Commit 6b97a20d authored Mar 17, 2016 by Sergey Senozhatsky Committed by Linus Torvalds Mar 17, 2016

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 8d91f8b1 ("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>

parent a8199371

kernel/printk/printk.c

+14 −9

Original line number	Diff line number	Diff line
		@@ -1757,13 +1757,6 @@ asmlinkage int vprintk_emit(int facility, int level,
		/* If called from the scheduler, we can not call up(). */
		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
		@@ -1771,7 +1764,6 @@ asmlinkage int vprintk_emit(int facility, int level,
		*/
		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);