diff options
| -rw-r--r-- | kernel/printk/printk.c | 181 |
1 files changed, 166 insertions, 15 deletions
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index bf2e6741ec12..63416bb2712a 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1548,6 +1548,146 @@ SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len) } /* + * Special console_lock variants that help to reduce the risk of soft-lockups. + * They allow to pass console_lock to another printk() call using a busy wait. + */ + +#ifdef CONFIG_LOCKDEP +static struct lockdep_map console_owner_dep_map = { + .name = "console_owner" +}; +#endif + +static DEFINE_RAW_SPINLOCK(console_owner_lock); +static struct task_struct *console_owner; +static bool console_waiter; + +/** + * console_lock_spinning_enable - mark beginning of code where another + * thread might safely busy wait + * + * This basically converts console_lock into a spinlock. This marks + * the section where the console_lock owner can not sleep, because + * there may be a waiter spinning (like a spinlock). Also it must be + * ready to hand over the lock at the end of the section. + */ +static void console_lock_spinning_enable(void) +{ + raw_spin_lock(&console_owner_lock); + console_owner = current; + raw_spin_unlock(&console_owner_lock); + + /* The waiter may spin on us after setting console_owner */ + spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_); +} + +/** + * console_lock_spinning_disable_and_check - mark end of code where another + * thread was able to busy wait and check if there is a waiter + * + * This is called at the end of the section where spinning is allowed. + * It has two functions. First, it is a signal that it is no longer + * safe to start busy waiting for the lock. Second, it checks if + * there is a busy waiter and passes the lock rights to her. + * + * Important: Callers lose the lock if there was a busy waiter. + * They must not touch items synchronized by console_lock + * in this case. + * + * Return: 1 if the lock rights were passed, 0 otherwise. + */ +static int console_lock_spinning_disable_and_check(void) +{ + int waiter; + + raw_spin_lock(&console_owner_lock); + waiter = READ_ONCE(console_waiter); + console_owner = NULL; + raw_spin_unlock(&console_owner_lock); + + if (!waiter) { + spin_release(&console_owner_dep_map, 1, _THIS_IP_); + return 0; + } + + /* The waiter is now free to continue */ + WRITE_ONCE(console_waiter, false); + + spin_release(&console_owner_dep_map, 1, _THIS_IP_); + + /* + * Hand off console_lock to waiter. The waiter will perform + * the up(). After this, the waiter is the console_lock owner. + */ + mutex_release(&console_lock_dep_map, 1, _THIS_IP_); + return 1; +} + +/** + * console_trylock_spinning - try to get console_lock by busy waiting + * + * This allows to busy wait for the console_lock when the current + * owner is running in specially marked sections. It means that + * the current owner is running and cannot reschedule until it + * is ready to lose the lock. + * + * Return: 1 if we got the lock, 0 othrewise + */ +static int console_trylock_spinning(void) +{ + struct task_struct *owner = NULL; + bool waiter; + bool spin = false; + unsigned long flags; + + if (console_trylock()) + return 1; + + printk_safe_enter_irqsave(flags); + + raw_spin_lock(&console_owner_lock); + owner = READ_ONCE(console_owner); + waiter = READ_ONCE(console_waiter); + if (!waiter && owner && owner != current) { + WRITE_ONCE(console_waiter, true); + spin = true; + } + raw_spin_unlock(&console_owner_lock); + + /* + * If there is an active printk() writing to the + * consoles, instead of having it write our data too, + * see if we can offload that load from the active + * printer, and do some printing ourselves. + * Go into a spin only if there isn't already a waiter + * spinning, and there is an active printer, and + * that active printer isn't us (recursive printk?). + */ + if (!spin) { + printk_safe_exit_irqrestore(flags); + return 0; + } + + /* We spin waiting for the owner to release us */ + spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_); + /* Owner will clear console_waiter on hand off */ + while (READ_ONCE(console_waiter)) + cpu_relax(); + spin_release(&console_owner_dep_map, 1, _THIS_IP_); + + printk_safe_exit_irqrestore(flags); + /* + * The owner passed the console lock to us. + * Since we did not spin on console lock, annotate + * this as a trylock. Otherwise lockdep will + * complain. + */ + mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_); + + return 1; +} + +/* * Call the console drivers, asking them to write out * log_buf[start] to log_buf[end - 1]. * The console_lock must be held. @@ -1753,12 +1893,19 @@ asmlinkage int vprintk_emit(int facility, int level, /* If called from the scheduler, we can not call up(). */ if (!in_sched) { /* + * Disable preemption to avoid being preempted while holding + * console_sem which would prevent anyone from printing to + * console + */ + preempt_disable(); + /* * Try to acquire and then immediately release the console * semaphore. The release will print out buffers and wake up * /dev/kmsg and syslog() users. */ - if (console_trylock()) + if (console_trylock_spinning()) console_unlock(); + preempt_enable(); } return printed_len; @@ -1859,6 +2006,8 @@ static ssize_t msg_print_ext_header(char *buf, size_t size, static ssize_t msg_print_ext_body(char *buf, size_t size, char *dict, size_t dict_len, char *text, size_t text_len) { return 0; } +static void console_lock_spinning_enable(void) { } +static int console_lock_spinning_disable_and_check(void) { return 0; } static void call_console_drivers(const char *ext_text, size_t ext_len, const char *text, size_t len) {} static size_t msg_print_text(const struct printk_log *msg, @@ -2084,20 +2233,7 @@ int console_trylock(void) return 0; } console_locked = 1; - /* - * When PREEMPT_COUNT disabled we can't reliably detect if it's - * safe to schedule (e.g. calling printk while holding a spin_lock), - * because preempt_disable()/preempt_enable() are just barriers there - * and preempt_count() is always 0. - * - * RCU read sections have a separate preemption counter when - * PREEMPT_RCU enabled thus we must take extra care and check - * rcu_preempt_depth(), otherwise RCU read sections modify - * preempt_count(). - */ - console_may_schedule = !oops_in_progress && - preemptible() && - !rcu_preempt_depth(); + console_may_schedule = 0; return 1; } EXPORT_SYMBOL(console_trylock); @@ -2247,14 +2383,29 @@ skip: console_seq++; raw_spin_unlock(&logbuf_lock); + /* + * While actively printing out messages, if another printk() + * were to occur on another CPU, it may wait for this one to + * finish. This task can not be preempted if there is a + * waiter waiting to take over. + */ + console_lock_spinning_enable(); + stop_critical_timings(); /* don't trace print latency */ call_console_drivers(ext_text, ext_len, text, len); start_critical_timings(); + + if (console_lock_spinning_disable_and_check()) { + printk_safe_exit_irqrestore(flags); + return; + } + printk_safe_exit_irqrestore(flags); if (do_cond_resched) cond_resched(); } + console_locked = 0; /* Release the exclusive_console once it is used */ |