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The commit42a0bb3f71("printk/nmi: generic solution for safe printk in NMI") caused that printk stores messages into a temporary buffer in NMI context. The buffer is per-CPU and therefore the size is rather limited. It works quite well for NMI backtraces. But there are longer logs that might get printed in NMI context, for example, lockdep warnings, ftrace_dump_on_oops. The temporary buffer is used to avoid deadlocks caused by logbuf_lock. Also it is needed to avoid races with the other temporary buffer that is used when PRINTK_SAFE_CONTEXT is entered. But the main buffer can be used in NMI if the lock is available and we did not interrupt PRINTK_SAFE_CONTEXT. The lock is checked using raw_spin_is_locked(). It might cause false negatives when the lock is taken on another CPU and this CPU is in the safe context from other reasons. Note that the safe context is used also to get console semaphore or when calling console drivers. For this reason, we do the check in printk_nmi_enter(). It makes the handling consistent for the entire NMI handler and avoids reshuffling of the messages. The patch also defines special printk context that allows to use printk_deferred() in NMI. Note that we could not flush the messages to the consoles because console drivers might use many other internal locks. The newly created vprintk_deferred() disables the preemption only around the irq work handling. It is needed there to keep the consistency between the two per-CPU variables. But there is no reason to disable preemption around vprintk_emit(). Finally, the patch puts back explicit serialization of the NMI backtraces from different CPUs. It was removed by the commita9edc88093("x86/nmi: Perform a safe NMI stack trace on all CPUs"). It was not needed because the flushing of the temporary per-CPU buffers was serialized. Link: http://lkml.kernel.org/r/1493912763-24873-1-git-send-email-pmladek@suse.com Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Russell King <rack+kernel@arm.linux.org.uk> Cc: Daniel Thompson <daniel.thompson@linaro.org> Cc: Ingo Molnar <mingo@redhat.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Chris Metcalf <cmetcalf@ezchip.com> Cc: x86@kernel.org Cc: linux-arm-kernel@lists.infradead.org Cc: linux-kernel@vger.kernel.org Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Signed-off-by: Petr Mladek <pmladek@suse.com>
73 lines
2.2 KiB
C
73 lines
2.2 KiB
C
/*
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* internal.h - printk internal definitions
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version 2
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* of the License, or (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, see <http://www.gnu.org/licenses/>.
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*/
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#include <linux/percpu.h>
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#ifdef CONFIG_PRINTK
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#define PRINTK_SAFE_CONTEXT_MASK 0x3fffffff
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#define PRINTK_NMI_DEFERRED_CONTEXT_MASK 0x40000000
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#define PRINTK_NMI_CONTEXT_MASK 0x80000000
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extern raw_spinlock_t logbuf_lock;
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__printf(1, 0) int vprintk_default(const char *fmt, va_list args);
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__printf(1, 0) int vprintk_deferred(const char *fmt, va_list args);
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__printf(1, 0) int vprintk_func(const char *fmt, va_list args);
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void __printk_safe_enter(void);
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void __printk_safe_exit(void);
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#define printk_safe_enter_irqsave(flags) \
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do { \
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local_irq_save(flags); \
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__printk_safe_enter(); \
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} while (0)
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#define printk_safe_exit_irqrestore(flags) \
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do { \
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__printk_safe_exit(); \
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local_irq_restore(flags); \
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} while (0)
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#define printk_safe_enter_irq() \
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do { \
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local_irq_disable(); \
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__printk_safe_enter(); \
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} while (0)
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#define printk_safe_exit_irq() \
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do { \
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__printk_safe_exit(); \
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local_irq_enable(); \
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} while (0)
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#else
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__printf(1, 0) int vprintk_func(const char *fmt, va_list args) { return 0; }
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/*
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* In !PRINTK builds we still export logbuf_lock spin_lock, console_sem
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* semaphore and some of console functions (console_unlock()/etc.), so
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* printk-safe must preserve the existing local IRQ guarantees.
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*/
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#define printk_safe_enter_irqsave(flags) local_irq_save(flags)
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#define printk_safe_exit_irqrestore(flags) local_irq_restore(flags)
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#define printk_safe_enter_irq() local_irq_disable()
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#define printk_safe_exit_irq() local_irq_enable()
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#endif /* CONFIG_PRINTK */
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