Re: [PATCH v2 3/3] printk/nmi: Prevent deadlock when accessing the main log buffer in NMI

On (06/27/18 16:20), Petr Mladek wrote:

+/*

• • Marks a code that might produce many messages in NMI context
• • and the risk of losing them is more critical than eventual
• • reordering.
• • It has effect only when called in NMI context. Then printk()
• • will try to store the messages into the main logbuf directly
• • and use the per-CPU buffers only as a fallback when the lock
• • is not available.
• */

+void printk_nmi_direct_enter(void) +{

• if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)

• this_cpu_or(printk_context, PRINTK_NMI_DIRECT_CONTEXT_MASK);
  

+}

A side note: This nesting also handles recursive printk-s for us.

NMI: printk_nmi_enter ftrace_dump printk_nmi_direct_enter vprintk_func spin_lock(logbuf_lock) vprintk_store vsprintf WARN_ON vprintk_func vprintk_nmi

__printf(1, 0) int vprintk_func(const char *fmt, va_list args) {

• /*

• * Try to use the main logbuf even in NMI. But avoid calling console
  

• * drivers that might have their own locks.
  

• */
  

• if ((this_cpu_read(printk_context) & PRINTK_NMI_DIRECT_CONTEXT_MASK) &&

•    raw_spin_trylock(&logbuf_lock)) {
  

• int len;
  

• len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, 0, fmt, args);
  

• raw_spin_unlock(&logbuf_lock);
  

• defer_console();
  

• return len;
  

• }

So, maybe, something a bit better than defer_console().

Otherwise, Acked-by: Sergey Senozhatsky sergey.senozhatsky@gmail.com

-ss