On Mon 2014-05-05 19:18:46, Steven Rostedt wrote: > To prevent deadlocks with doing a printk inside the scheduler, > printk_sched() was created. The issue is that printk has a console_sem > that it can grab and release. The release does a wake up if there's a > task pending on the sem, and this wake up grabs the rq locks that is > held in the scheduler. This leads to a possible deadlock if the wake up > uses the same rq as the one with the rq lock held already. > > What printk_sched() does is to save the printk write in a per cpu buffer > and sets the PRINTK_PENDING_SCHED flag. On a timer tick, if this flag is > set, the printk() is done against the buffer. > > There's a couple of issues with this approach. > > 1) If two printk_sched()s are called before the tick, the second one > will overwrite the first one. > > 2) The temporary buffer is 512 bytes and is per cpu. This is a quite a > bit of space wasted for something that is seldom used. > > In order to remove this, the printk_sched() can use the printk buffer > instead, and delay the console_trylock()/console_unlock() to the queued > work. > > Because printk_sched() would then be taking the logbuf_lock, the > logbuf_lock must not be held while doing anything that may call into the > scheduler functions, which includes wake ups. Unfortunately, printk() > also has a console_sem that it uses, and on release, the > up(&console_sem) may do a wake up of any pending waiters. This must be > avoided while holding the logbuf_lock. > > Signed-off-by: Steven Rostedt <rost...@goodmis.org> > --- > This version has been forward ported to the 3.15-rc releases. > --- > kernel/printk/printk.c | 87 > +++++++++++++++++++++++++++++++------------------ > 1 file changed, 56 insertions(+), 31 deletions(-) > > Index: linux-trace.git/kernel/printk/printk.c > =================================================================== > --- linux-trace.git.orig/kernel/printk/printk.c 2014-05-05 > 16:46:17.280815365 -0400 > +++ linux-trace.git/kernel/printk/printk.c 2014-05-05 16:52:23.398378897 > -0400 > @@ -208,6 +208,9 @@ > /* > * The logbuf_lock protects kmsg buffer, indices, counters. It is also > * used in interesting ways to provide interlocking in console_unlock(); > + * This can be taken within the scheduler's rq lock. It must be released > + * before calling console_unlock() or anything else that might wake up > + * a process. > */ > static DEFINE_RAW_SPINLOCK(logbuf_lock); > > @@ -1338,27 +1341,43 @@ > * interrupts disabled. It should return with 'lockbuf_lock' > * released but interrupts still disabled. > */ > -static int console_trylock_for_printk(unsigned int cpu) > +static int console_trylock_for_printk(unsigned int cpu, bool in_sched) > __releases(&logbuf_lock) > { > int retval = 0, wake = 0; > > - if (console_trylock()) { > - retval = 1; > + /* if called from the scheduler, we can not call up() */ > + if (in_sched) > + goto out; > > - /* > - * If we can't use the console, we need to release > - * the console semaphore by hand to avoid flushing > - * the buffer. We need to hold the console semaphore > - * in order to do this test safely. > - */ > - if (!can_use_console(cpu)) { > - console_locked = 0; > - wake = 1; > - retval = 0; > - } > + if (down_trylock(&console_sem)) > + goto out; > + > + /* > + * If we can't use the console, we need to release > + * the console semaphore by hand to avoid flushing > + * the buffer. We need to hold the console semaphore > + * in order to do this test safely. > + */ > + if (console_suspended || !can_use_console(cpu)) { > + wake = 1; > + goto out; > } > + > + /* console is now locked */ > + > + console_locked = 1; > + console_may_schedule = 0; > + mutex_acquire(&console_lock_dep_map, 0, 1, _RET_IP_); > + > + retval = 1; > + > +out: > logbuf_cpu = UINT_MAX; > + /* > + * The logbuf_lock must not be held when doing a wake up, > + * which the up(&console_sem) can do. > + */ > raw_spin_unlock(&logbuf_lock); > if (wake) > up(&console_sem); > @@ -1490,11 +1509,17 @@ > static int recursion_bug; > static char textbuf[LOG_LINE_MAX]; > char *text = textbuf; > - size_t text_len; > + size_t text_len = 0; > enum log_flags lflags = 0; > unsigned long flags; > int this_cpu; > int printed_len = 0; > + bool in_sched = false; > + > + if (level == -2) { > + level = -1; > + in_sched = true; > + } > > boot_delay_msec(level); > printk_delay(); > @@ -1540,7 +1565,12 @@ > * The printf needs to come first; we need the syslog > * prefix which might be passed-in as a parameter. > */ > - text_len = vscnprintf(text, sizeof(textbuf), fmt, args); > + if (in_sched) > + text_len = scnprintf(text, sizeof(textbuf), > + KERN_WARNING "[sched_delayed] "); > + > + text_len += vscnprintf(text + text_len, > + sizeof(textbuf) - text_len, fmt, args); > > /* mark and strip a trailing newline */ > if (text_len && text[text_len-1] == '\n') { > @@ -1621,7 +1651,7 @@ > * The console_trylock_for_printk() function will release 'logbuf_lock' > * regardless of whether it actually gets the console semaphore or not. > */ > - if (console_trylock_for_printk(this_cpu)) > + if (console_trylock_for_printk(this_cpu, in_sched)) > console_unlock(); > > lockdep_on(); > @@ -2440,18 +2470,20 @@ > #define PRINTK_BUF_SIZE 512 > > #define PRINTK_PENDING_WAKEUP 0x01 > -#define PRINTK_PENDING_SCHED 0x02 > +#define PRINTK_PENDING_OUTPUT 0x02 > > static DEFINE_PER_CPU(int, printk_pending); > -static DEFINE_PER_CPU(char [PRINTK_BUF_SIZE], printk_sched_buf); > > static void wake_up_klogd_work_func(struct irq_work *irq_work) > { > int pending = __this_cpu_xchg(printk_pending, 0); > > - if (pending & PRINTK_PENDING_SCHED) { > - char *buf = __get_cpu_var(printk_sched_buf); > - pr_warn("[sched_delayed] %s", buf); > + if (pending & PRINTK_PENDING_OUTPUT) { > + if (console_trylock()) > + console_unlock();
I wonder if we should call here console_trylock_for_printk() which checks whether the console is really usable. The check for usable console was introduced in the commit 76a8ad293912cd2f (Make printk work for really early debugging). I think that this IRQ work could get called during early boot, so the check would make sense here as well. Or have I missed something? Best Regards, Petr > + else > + /* Try again later */ > + __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT); > } > > if (pending & PRINTK_PENDING_WAKEUP) > @@ -2475,21 +2507,14 @@ > > int printk_sched(const char *fmt, ...) > { > - unsigned long flags; > va_list args; > - char *buf; > int r; > > - local_irq_save(flags); > - buf = __get_cpu_var(printk_sched_buf); > - > va_start(args, fmt); > - r = vsnprintf(buf, PRINTK_BUF_SIZE, fmt, args); > + r = vprintk_emit(0, -2, NULL, 0, fmt, args); > va_end(args); > > - __this_cpu_or(printk_pending, PRINTK_PENDING_SCHED); > - irq_work_queue(&__get_cpu_var(wake_up_klogd_work)); > - local_irq_restore(flags); > + __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT); > > return r; > } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/