On Fri, 12 Jan 2018 17:08:37 +0100 Petr Mladek <[email protected]> wrote:
> On Fri 2018-01-12 10:37:54, Steven Rostedt wrote: > > On Thu, 11 Jan 2018 13:03:41 +0100 > > Petr Mladek <[email protected]> wrote: > > > All the other changes look good to me. I will use them in the next > > > version. > > > > Great. > > Please, find below the updated version. If I get Ack at least from > Steven and no nack's, I will put it into linux-next next week. > Typos below. > > >From f67f70d910d9cf310a7bc73e97bf14097d31b059 Mon Sep 17 00:00:00 2001 > From: Petr Mladek <[email protected]> > Date: Fri, 22 Dec 2017 18:58:46 +0100 > Subject: [PATCH v6 2/4] printk: Hide console waiter logic into helpers > > The commit ("printk: Add console owner and waiter logic to load balance > console writes") made vprintk_emit() and console_unlock() even more > complicated. > > This patch extracts the new code into 3 helper functions. They should > help to keep it rather self-contained. It will be easier to use and > maintain. > > This patch just shuffles the existing code. It does not change > the functionality. > > Signed-off-by: Petr Mladek <[email protected]> > --- > Changes against v5: > > + updated some comments (Steven) > + do console_trylock() in console_trylock_spinning() (Steven) > > kernel/printk/printk.c | 245 > +++++++++++++++++++++++++++++-------------------- > 1 file changed, 148 insertions(+), 97 deletions(-) > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 7e6459abba43..3057dbc69b4f 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -86,15 +86,8 @@ EXPORT_SYMBOL_GPL(console_drivers); > static struct lockdep_map console_lock_dep_map = { > .name = "console_lock" > }; > -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; > - > enum devkmsg_log_bits { > __DEVKMSG_LOG_BIT_ON = 0, > __DEVKMSG_LOG_BIT_OFF, > @@ -1551,6 +1544,146 @@ 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 not longer "it is no longer safe" > + * 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 the busy waiter. "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 a special marked sections. It means that "running in specially marked sections." > + * the current owner is running and cannot reschedule until it > + * is ready to loose the lock. "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. > @@ -1760,56 +1893,8 @@ asmlinkage int vprintk_emit(int facility, int level, > * semaphore. The release will print out buffers and wake up > * /dev/kmsg and syslog() users. > */ > - if (console_trylock()) { > + if (console_trylock_spinning()) > console_unlock(); > - } else { > - struct task_struct *owner = NULL; > - bool waiter; > - bool spin = false; > - > - 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) { > - /* 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_); > - console_unlock(); > - printk_safe_enter_irqsave(flags); > - } > - printk_safe_exit_irqrestore(flags); > - > - } > } > > return printed_len; > @@ -1910,6 +1995,8 @@ 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, > @@ -2196,7 +2283,6 @@ void console_unlock(void) > static u64 seen_seq; > unsigned long flags; > bool wake_klogd = false; > - bool waiter = false; > bool do_cond_resched, retry; > > if (console_suspended) { > @@ -2291,31 +2377,16 @@ void console_unlock(void) > * finish. This task can not be preempted if there is a > * waiter waiting to take over. > */ > - 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_enable(); > > stop_critical_timings(); /* don't trace print latency */ > call_console_drivers(ext_text, ext_len, text, len); > start_critical_timings(); > > - raw_spin_lock(&console_owner_lock); > - waiter = READ_ONCE(console_waiter); > - console_owner = NULL; > - raw_spin_unlock(&console_owner_lock); > - > - /* > - * If there is a waiter waiting for us, then pass the > - * rest of the work load over to that waiter. > - */ > - if (waiter) > - break; > - > - /* There was no waiter, and nothing will spin on us here */ > - spin_release(&console_owner_dep_map, 1, _THIS_IP_); > + if (console_lock_spinning_disable_and_check()) { > + printk_safe_exit_irqrestore(flags); > + return; > + } > > printk_safe_exit_irqrestore(flags); > > @@ -2323,26 +2394,6 @@ void console_unlock(void) > cond_resched(); > } > > - /* > - * If there is an active waiter waiting on the console_lock. > - * Pass off the printing to the waiter, and the waiter > - * will continue printing on its CPU, and when all writing > - * has finished, the last printer will wake up klogd. > - */ > - if (waiter) { > - WRITE_ONCE(console_waiter, false); > - /* The waiter is now free to continue */ > - 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_); > - printk_safe_exit_irqrestore(flags); > - /* Note, if waiter is set, logbuf_lock is not held */ > - return; > - } > - > console_locked = 0; > > /* Release the exclusive_console once it is used */ Besides the typos (which should be fixed)... Reviewed-by: Steven Rostedt (VMware) <[email protected]> -- Steve
