This fixes a race where a thread acquires the console while theconsole is suspended, and the console is resumed before thisthread releases it. In this case, the secondary consolesemaphore would be left locked, and the primary semaphore wouldbe released twice. This in turn would cause the console switchon suspend or resume to hang forever.
Note that suspend_console does not actually lock the consolefor clients that use acquire_console_sem, it only locks it forclients that use try_acquire_console_sem. If we changesuspend_console to fully lock the console, then the kernelmay deadlock on suspend. One client of try_acquire_console_semis acquire_console_semaphore_for_printk, which uses it toprevent printk from using the console while it is suspended.
Signed-off-by: Arve Hjønnevåg <arve@xxxxxxxxxxx>--- kernel/printk.c | 15 +++++++++------ 1 files changed, 9 insertions(+), 6 deletions(-)
diff --git a/kernel/printk.c b/kernel/printk.cindex 69188f2..e3602d0 100644--- a/kernel/printk.c+++ b/kernel/printk.c@@ -73,7 +73,6 @@ EXPORT_SYMBOL(oops_in_progress); * driver system. */ static DECLARE_MUTEX(console_sem);-static DECLARE_MUTEX(secondary_console_sem); struct console *console_drivers; EXPORT_SYMBOL_GPL(console_drivers); @@ -891,12 +890,14 @@ void suspend_console(void) printk("Suspending console(s) (use no_console_suspend to debug)\n"); acquire_console_sem(); console_suspended = 1;+ up(&console_sem); } void resume_console(void) { if (!console_suspend_enabled) return;+ down(&console_sem); console_suspended = 0; release_console_sem(); }@@ -912,11 +913,9 @@ void resume_console(void) void acquire_console_sem(void) { BUG_ON(in_interrupt());- if (console_suspended) {- down(&secondary_console_sem);- return;- } down(&console_sem);+ if (console_suspended)+ return; console_locked = 1; console_may_schedule = 1; }@@ -926,6 +925,10 @@ int try_acquire_console_sem(void) { if (down_trylock(&console_sem)) return -1;+ if (console_suspended) {+ up(&console_sem);+ return -1;+ } console_locked = 1; console_may_schedule = 0; return 0;@@ -979,7 +982,7 @@ void release_console_sem(void) unsigned wake_klogd = 0; if (console_suspended) {- up(&secondary_console_sem);+ up(&console_sem); return; } -- 1.6.1
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