• Petr Mladek's avatar
    printk: Correctly handle preemption in console_unlock() · 257ab443
    Petr Mladek authored
    Some console drivers code calls console_conditional_schedule()
    that looks at @console_may_schedule. The value must be cleared
    when the drivers are called from console_unlock() with
    interrupts disabled. But rescheduling is fine when the same
    code is called, for example, from tty operations where the
    console semaphore is taken via console_lock().
    
    This is why @console_may_schedule is cleared before calling console
    drivers. The original value is stored to decide if we could sleep
    between lines.
    
    Now, @console_may_schedule is not cleared when we call
    console_trylock() and jump back to the "again" goto label.
    This has become a problem, since the commit 6b97a20d
    ("printk: set may_schedule for some of console_trylock() callers").
    @console_may_schedule might get enabled now.
    
    There is also the opposite problem. console_lock() can be called
    only from preemptive context. It can always enable scheduling in
    the console code. But console_trylock() is not able to detect it
    when CONFIG_PREEMPT_COUNT is disabled. Therefore we should use the
    original @console_may_schedule value after re-acquiring
    the console semaphore in console_unlock().
    
    This patch solves both problems by moving the "again" goto label.
    
    Alternative solution was to clear and restore the value around
    call_console_drivers(). Then console_conditional_schedule() could
    be used also inside console_unlock(). But there was a potential race
    with console_flush_on_panic() as reported by Sergey Senozhatsky.
    That function should be called only where there is only one CPU
    and with interrupts disabled. But better be on the safe side
    because stopping CPUs might fail.
    
    Fixes: 6b97a20d ("printk: set may_schedule for some of console_trylock() callers")
    Link: http://lkml.kernel.org/r/1490372045-22288-1-git-send-email-pmladek@suse.com
    
    Suggested-by: default avatarTetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Cc: Jiri Slaby <jslaby@suse.cz>
    Cc: linux-fbdev@vger.kernel.org
    Cc: linux-kernel@vger.kernel.org
    Reviewed-by: default avatarSergey Senozhatsky <sergey.senozhatsky@gmail.com>
    Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
    257ab443
printk.c 78.5 KB