Commit 093a07e2 authored Apr 15, 2008 by Linus Torvalds

Fix locking bug in "acquire_console_semaphore_for_printk()"

When I cleaned up printk() and split up the printk locking logic in
commit 266c2e0a ("Make printk() console
semaphore accesses sensible") I had incorrectly moved the call to
have_callable_console() outside of the console semaphore.

That was buggy.  The console semaphore protects the console_drivers list
that is used by have_callable_console().

Thanks go to Bongani Hlope who saw this as a hang on shutdown and reboot
and bisected the bug to the right commit, and tested this patch. See

	http://lkml.org/lkml/2008/4/11/315



Bisected-and-tested-by: Bongani Hlope <bonganilinux@mweb.co.za>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 0de19a45

kernel/printk.c

+15 −2

Original line number	Diff line number	Diff line
		@@ -643,8 +643,21 @@ static int acquire_console_semaphore_for_printk(unsigned int cpu)
		{
		int retval = 0;

		if (can_use_console(cpu))
		retval = !try_acquire_console_sem();
		if (!try_acquire_console_sem()) {
		retval = 1;

		/*
		* If we can't use the console, we need to release
		* the console semaphore by hand to avoid flushing
		* the buffer. We need to hold the console semaphore
		* in order to do this test safely.
		*/
		if (!can_use_console(cpu)) {
		console_locked = 0;
		up(&console_sem);
		retval = 0;
		}
		}
		printk_cpu = UINT_MAX;
		spin_unlock(&logbuf_lock);
		return retval;

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