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SubjectRe: [PATCHv7 6/8] printk: use printk_safe buffers in printk
On Tue, 27 Dec 2016 23:16:09 +0900
Sergey Senozhatsky <sergey.senozhatsky@gmail.com> 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@gmail.com>
> Reviewed-by: Petr Mladek <pmladek@suse.com>
> ---

Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

-- Steve

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