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SubjectRe: [PATCH v4] kmod: Avoid deadlock by recursive kmod call.
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Andrew Morton wrote:
> > Since __call_usermodehelper() is exclusively called (am I right?), we don't
> > need to use per "struct task_struct" flag.
>
> The locking for the new kmod_thread_locker global is quite unobvious.
> From a quick look I can't say that I obviously agree with the above
> claim.

Should we use semaphore in order to utilize lockdep?

>
> Maybe it relies upon there only ever being a single khelper thread,
> system wide?

Yes. This patch relies on khelper being singlethreaded.

khelper_wq = create_singlethread_workqueue("khelper");

Below output (taken without this patch) shows that __call_usermodehelper() is
called with khelper lock held.

[ 95.333533] INFO: task kworker/u:0:5 blocked for more than 20 seconds.
[ 95.342879] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 95.493057] kworker/u:0 D 67af850a 6084 5 2 0x00000000
[ 95.570863] f61a1d1c 00000046 00000000 67af850a 0000011c 0000019a 00000000 f61a1ca8
[ 95.585696] c1070315 f619e120 f62e8560 00000003 00000002 f619e5d0 00000330 00000000
[ 95.669557] b194689f c174b8e0 f23ffd78 00000000 f6b458e0 f6b458e0 f619e120 0000019a
[ 95.752002] Call Trace:
[ 95.823457] [<c1070315>] ? validate_chain+0x3d5/0x520
[ 95.830678] [<c1070315>] ? validate_chain+0x3d5/0x520
[ 95.906497] [<c139efa5>] schedule+0x55/0x60
[ 95.980630] [<c139f6ec>] schedule_timeout+0x19c/0x1b0
[ 96.059353] [<c106d833>] ? lock_release_holdtime+0x73/0xb0
[ 96.134561] [<c1071082>] ? mark_held_locks+0x92/0xf0
[ 96.141343] [<c13a17a2>] ? _raw_spin_unlock_irq+0x22/0x30
[ 96.217468] [<c1071220>] ? trace_hardirqs_on_caller+0x90/0x100
[ 96.293072] [<c107129b>] ? trace_hardirqs_on+0xb/0x10
[ 96.369510] [<c139f094>] wait_for_common+0xe4/0x120
[ 96.443832] [<c1038fd0>] ? put_prev_task+0x40/0x40
[ 96.450638] [<c1071220>] ? trace_hardirqs_on_caller+0x90/0x100
[ 96.526705] [<c1038fd0>] ? put_prev_task+0x40/0x40
[ 96.601386] [<c1037f56>] ? wake_up_new_task+0xe6/0x1c0
[ 96.680872] [<c139f0e2>] wait_for_completion+0x12/0x20
[ 96.755649] [<c103f429>] do_fork+0x169/0x250
[ 96.761904] [<c139efce>] ? wait_for_common+0x1e/0x120
[ 96.851797] [<c100a488>] kernel_thread+0x88/0xa0
[ 96.925236] [<c1054310>] ? __request_module+0x130/0x130
[ 96.935826] [<c1054310>] ? __request_module+0x130/0x130
[ 97.013681] [<c13a2db4>] ? common_interrupt+0x34/0x34
[ 97.086563] [<c1054558>] __call_usermodehelper+0x28/0x90
[ 97.152805] [<c1056099>] process_one_work+0x149/0x3b0
[ 97.158095] [<c1056028>] ? process_one_work+0xd8/0x3b0
[ 97.217462] [<c1074149>] ? __lock_acquired+0x119/0x1c0
[ 97.278057] [<c1054530>] ? wait_for_helper+0xa0/0xa0
[ 97.283217] [<c105635c>] ? worker_thread+0x1c/0x200
[ 97.342278] [<c10563e6>] worker_thread+0xa6/0x200
[ 97.347193] [<c105b8c5>] kthread+0x75/0x80
[ 97.405213] [<c1056340>] ? process_scheduled_works+0x40/0x40
[ 97.463717] [<c105b850>] ? kthread_data+0x20/0x20
[ 97.468829] [<c13a2dba>] kernel_thread_helper+0x6/0xd
[ 97.528077] 2 locks held by kworker/u:0/5:
[ 97.532442] #0: (khelper){.+.+.+}, at: [<c1056028>] process_one_work+0xd8/0x3b0
[ 97.593214] #1: ((&sub_info->work)){+.+.+.}, at: [<c1056028>] process_one_work+0xd8/0x3b0
[ 97.656084] INFO: task modprobe:1158 blocked for more than 20 seconds.
[ 97.716441] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 97.778390] modprobe D 0000011c 6088 1158 1 0x00000000
[ 97.838302] f1c13d68 00000046 67abe0ee 0000011c 0000019a 00000000 34dfa000 c16074e0
[ 97.850432] 000002dc f3fd25a0 f62142a0 00000000 67abe8f0 0000011c 0000019a 00000000
[ 97.913373] 34dfa000 c174b8e0 f63edd78 00000000 f65458e0 f65458e0 f3fd25a0 0000019a
[ 97.976795] Call Trace:
[ 97.979423] [<c1070315>] ? validate_chain+0x3d5/0x520
[ 98.038568] [<c139efa5>] schedule+0x55/0x60
[ 98.095457] [<c139f6ec>] schedule_timeout+0x19c/0x1b0
[ 98.100976] [<c106d833>] ? lock_release_holdtime+0x73/0xb0
[ 98.159662] [<c139f08d>] ? wait_for_common+0xdd/0x120
[ 98.164966] [<c13a17a2>] ? _raw_spin_unlock_irq+0x22/0x30
[ 98.223160] [<c1071220>] ? trace_hardirqs_on_caller+0x90/0x100
[ 98.283620] [<c107129b>] ? trace_hardirqs_on+0xb/0x10
[ 98.289086] [<c139f094>] wait_for_common+0xe4/0x120
[ 98.342483] [<c1038fd0>] ? put_prev_task+0x40/0x40
[ 98.346653] [<c1038fd0>] ? put_prev_task+0x40/0x40
[ 98.394918] [<c1055389>] ? queue_work_on+0x39/0x40
[ 98.399412] [<c139f0e2>] wait_for_completion+0x12/0x20
[ 98.447573] [<c1054838>] call_usermodehelper_exec+0x88/0x90
[ 98.452396] [<c1070101>] ? validate_chain+0x1c1/0x520
[ 98.501567] [<c139efce>] ? wait_for_common+0x1e/0x120
[ 98.506082] [<c105475f>] ? call_usermodehelper_setup+0x5f/0x90
[ 98.554976] [<c11b91d8>] kobject_uevent_env+0x3d8/0x490
[ 98.559546] [<c11b8d70>] ? kobject_action_type+0x90/0x90
[ 98.608237] [<c11b929a>] kobject_uevent+0xa/0x10
[ 98.612311] [<c107e3c8>] mod_sysfs_setup+0x88/0xc0
[ 98.660562] [<c107ff6d>] load_module+0x1ad/0x240
[ 98.664596] [<c1080051>] sys_init_module+0x41/0x1c0
[ 98.715885] [<c1071220>] ? trace_hardirqs_on_caller+0x90/0x100
[ 98.764376] [<c11c21e4>] ? trace_hardirqs_on_thunk+0xc/0x10
[ 98.769215] [<c13a2049>] syscall_call+0x7/0xb
[ 98.817009] no locks held by modprobe/1158.
[ 98.820564] INFO: task kworker/u:0:1159 blocked for more than 20 seconds.
[ 98.870372] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 98.877038] kworker/u:0 D 0000011c 6644 1159 5 0x00000000
[ 98.926711] f22a1cf4 00000046 c18cf200 0000011c 0000019a 00000000 34dfa000 c16074e0
[ 98.970856] 00000000 f38b08e0 c15557e0 00000000 f38b0908 00000001 f38b0d40 f22a1c88
[ 98.977575] c106febb c174b8e0 f63edd78 00000000 f65458e0 f65458e0 f38b08e0 00000001
[ 99.022205] Call Trace:
[ 99.024099] [<c106febb>] ? check_prevs_add+0xab/0x100
[ 99.065494] [<c10701e7>] ? validate_chain+0x2a7/0x520
[ 99.069244] [<c139efa5>] schedule+0x55/0x60
[ 99.072374] [<c139f6ec>] schedule_timeout+0x19c/0x1b0
[ 99.113619] [<c106d833>] ? lock_release_holdtime+0x73/0xb0
[ 99.117657] [<c1071082>] ? mark_held_locks+0x92/0xf0
[ 99.159953] [<c13a17a2>] ? _raw_spin_unlock_irq+0x22/0x30
[ 99.163931] [<c1071220>] ? trace_hardirqs_on_caller+0x90/0x100
[ 99.168162] [<c107129b>] ? trace_hardirqs_on+0xb/0x10
[ 99.209277] [<c139f094>] wait_for_common+0xe4/0x120
[ 99.212890] [<c1038fd0>] ? put_prev_task+0x40/0x40
[ 99.253976] [<c1038fd0>] ? put_prev_task+0x40/0x40
[ 99.257568] [<c1055389>] ? queue_work_on+0x39/0x40
[ 99.261105] [<c139f0e2>] wait_for_completion+0x12/0x20
[ 99.302652] [<c1054838>] call_usermodehelper_exec+0x88/0x90
[ 99.306783] [<c1070101>] ? validate_chain+0x1c1/0x520
[ 99.348505] [<c139efce>] ? wait_for_common+0x1e/0x120
[ 99.353139] [<c105475f>] ? call_usermodehelper_setup+0x5f/0x90
[ 99.395366] [<c10542c2>] __request_module+0xe2/0x130
[ 99.400335] [<c10dabc8>] ? search_binary_handler+0x158/0x2a0
[ 99.442201] [<c10738b7>] ? __lock_release+0x47/0x70
[ 99.445882] [<c10dabc8>] ? search_binary_handler+0x158/0x2a0
[ 99.449998] [<c11151c0>] ? randomize_stack_top+0x50/0x50
[ 99.753876] [<c11151c0>] ? randomize_stack_top+0x50/0x50
[ 99.757523] [<c10dac64>] search_binary_handler+0x1f4/0x2a0
[ 99.794255] [<c10daaae>] ? search_binary_handler+0x3e/0x2a0
[ 99.797829] [<c10daeab>] do_execve_common+0x19b/0x270
[ 99.801079] [<c10daf94>] do_execve+0x14/0x20
[ 99.837998] [<c100a4e2>] sys_execve+0x42/0x60
[ 99.840850] [<c13a2903>] ptregs_execve+0x13/0x18
[ 99.843811] [<c13a2049>] ? syscall_call+0x7/0xb
[ 99.879826] [<c1007182>] ? kernel_execve+0x22/0x40
[ 99.883203] [<c105444a>] ? ____call_usermodehelper+0x13a/0x160
[ 99.887070] [<c1054310>] ? __request_module+0x130/0x130
[ 99.923601] [<c13a2dba>] ? kernel_thread_helper+0x6/0xd
[ 99.926983] 1 lock held by kworker/u:0/1159:
[ 99.929732] #0: (&sig->cred_guard_mutex){+.+.+.}, at: [<c10da618>] prepare_bprm_creds+0x28/0x70

If khelper becomes multithreaded, this patch will become unnecessary.

> If so then, err, maybe this is OK. But I think the
> analysis should be fully spelled out in the changelog and described in
> the code in some manner which will prevent us from accidentally
> breaking this exclusivity as the code evolves.
>
> Does this look truthful and complete?
>
Yes, thank you.

Maybe kmod_thread_blocker conveys better than kmod_thread_locker?

> --- a/kernel/kmod.c~kmod-avoid-deadlock-by-recursive-kmod-call-fix
> +++ a/kernel/kmod.c
> @@ -44,6 +44,12 @@
> extern int max_threads;
>
> static struct workqueue_struct *khelper_wq;
> +
> +/*
> + * kmod_thread_locker is used for deadlock avoidance. There is no explicit
> + * locking to protect this global - it is private to the singleton khelper
> + * thread and should only ever be modified by that thread.
> + */
> static const struct task_struct *kmod_thread_locker;
>
> #define CAP_BSET (void *)1
> _
>


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