path: root/kernel/printk/printk_safe.c
diff options
authorPetr Mladek <>2017-04-20 10:52:31 +0200
committerPetr Mladek <>2017-05-19 14:42:19 +0200
commit719f6a7040f1bdaf96fcc709d272548facb88e90 (patch)
tree050a91d990c5e870a5b19a9d77d0965e599544ef /kernel/printk/printk_safe.c
parent9c35baf6cee9a5745d55de6f9995916dde642517 (diff)
printk: Use the main logbuf in NMI when logbuf_lock is available
The commit 42a0bb3f71383b457a7d ("printk/nmi: generic solution for safe printk in NMI") caused that printk stores messages into a temporary buffer in NMI context. The buffer is per-CPU and therefore the size is rather limited. It works quite well for NMI backtraces. But there are longer logs that might get printed in NMI context, for example, lockdep warnings, ftrace_dump_on_oops. The temporary buffer is used to avoid deadlocks caused by logbuf_lock. Also it is needed to avoid races with the other temporary buffer that is used when PRINTK_SAFE_CONTEXT is entered. But the main buffer can be used in NMI if the lock is available and we did not interrupt PRINTK_SAFE_CONTEXT. The lock is checked using raw_spin_is_locked(). It might cause false negatives when the lock is taken on another CPU and this CPU is in the safe context from other reasons. Note that the safe context is used also to get console semaphore or when calling console drivers. For this reason, we do the check in printk_nmi_enter(). It makes the handling consistent for the entire NMI handler and avoids reshuffling of the messages. The patch also defines special printk context that allows to use printk_deferred() in NMI. Note that we could not flush the messages to the consoles because console drivers might use many other internal locks. The newly created vprintk_deferred() disables the preemption only around the irq work handling. It is needed there to keep the consistency between the two per-CPU variables. But there is no reason to disable preemption around vprintk_emit(). Finally, the patch puts back explicit serialization of the NMI backtraces from different CPUs. It was removed by the commit a9edc88093287183ac934b ("x86/nmi: Perform a safe NMI stack trace on all CPUs"). It was not needed because the flushing of the temporary per-CPU buffers was serialized. Link: Cc: Steven Rostedt <> Cc: Andrew Morton <> Cc: Peter Zijlstra <> Cc: Russell King <> Cc: Daniel Thompson <> Cc: Ingo Molnar <> Cc: Thomas Gleixner <> Cc: Chris Metcalf <> Cc: Cc: Cc: Suggested-by: Sergey Senozhatsky <> Acked-by: Sergey Senozhatsky <> Signed-off-by: Petr Mladek <>
Diffstat (limited to 'kernel/printk/printk_safe.c')
1 files changed, 24 insertions, 2 deletions
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index 033e50a7d706..03a42a539b20 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -308,12 +308,24 @@ static int vprintk_nmi(const char *fmt, va_list args)
void printk_nmi_enter(void)
- this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
+ /*
+ * The size of the extra per-CPU buffer is limited. Use it only when
+ * the main one is locked. If this CPU is not in the safe context,
+ * the lock must be taken on another CPU and we could wait for it.
+ */
+ if ((this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK) &&
+ raw_spin_is_locked(&logbuf_lock)) {
+ this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
+ } else {
+ this_cpu_or(printk_context, PRINTK_NMI_DEFERRED_CONTEXT_MASK);
+ }
void printk_nmi_exit(void)
- this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
+ this_cpu_and(printk_context,
@@ -351,12 +363,22 @@ void __printk_safe_exit(void)
__printf(1, 0) int vprintk_func(const char *fmt, va_list args)
+ /* Use extra buffer in NMI when logbuf_lock is taken or in safe mode. */
if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
return vprintk_nmi(fmt, args);
+ /* Use extra buffer to prevent a recursion deadlock in safe mode. */
if (this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK)
return vprintk_safe(fmt, args);
+ /*
+ * Use the main logbuf when logbuf_lock is available in NMI.
+ * But avoid calling console drivers that might have their own locks.
+ */
+ if (this_cpu_read(printk_context) & PRINTK_NMI_DEFERRED_CONTEXT_MASK)
+ return vprintk_deferred(fmt, args);
+ /* No obstacles. */
return vprintk_default(fmt, args);