From: Linus Torvalds Date: Tue, 15 Apr 2008 20:09:54 +0000 (-0700) Subject: Fix locking bug in "acquire_console_semaphore_for_printk()" X-Git-Tag: v2.6.25~27 X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=093a07e2fdfaddab7fc7d4adc76cc569c86603d7;p=linux-2.6 Fix locking bug in "acquire_console_semaphore_for_printk()" When I cleaned up printk() and split up the printk locking logic in commit 266c2e0abeca649fa6667a1a427ad1da507c6375 ("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 Cc: Alexey Dobriyan Signed-off-by: Linus Torvalds --- diff --git a/kernel/printk.c b/kernel/printk.c index c46a20a19a..bdd4ea8c3f 100644 --- a/kernel/printk.c +++ b/kernel/printk.c @@ -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;