• sergey-senozhatsky's avatar
    printk: introduce per-cpu safe_print seq buffer · 099f1c84
    sergey-senozhatsky authored
    This patch extends the idea of NMI per-cpu buffers to regions
    that may cause recursive printk() calls and possible deadlocks.
    Namely, printk() can't handle printk calls from schedule code
    or printk() calls from lock debugging code (spin_dump() for instance);
    because those may be called with `sem->lock' already taken or any
    other `critical' locks (p->pi_lock, etc.). An example of deadlock
    can be
    
     vprintk_emit()
      console_unlock()
       up()                        << raw_spin_lock_irqsave(&sem->lock, flags);
        wake_up_process()
         try_to_wake_up()
          ttwu_queue()
           ttwu_activate()
            activate_task()
             enqueue_task()
              enqueue_task_fair()
               cfs_rq_of()
                task_of()
                 WARN_ON_ONCE(!entity_is_task(se))
                  vprintk_emit()
                   console_trylock()
                    down_trylock()
                     raw_spin_lock_irqsave(&sem->lock, flags)
                     ^^^^ deadlock
    
    and some other cases.
    
    Just like in NMI implementation, the solution uses a per-cpu
    `printk_func' pointer to 'redirect' printk() calls to a 'safe'
    callback, that store messages in a per-cpu buffer and flushes
    them back to logbuf buffer later.
    
    Usage example:
    
     printk()
      printk_safe_enter_irqsave(flags)
      //
      //  any printk() call from here will endup in vprintk_safe(),
      //  that stores messages in a special per-CPU buffer.
      //
      printk_safe_exit_irqrestore(flags)
    
    The 'redirection' mechanism, though, has been reworked, as suggested
    by Petr Mladek. Instead of using a per-cpu @print_func callback we now
    keep a per-cpu printk-context variable and call either default or nmi
    vprintk function depending on its value. printk_nmi_entrer/exit and
    printk_safe_enter/exit, thus, just set/celar corresponding bits in
    printk-context functions.
    
    The patch only adds printk_safe support, we don't use it yet.
    
    Link: http://lkml.kernel.org/r/20161227141611.940-4-sergey.senozhatsky@gmail.com
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Jan Kara <jack@suse.cz>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: Calvin Owens <calvinowens@fb.com>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Peter Hurley <peter@hurleysoftware.com>
    Cc: linux-kernel@vger.kernel.org
    Signed-off-by: sergey-senozhatsky's avatarSergey Senozhatsky <sergey.senozhatsky@gmail.com>
    Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
    Reviewed-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
    099f1c84
Makefile 103 Bytes