printk: queue wake_up_klogd irq_work only if per-CPU areas are ready

printk_deferred(), similarly to printk_safe/printk_nmi, does not
immediately attempt to print a new message on the consoles, avoiding
calls into non-reentrant kernel paths, e.g. scheduler or timekeeping,
which potentially can deadlock the system.

Those printk() flavors, instead, rely on per-CPU flush irq_work to print
messages from safer contexts.  For same reasons (recursive scheduler or
timekeeping calls) printk() uses per-CPU irq_work in order to wake up
user space syslog/kmsg readers.

However, only printk_safe/printk_nmi do make sure that per-CPU areas
have been initialised and that it's safe to modify per-CPU irq_work.
This means that, for instance, should printk_deferred() be invoked "too
early", that is before per-CPU areas are initialised, printk_deferred()
will perform illegal per-CPU access.

Lech Perczak [0] reports that after commit 1b710b1b10ef ("char/random:
silence a lockdep splat with printk()") user-space syslog/kmsg readers
are not able to read new kernel messages.

The reason is printk_deferred() being called too early (as was pointed
out by Petr and John).

Fix printk_deferred() and do not queue per-CPU irq_work before per-CPU
areas are initialized.

Link: https://lore.kernel.org/lkml/aa0732c6-5c4e-8a8b-a1c1-75ebe3dca05b@camlintechnologies.com/
Reported-by: Lech Perczak <l.perczak@camlintechnologies.com>
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Tested-by: Jann Horn <jannh@google.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 10 deletions

diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index b4045e782743..d9a659a686f3 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -27,7 +27,6 @@
  * There are situations when we want to make sure that all buffers
  * were handled or when IRQs are blocked.
  */
-static int printk_safe_irq_ready __read_mostly;
 
 #define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) -	\
 				sizeof(atomic_t) -			\
@@ -51,7 +50,7 @@ static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
 /* Get flushed in a more safe context. */
 static void queue_flush_work(struct printk_safe_seq_buf *s)
 {
-	if (printk_safe_irq_ready)
+	if (printk_percpu_data_ready())
 		irq_work_queue(&s->work);
 }
 
@@ -402,14 +401,6 @@ void __init printk_safe_init(void)
 #endif
 	}
 
-	/*
-	 * In the highly unlikely event that a NMI were to trigger at
-	 * this moment. Make sure IRQ work is set up before this
-	 * variable is set.
-	 */
-	barrier();
-	printk_safe_irq_ready = 1;
-
 	/* Flush pending messages that did not have scheduled IRQ works. */
 	printk_safe_flush();
 }