linux-kernel - Re: [RFC][PATCH -next 2/2] printk: set may_schedule for some of console

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Date:	Sun, 17 Jan 2016 23:11:37 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Jan Kara <jack@...e.com>,
	Petr Mladek <pmladek@...e.com>,
	Kyle McMartin <kyle@...nel.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Calvin Owens <calvinowens@...com>,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC][PATCH -next 2/2] printk: set may_schedule for some of
 console_trylock callers

On (01/14/16 13:57), Sergey Senozhatsky wrote:
[..]
> @@ -2138,7 +2130,10 @@ int console_trylock(void)
>  		return 0;
>  	}
>  	console_locked = 1;
> -	console_may_schedule = 0;
> +	console_may_schedule = !(oops_in_progress ||
> +			in_interrupt() ||
> +			irqs_disabled() ||
> +			in_atomic());
>  	return 1;
>  }

should check rcu_preempt_depth() on preempt RCU kernels.
and in_interrupt() can be dropped, in_atomic() does
preempt_count != 0.

===8<===8<===

 console_unlock() allows to cond_resched() if its caller has
set `console_may_schedule' to 1 (this functionality present
since commit '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()) are
not necessarily executing in atomic contexts, and some of them
can cond_resched() in console_unlock(). So console_trylock()
can set `console_may_schedule' to 0 only if cond_resched() is
invalid in the current context, and set it to 1 otherwise.

The patch also drops explicit preempt_disable()/preempt_enable()
calls in vprintk_emit().

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
---
V2: rcu_preempt_depth() on preempt RCU kernels

 kernel/printk/printk.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index ae641d7..5752c0e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1777,20 +1777,12 @@ 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_for_printk())
 			console_unlock();
-		preempt_enable();
 		lockdep_on();
 	}
 
@@ -2123,7 +2115,10 @@ int console_trylock(void)
 		return 0;
 	}
 	console_locked = 1;
-	console_may_schedule = 0;
+	console_may_schedule = !(oops_in_progress ||
+			irqs_disabled() ||
+			in_atomic(),
+			rcu_preempt_depth());
 	return 1;
 }
 EXPORT_SYMBOL(console_trylock);
-- 
2.7.0