| Message ID | 1485353325-26591-1-git-send-email-pmladek@suse.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On (01/25/17 15:08), Petr Mladek wrote: [..] > Fixes: 6b97a20d3a7909 ("printk: set may_schedule for some of console_trylock() callers") > Suggested-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> > Signed-off-by: Petr Mladek <pmladek@suse.com> Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> -ss -- To unsubscribe from this list: send the line "unsubscribe linux-fbdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Wed, 25 Jan 2017 15:08:45 +0100 Petr Mladek <pmladek@suse.com> wrote: > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 7180088cbb23..cc90c0a5ae21 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -2158,19 +2158,18 @@ void console_unlock(void) > } > > /* > - * Console drivers are called under logbuf_lock, so > - * @console_may_schedule should be cleared before; however, we may > - * end up dumping a lot of lines, for example, if called from > - * console registration path, and should invoke cond_resched() > - * between lines if allowable. Not doing so can cause a very long > - * scheduling stall on a slow console leading to RCU stall and > - * softlockup warnings which exacerbate the issue with more > - * messages practically incapacitating the system. Why did you remove the comment about invoking cond_resched()? It's still pertinent to the code, as there still exists a: if (do_cond_resched) cond_resched(); And the rational in the comment is still correct. -- Steve > + * Console drivers are called with interrupts disabled, so > + * @console_may_schedule must be cleared before. The original > + * value must be stored so that we could schedule between lines. > + * > + * console_trylock() is not able to detect the preemtible > + * context reliably. Therefore the value must be stored before > + * and cleared after the the "again" goto label. > */ > do_cond_resched = console_may_schedule; > +again: > console_may_schedule = 0; > > -again: > /* > * We released the console_sem lock, so we need to recheck if > * cpu is online and (if not) is there at least one CON_ANYTIME -- To unsubscribe from this list: send the line "unsubscribe linux-fbdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu 2017-02-02 09:30:11, Steven Rostedt wrote: > On Wed, 25 Jan 2017 15:08:45 +0100 > Petr Mladek <pmladek@suse.com> wrote: > > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > > index 7180088cbb23..cc90c0a5ae21 100644 > > --- a/kernel/printk/printk.c > > +++ b/kernel/printk/printk.c > > @@ -2158,19 +2158,18 @@ void console_unlock(void) > > } > > > > /* > > - * Console drivers are called under logbuf_lock, so > > - * @console_may_schedule should be cleared before; however, we may > > - * end up dumping a lot of lines, for example, if called from > > - * console registration path, and should invoke cond_resched() > > - * between lines if allowable. Not doing so can cause a very long > > - * scheduling stall on a slow console leading to RCU stall and > > - * softlockup warnings which exacerbate the issue with more > > - * messages practically incapacitating the system. > > Why did you remove the comment about invoking cond_resched()? It's > still pertinent to the code, as there still exists a: > > if (do_cond_resched) > cond_resched(); > > And the rational in the comment is still correct. The comment was above an assignment to a variable called "do_cond_resched". The variable was later used only on one place. I hoped that the purpose of cond_resched() was a common knowledge. The new comment still mentions scheduling between lines. But it newly explains why it has to be done here and the relation to the "again" label. Would you prefer to keep the original text as is (modulo the context in which the drivers are called) and just add the second paragraph? Anyway, thank a lot for review. Best Regards, Petr > -- Steve > > > + * Console drivers are called with interrupts disabled, so > > + * @console_may_schedule must be cleared before. The original > > + * value must be stored so that we could schedule between lines. > > + * > > + * console_trylock() is not able to detect the preemtible > > + * context reliably. Therefore the value must be stored before > > + * and cleared after the the "again" goto label. > > */ > > do_cond_resched = console_may_schedule; > > +again: > > console_may_schedule = 0; > > > > -again: > > /* > > * We released the console_sem lock, so we need to recheck if > > * cpu is online and (if not) is there at least one CON_ANYTIME > -- To unsubscribe from this list: send the line "unsubscribe linux-fbdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 7180088cbb23..cc90c0a5ae21 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2158,19 +2158,18 @@ void console_unlock(void) } /* - * Console drivers are called under logbuf_lock, so - * @console_may_schedule should be cleared before; however, we may - * end up dumping a lot of lines, for example, if called from - * console registration path, and should invoke cond_resched() - * between lines if allowable. Not doing so can cause a very long - * scheduling stall on a slow console leading to RCU stall and - * softlockup warnings which exacerbate the issue with more - * messages practically incapacitating the system. + * Console drivers are called with interrupts disabled, so + * @console_may_schedule must be cleared before. The original + * value must be stored so that we could schedule between lines. + * + * console_trylock() is not able to detect the preemtible + * context reliably. Therefore the value must be stored before + * and cleared after the the "again" goto label. */ do_cond_resched = console_may_schedule; +again: console_may_schedule = 0; -again: /* * We released the console_sem lock, so we need to recheck if * cpu is online and (if not) is there at least one CON_ANYTIME
Some console drivers code calls console_conditional_schedule() that looks at @console_may_schedule. The value must be cleared when the drivers are called from console_unlock() with interrupts disabled. But rescheduling is fine when the same code is called, for example, from tty operations where the console semaphore is taken via console_lock(). This is why @console_may_schedule is cleared before calling console drivers. The original value is stored to decide if we could sleep between lines. Now, @console_may_schedule is not cleared when we call console_trylock() and jump back to the "again" goto label. This has become a problem, since the commit 6b97a20d3a7909daa066 ("printk: set may_schedule for some of console_trylock() callers"). @console_may_schedule might get enabled now. There is also the opposite problem. console_lock() can be called only from preemptive context. It can always enable scheduling in the console code. But console_trylock() is not able to detect it when CONFIG_PREEMPT_COUNT is disabled. Therefore we should use the original @console_may_schedule value after re-acquiring the console semaphore in console_unlock(). This patch solves both problems by moving the "again" goto label. Alternative solution was to clear and restore the value around call_console_drivers(). Then console_conditional_schedule() could be used also inside console_unlock(). But there was a potential race with console_flush_on_panic() as reported by Sergey Senozhatsky. That function should be called only where there is only one CPU and with interrupts disabled. But better be on the safe side because stopping CPUs might fail. Fixes: 6b97a20d3a7909 ("printk: set may_schedule for some of console_trylock() callers") Suggested-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Signed-off-by: Petr Mladek <pmladek@suse.com> --- Changes against v2: + use conservative solution with the following rules: + always clear console_may_schedule after again goto label + save and use the original value to decide about sleeping inside console unlock + do not set console_may_schedule using the saved value; it avoids potential race with console_flush_on_panic(); also it avoids breaking the complex logic used in other functions manipulating this variable. kernel/printk/printk.c | 17 ++++++++--------- 1 file changed, 8 insertions(+), 9 deletions(-)