Re: [PATCH RESEND] sched: fix false lockdep warning in set_task_cpu()

[Mika Westerberg]

On Tue, Jul 26, 2011 at 02:36:01PM +0300, Mika Westerberg wrote:
> When we have CONFIG_LOCKDEP enabled we get occasionally following warning
> on the system console at boot:

Ping.

Peter, Ingo: any comments on this patch?

> 
>  WARNING: at kernel/sched.c:2204 set_task_cpu+0x1a1/0x390()
>  Modules linked in:
>  Pid: 133, comm: mount Not tainted 3.0.0-rc2+ #70
>  Call Trace:
>   [<c104137d>] warn_slowpath_common+0x6d/0xa0
>   [<c103d3b1>] ? set_task_cpu+0x1a1/0x390
>   [<c103d3b1>] ? set_task_cpu+0x1a1/0x390
>   [<c10413cd>] warn_slowpath_null+0x1d/0x20
>   [<c103d3b1>] set_task_cpu+0x1a1/0x390
>   [<c103e0a7>] try_to_wake_up+0x127/0x250
>   [<c103e1db>] default_wake_function+0xb/0x10
>   [<c106290e>] autoremove_wake_function+0x1e/0x50
>   [<c102a2b0>] __wake_up_common+0x40/0x70
>   [<c102f2d7>] __wake_up+0x37/0x50
>   [<c12d36c0>] ? serial_m3110_enable_ms+0x10/0x10
>   [<c12d3715>] serial_m3110_con_write+0x55/0x60
>   [<c1041575>] __call_console_drivers+0x75/0x90
>   [<c10415d9>] _call_console_drivers+0x49/0x80
>   [<c1041baa>] console_unlock+0xca/0x1f0
>   [<c10420ef>] vprintk+0x18f/0x4f0
>   [<c10787cb>] ? trace_hardirqs_on+0xb/0x10
>   [<c107985e>] ? lockdep_init_map+0x4e/0x4c0
>   [<c10787cb>] ? trace_hardirqs_on+0xb/0x10
>   [<c14928a3>] printk+0x18/0x1a
>   [<c115dd4e>] ext3_msg+0x3e/0x40
>   [<c116046a>] ext3_fill_super+0x16ba/0x1960
>   [<c11478b8>] ? disk_name+0x88/0xc0
>   [<c10fa88d>] mount_bdev+0x16d/0x1b0
>   [<c10ed667>] ? __kmalloc_track_caller+0xf7/0x280
>   [<c1112021>] ? alloc_vfsmnt+0x81/0x140
>   [<c115d20a>] ext3_mount+0x1a/0x20
>   [<c115edb0>] ? ext3_clear_journal_err+0xa0/0xa0
>   [<c10f95dc>] mount_fs+0x1c/0xc0
>   [<c10d763a>] ? __alloc_percpu+0xa/0x10
>   [<c111203b>] ? alloc_vfsmnt+0x9b/0x140
>   [<c1112896>] vfs_kern_mount+0x46/0xa0
>   [<c1113609>] do_kern_mount+0x39/0xd0
>   [<c1114087>] do_mount+0x2c7/0x680
>   [<c10d31f9>] ? strndup_user+0x49/0x60
>   [<c11144a6>] sys_mount+0x66/0xa0
>   [<c14976d0>] sysenter_do_call+0x12/0x36
> 
> This is due the fact that printk() disables lockdep before the output is
> passed to the actual console driver. If the console driver happens to call
> some functions which eventually calls set_task_cpu() we end up failing the
> following test:
> 
>        WARN_ON_ONCE(debug_locks && !(lockdep_is_held(&p->pi_lock) ||
>                                      lockdep_is_held(&task_rq(p)->lock)));
> 
> This is because lockdep_is_held() returns 0 if lockdep is temporarily
> disabled.
> 
> So we check whether lockdep is enabled and only then validate the locks.
> 
> Signed-off-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxx>
> ---
>  kernel/sched.c |    5 +++--
>  1 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 9769c75..1177f81 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2211,8 +2211,9 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
>  	 * Furthermore, all task_rq users should acquire both locks, see
>  	 * task_rq_lock().
>  	 */
> -	WARN_ON_ONCE(debug_locks && !(lockdep_is_held(&p->pi_lock) ||
> -				      lockdep_is_held(&task_rq(p)->lock)));
> +	WARN_ON_ONCE(debug_locks && !current->lockdep_recursion &&
> +			!(lockdep_is_held(&p->pi_lock) ||
> +			  lockdep_is_held(&task_rq(p)->lock)));
>  #endif
>  #endif
>  
> -- 
> 1.7.5.4
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