linux-kernel - Re: [PATCH 2/8] printk: Release lockbuf_lock before calling console_trylock_for

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Message-Id: <20140423135606.d727fe5eb38bd8805ad3a5c3@linux-foundation.org>
Date:	Wed, 23 Apr 2014 13:56:06 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jan Kara <jack@...e.cz>
Cc:	LKML <linux-kernel@...r.kernel.org>, pmladek@...e.cz,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 2/8] printk: Release lockbuf_lock before calling
 console_trylock_for_printk()

On Tue, 25 Mar 2014 18:54:55 +0100 Jan Kara <jack@...e.cz> wrote:

> There's no reason to hold lockbuf_lock when entering
> console_trylock_for_printk(). The first thing this function does is
> calling down_trylock(console_sem) and if that fails it immediately
> unlocks lockbuf_lock. So lockbuf_lock isn't needed for that branch.
> When down_trylock() succeeds, the rest of console_trylock() is OK
> without lockbuf_lock (it is called without it from other places), and
> the only remaining thing in console_trylock_for_printk() is
> can_use_console() call. For that call console_sem is enough (it
> iterates all consoles and checks CON_ANYTIME flag).
> 
> So we drop logbuf_lock before entering console_trylock_for_printk()
> which simplifies the code.
> 

I suppose we should document have_callable_console()'s locking
requirements, and I don't think that "early in boot" part is true?

--- a/kernel/printk/printk.c~printk-release-lockbuf_lock-before-calling-console_trylock_for_printk-fix
+++ a/kernel/printk/printk.c
@@ -1377,7 +1377,9 @@ static void zap_locks(void)
 	sema_init(&console_sem, 1);
 }
 
-/* Check if we have any console registered that can be called early in boot. */
+/*
+ * Check if we have any console registered.  Requires console_sem.
+ */
 static int have_callable_console(void)
 {
 	struct console *con;
_

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