This fixes a race where a thread acquires the console while the console is suspended, and the console is resumed before this thread releases it. In this case, the secondary console semaphore would be left locked, and the primary semaphore would be released twice. This in turn would cause the console switch on suspend or resume to hang forever. Note that suspend_console does not actually lock the console for clients that use acquire_console_sem, it only locks it for clients that use try_acquire_console_sem. If we change suspend_console to fully lock the console, then the kernel may deadlock on suspend. One client of try_acquire_console_sem is acquire_console_semaphore_for_printk, which uses it to prevent printk from using the console while it is suspended.
Showing the changes to kernel (1 of 1 file). Show all
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| @@ -73,7 +73,6 @@ EXPORT_SYMBOL(oops_in_progress); · 72 unmodified lines | ||
| 73 | 73 | * driver system. |
| 74 | 74 | */ |
| 75 | 75 | static DECLARE_MUTEX(console_sem); |
| 76 | −static DECLARE_MUTEX(secondary_console_sem); | |
| 77 | 76 | struct console *console_drivers; |
| 78 | 77 | EXPORT_SYMBOL_GPL(console_drivers); |
| 79 | 78 | |
| @@ -891,12 +890,14 @@ void suspend_console(void) · 811 unmodified lines | ||
| 891 | 890 | printk("Suspending console(s) (use no_console_suspend to debug)\n"); |
| 892 | 891 | acquire_console_sem(); |
| 893 | 892 | console_suspended = 1; |
| 893 | + up(&console_sem); | |
| 894 | 894 | } |
| 895 | 895 | |
| 896 | 896 | void resume_console(void) |
| 897 | 897 | { |
| 898 | 898 | if (!console_suspend_enabled) |
| 899 | 899 | return; |
| 900 | + down(&console_sem); | |
| 900 | 901 | console_suspended = 0; |
| 901 | 902 | release_console_sem(); |
| 902 | 903 | } |
| @@ -912,11 +913,9 @@ void resume_console(void) · 9 unmodified lines | ||
| 912 | 913 | void acquire_console_sem(void) |
| 913 | 914 | { |
| 914 | 915 | BUG_ON(in_interrupt()); |
| 915 | − if (console_suspended) { | |
| 916 | − down(&secondary_console_sem); | |
| 917 | − return; | |
| 918 | − } | |
| 919 | 916 | down(&console_sem); |
| 917 | + if (console_suspended) | |
| 918 | + return; | |
| 920 | 919 | console_locked = 1; |
| 921 | 920 | console_may_schedule = 1; |
| 922 | 921 | } |
| @@ -926,6 +925,10 @@ int try_acquire_console_sem(void) · 3 unmodified lines | ||
| 926 | 925 | { |
| 927 | 926 | if (down_trylock(&console_sem)) |
| 928 | 927 | return -1; |
| 928 | + if (console_suspended) { | |
| 929 | + up(&console_sem); | |
| 930 | + return -1; | |
| 931 | + } | |
| 929 | 932 | console_locked = 1; |
| 930 | 933 | console_may_schedule = 0; |
| 931 | 934 | return 0; |
| @@ -979,7 +982,7 @@ void release_console_sem(void) · 47 unmodified lines | ||
| 979 | 982 | unsigned wake_klogd = 0; |
| 980 | 983 | |
| 981 | 984 | if (console_suspended) { |
| 982 | − up(&secondary_console_sem); | |
| 985 | + up(&console_sem); | |
| 983 | 986 | return; |
| 984 | 987 | } |
| 985 | 988 | |