From: Petr Mladek <pmladek@suse.com> To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>, Steven Rostedt <rostedt@goodmis.org>, Peter Zijlstra <peterz@infradead.org>, Andrew Morton <akpm@linux-foundation.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Jiri Slaby <jslaby@suse.cz>, linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org, Petr Mladek <pmladek@suse.com> Subject: [PATCH] printk: Correctly handle preemption in console_unlock() Date: Fri, 13 Jan 2017 14:15:21 +0100 [thread overview] Message-ID: <1484313321-17196-1-git-send-email-pmladek@suse.com> (raw) 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 clearing and restoring the very original @may_schedule setting only around call_console_drivers(). Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Signed-off-by: Petr Mladek <pmladek@suse.com> --- This is related to the thread https://lkml.kernel.org/r/201612261954.FJE69201.OFLVtFJSQFOHMO@I-love.SAKURA.ne.jp kernel/printk/printk.c | 25 ++++++++++++------------- 1 file changed, 12 insertions(+), 13 deletions(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 7180088cbb23..2ac54291230d 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2150,7 +2150,7 @@ void console_unlock(void) static u64 seen_seq; unsigned long flags; bool wake_klogd = false; - bool do_cond_resched, retry; + bool may_schedule_orig, retry; if (console_suspended) { up_console_sem(); @@ -2158,17 +2158,15 @@ 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 restored so that we could schedule between lines. + * + * console_trylock() is not able to detect the preemptive context when + * CONFIG_PREEMPT_COUNT is disabled. Therefore the value must be + * stored before the "again" goto label. */ - do_cond_resched = console_may_schedule; - console_may_schedule = 0; + may_schedule_orig = console_may_schedule; again: /* @@ -2235,12 +2233,13 @@ void console_unlock(void) raw_spin_unlock(&logbuf_lock); stop_critical_timings(); /* don't trace print latency */ + console_may_schedule = 0; call_console_drivers(ext_text, ext_len, text, len); + console_may_schedule = may_schedule_orig; start_critical_timings(); printk_safe_exit_irqrestore(flags); - if (do_cond_resched) - cond_resched(); + console_conditional_schedule(); } console_locked = 0; -- 1.8.5.6
WARNING: multiple messages have this Message-ID (diff)
From: Petr Mladek <pmladek@suse.com> To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>, Steven Rostedt <rostedt@goodmis.org>, Peter Zijlstra <peterz@infradead.org>, Andrew Morton <akpm@linux-foundation.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Jiri Slaby <jslaby@suse.cz>, linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org, Petr Mladek <pmladek@suse.com> Subject: [PATCH] printk: Correctly handle preemption in console_unlock() Date: Fri, 13 Jan 2017 13:15:21 +0000 [thread overview] Message-ID: <1484313321-17196-1-git-send-email-pmladek@suse.com> (raw) 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 clearing and restoring the very original @may_schedule setting only around call_console_drivers(). Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Signed-off-by: Petr Mladek <pmladek@suse.com> --- This is related to the thread https://lkml.kernel.org/r/201612261954.FJE69201.OFLVtFJSQFOHMO@I-love.SAKURA.ne.jp kernel/printk/printk.c | 25 ++++++++++++------------- 1 file changed, 12 insertions(+), 13 deletions(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 7180088cbb23..2ac54291230d 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2150,7 +2150,7 @@ void console_unlock(void) static u64 seen_seq; unsigned long flags; bool wake_klogd = false; - bool do_cond_resched, retry; + bool may_schedule_orig, retry; if (console_suspended) { up_console_sem(); @@ -2158,17 +2158,15 @@ 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 restored so that we could schedule between lines. + * + * console_trylock() is not able to detect the preemptive context when + * CONFIG_PREEMPT_COUNT is disabled. Therefore the value must be + * stored before the "again" goto label. */ - do_cond_resched = console_may_schedule; - console_may_schedule = 0; + may_schedule_orig = console_may_schedule; again: /* @@ -2235,12 +2233,13 @@ void console_unlock(void) raw_spin_unlock(&logbuf_lock); stop_critical_timings(); /* don't trace print latency */ + console_may_schedule = 0; call_console_drivers(ext_text, ext_len, text, len); + console_may_schedule = may_schedule_orig; start_critical_timings(); printk_safe_exit_irqrestore(flags); - if (do_cond_resched) - cond_resched(); + console_conditional_schedule(); } console_locked = 0; -- 1.8.5.6
next reply other threads:[~2017-01-13 13:15 UTC|newest] Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-01-13 13:15 Petr Mladek [this message] 2017-01-13 13:15 ` [PATCH] printk: Correctly handle preemption in console_unlock() Petr Mladek 2017-01-13 16:05 ` Steven Rostedt 2017-01-13 16:05 ` Steven Rostedt 2017-01-16 11:00 ` Petr Mladek 2017-01-16 11:00 ` Petr Mladek 2017-01-18 5:45 ` Sergey Senozhatsky 2017-01-18 5:45 ` Sergey Senozhatsky 2017-01-18 7:21 ` Sergey Senozhatsky 2017-01-18 7:21 ` Sergey Senozhatsky 2017-01-25 12:34 ` Petr Mladek 2017-01-25 12:34 ` Petr Mladek 2017-01-14 6:28 ` Sergey Senozhatsky 2017-01-14 6:28 ` Sergey Senozhatsky 2017-01-16 11:38 ` Petr Mladek 2017-01-16 11:38 ` Petr Mladek 2017-01-16 11:58 ` Sergey Senozhatsky 2017-01-16 11:58 ` Sergey Senozhatsky 2017-01-16 12:48 ` Petr Mladek 2017-01-16 12:48 ` Petr Mladek 2017-01-16 13:26 ` Sergey Senozhatsky 2017-01-16 13:26 ` Sergey Senozhatsky 2017-01-16 13:43 ` Sergey Senozhatsky 2017-01-16 13:43 ` Sergey Senozhatsky 2017-01-16 14:14 ` Petr Mladek 2017-01-16 14:14 ` Petr Mladek 2017-01-16 15:19 ` Sergey Senozhatsky 2017-01-16 15:19 ` Sergey Senozhatsky 2017-01-16 15:43 ` Sergey Senozhatsky 2017-01-16 15:43 ` Sergey Senozhatsky 2017-01-16 16:35 ` Petr Mladek 2017-01-16 16:35 ` Petr Mladek 2017-01-16 13:41 ` Tetsuo Handa 2017-01-16 13:41 ` Tetsuo Handa
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