printk: set may_schedule for some of console_trylock() callers

console_unlock() allows to cond_resched() if its caller has set
`console_may_schedule' to 1, since 8d91f8b15361 ("printk: do
cond_resched() between lines while outputting to consoles").

The rules are:
-- console_lock() always sets `console_may_schedule' to 1
-- console_trylock() always sets `console_may_schedule' to 0

However, console_trylock() callers (among them is printk()) do not
always call printk() from atomic contexts, and some of them can
cond_resched() in console_unlock(), so console_trylock() can set
`console_may_schedule' to 1 for such processes.

For !CONFIG_PREEMPT_COUNT kernels, however, console_trylock() always
sets `console_may_schedule' to 0.

It's possible to drop explicit preempt_disable()/preempt_enable() in
vprintk_emit(), because console_unlock() and console_trylock() are now
smart enough:
 a) console_unlock() does not cond_resched() when it's unsafe
    (console_trylock() takes care of that)
 b) console_unlock() does can_use_console() check.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Cc: Jan Kara <jack@suse.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Kyle McMartin <kyle@kernel.org>
Cc: Dave Jones <davej@codemonkey.org.uk>
Cc: Calvin Owens <calvinowens@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 9 deletions

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 2523332bd998..a6d023c3b852 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1758,20 +1758,12 @@ asmlinkage int vprintk_emit(int facility, int level,
         if (!in_sched) {
                 lockdep_off();
                 /*
-                 * Disable preemption to avoid being preempted while holding
-                 * console_sem which would prevent anyone from printing to
-                 * console
-                 */
-                preempt_disable();
-
-                /*
                  * Try to acquire and then immediately release the console
                  * semaphore.  The release will print out buffers and wake up
                  * /dev/kmsg and syslog() users.
                  */
                 if (console_trylock())
                         console_unlock();
-                preempt_enable();
                 lockdep_on();
         }
 
@@ -2122,7 +2114,20 @@ int console_trylock(void)
                 return 0;
         }
         console_locked = 1;
-        console_may_schedule = 0;
+        /*
+         * When PREEMPT_COUNT disabled we can't reliably detect if it's
+         * safe to schedule (e.g. calling printk while holding a spin_lock),
+         * because preempt_disable()/preempt_enable() are just barriers there
+         * and preempt_count() is always 0.
+         *
+         * RCU read sections have a separate preemption counter when
+         * PREEMPT_RCU enabled thus we must take extra care and check
+         * rcu_preempt_depth(), otherwise RCU read sections modify
+         * preempt_count().
+         */
+        console_may_schedule = !oops_in_progress &&
+                        preemptible() &&
+                        !rcu_preempt_depth();
         return 1;
 }
 EXPORT_SYMBOL(console_trylock);