Messages in this thread | | | Date | Wed, 1 Feb 2017 12:04:47 -0500 | From | Steven Rostedt <> | Subject | Re: [PATCHv7 6/8] printk: use printk_safe buffers in printk |
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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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