Commit bb4f552a authored by Petr Mladek's avatar Petr Mladek

Merge branch 'for-4.16-console-waiter-logic' into for-4.16

parents 51ccbb0a fd5f7cde
......@@ -1547,6 +1547,146 @@ SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
return do_syslog(type, buf, len, SYSLOG_FROM_READER);
}
/*
* 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].
......@@ -1752,13 +1892,20 @@ 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 @@ void console_unlock(void)
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 */
......
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