on v3 then it’s v4 issue.

-Ajay

>
> I moved the code around a bit, and it appears that kprobes is getting
> dput() more than once.
>
> I moved the d_invalidate() and dput() into the workqueue function, and on
> kprobes going away, d_invalidate() frees it, and dput() is now corrupted.
>
> Still investigating. The VFS layer is a magic box that needs the right
> wizard hat to deal with, but I unfortunately am waiting on back order to
> retrieve that specific hat :-p
>
> -- Steve
>
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