Re: [PATCHv7 6/8] printk: use printk_safe buffers in printk

[Steven Rostedt]

On Tue, 27 Dec 2016 23:16:09 +0900
Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx> wrote:

> Use printk_safe per-CPU buffers in printk recursion-prone blocks:
> -- around logbuf_lock protected sections in vprintk_emit() and
>    console_unlock()
> -- around down_trylock_console_sem() and up_console_sem()
> 
> Note that this solution addresses deadlocks caused by printk()
> recursive calls only. That is vprintk_emit() and console_unlock().
> The rest will be converted in a followup patch.
> 
> Another thing to note is that we now keep lockdep enabled in printk,
> because we are protected against the printk recursion caused by
> lockdep in vprintk_emit() by the printk-safe mechanism - we first
> switch to per-CPU buffers and only then access the deadlock-prone
> locks.
> 
> Examples:
> 
> 1) printk() from logbuf_lock spin_lock section
> 
> Assume the following code:
>   printk()
>     raw_spin_lock(&logbuf_lock);
>     WARN_ON(1);
>     raw_spin_unlock(&logbuf_lock);
> 
> which now produces:
> 
>  ------------[ cut here ]------------
>  WARNING: CPU: 0 PID: 366 at kernel/printk/printk.c:1811 vprintk_emit
>  CPU: 0 PID: 366 Comm: bash
>  Call Trace:
>    warn_slowpath_null+0x1d/0x1f
>    vprintk_emit+0x1cd/0x438
>    vprintk_default+0x1d/0x1f
>    printk+0x48/0x50
>   [..]
> 
> 2) printk() from semaphore sem->lock spin_lock section
> 
> Assume the following code
> 
>   printk()
>     console_trylock()
>       down_trylock()
>         raw_spin_lock_irqsave(&sem->lock, flags);
>         WARN_ON(1);
>         raw_spin_unlock_irqrestore(&sem->lock, flags);
> 
> which now produces:
> 
>  ------------[ cut here ]------------
>  WARNING: CPU: 1 PID: 363 at kernel/locking/semaphore.c:141 down_trylock
>  CPU: 1 PID: 363 Comm: bash
>  Call Trace:
>    warn_slowpath_null+0x1d/0x1f
>    down_trylock+0x3d/0x62
>    ? vprintk_emit+0x3f9/0x414
>    console_trylock+0x31/0xeb
>    vprintk_emit+0x3f9/0x414
>    vprintk_default+0x1d/0x1f
>    printk+0x48/0x50
>   [..]
> 
> 3) printk() from console_unlock()
> 
> Assume the following code:
> 
>   printk()
>     console_unlock()
>       raw_spin_lock(&logbuf_lock);
>       WARN_ON(1);
>       raw_spin_unlock(&logbuf_lock);
> 
> which now produces:
> 
>  ------------[ cut here ]------------
>  WARNING: CPU: 1 PID: 329 at kernel/printk/printk.c:2384 console_unlock
>  CPU: 1 PID: 329 Comm: bash
>  Call Trace:
>    warn_slowpath_null+0x18/0x1a
>    console_unlock+0x12d/0x559
>    ? trace_hardirqs_on_caller+0x16d/0x189
>    ? trace_hardirqs_on+0xd/0xf
>    vprintk_emit+0x363/0x374
>    vprintk_default+0x18/0x1a
>    printk+0x43/0x4b
>   [..]
> 
> 4) printk() from try_to_wake_up()
> 
> Assume the following code:
> 
>   printk()
>     console_unlock()
>       up()
>         try_to_wake_up()
>           raw_spin_lock_irqsave(&p->pi_lock, flags);
>           WARN_ON(1);
>           raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> 
> which now produces:
> 
>  ------------[ cut here ]------------
>  WARNING: CPU: 3 PID: 363 at kernel/sched/core.c:2028 try_to_wake_up
>  CPU: 3 PID: 363 Comm: bash
>  Call Trace:
>    warn_slowpath_null+0x1d/0x1f
>    try_to_wake_up+0x7f/0x4f7
>    wake_up_process+0x15/0x17
>    __up.isra.0+0x56/0x63
>    up+0x32/0x42
>    __up_console_sem+0x37/0x55
>    console_unlock+0x21e/0x4c2
>    vprintk_emit+0x41c/0x462
>    vprintk_default+0x1d/0x1f
>    printk+0x48/0x50
>   [..]
> 
> 5) printk() from call_console_drivers()
> 
> Assume the following code:
>   printk()
>     console_unlock()
>       call_console_drivers()
>       ...
>           WARN_ON(1);
> 
> which now produces:
> 
>  ------------[ cut here ]------------
>  WARNING: CPU: 2 PID: 305 at kernel/printk/printk.c:1604 call_console_drivers
>  CPU: 2 PID: 305 Comm: bash
>  Call Trace:
>    warn_slowpath_null+0x18/0x1a
>    call_console_drivers.isra.6.constprop.16+0x3a/0xb0
>    console_unlock+0x471/0x48e
>    vprintk_emit+0x1f4/0x206
>    vprintk_default+0x18/0x1a
>    vprintk_func+0x6e/0x70
>    printk+0x3e/0x46
>   [..]
> 
> 6) unsupported placeholder in printk() format now prints an actual
>    warning from vscnprintf(), instead of
>    	'BUG: recent printk recursion!'.
> 
>  ------------[ cut here ]------------
>  WARNING: CPU: 5 PID: 337 at lib/vsprintf.c:1900 format_decode
>  Please remove unsupported %
>   in format string
>  CPU: 5 PID: 337 Comm: bash
>  Call Trace:
>    dump_stack+0x4f/0x65
>    __warn+0xc2/0xdd
>    warn_slowpath_fmt+0x4b/0x53
>    format_decode+0x22c/0x308
>    vsnprintf+0x89/0x3b7
>    vscnprintf+0xd/0x26
>    vprintk_emit+0xb4/0x238
>    vprintk_default+0x1d/0x1f
>    vprintk_func+0x6c/0x73
>    printk+0x43/0x4b
>   [..]
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
> Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
> ---

Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

-- Steve