From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f67.google.com ([209.85.160.67]:43632 "EHLO mail-pl0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754488AbeFRGhn (ORCPT ); Mon, 18 Jun 2018 02:37:43 -0400 Date: Mon, 18 Jun 2018 15:37:38 +0900 From: Sergey Senozhatsky To: Petr Mladek Cc: Sergey Senozhatsky , Steven Rostedt , Peter Zijlstra , Tetsuo Handa , Sergey Senozhatsky , linux-kernel@vger.kernel.org, "4 . 13+" Subject: Re: [PATCH] printk/nmi: Prevent deadlock when serializing NMI backtraces Message-ID: <20180618063738.GC27996@jagdpanzerIV> References: <20180517143903.19339-1-pmladek@suse.com> <20180605124729.3vix5nlkjpjzdljx@pathway.suse.cz> <20180606051029.GA19211@tigerII.localdomain> <20180606103356.GB19211@tigerII.localdomain> <20180608104825.e7xoxteelaxnwx66@pathway.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180608104825.e7xoxteelaxnwx66@pathway.suse.cz> Sender: stable-owner@vger.kernel.org List-ID: On (06/08/18 12:48), Petr Mladek wrote: [..] > diff --git a/include/linux/printk.h b/include/linux/printk.h > index 6d7e800affd8..872fbdf8df26 100644 > --- a/include/linux/printk.h > +++ b/include/linux/printk.h > @@ -148,9 +148,13 @@ void early_printk(const char *s, ...) { } > #ifdef CONFIG_PRINTK_NMI > extern void printk_nmi_enter(void); > extern void printk_nmi_exit(void); > +extern void printk_nmi_direct_enter(void); > +extern void printk_nmi_direct_exit(void); > #else > static inline void printk_nmi_enter(void) { } > static inline void printk_nmi_exit(void) { } > +static void printk_nmi_direct_enter(void) { } > +static void printk_nmi_direct_exit(void) { } Can we have better names may be? Since direct printk_nmi is not in fact always `direct'. > +#ifdef CONFIG_PRINTK_NMI > +__printf(1, 0) int vprintk_nmi(const char *fmt, va_list args); > +#else > +__printf(1, 0) int vprintk_nmi(const char *fmt, va_list args) { return 0; } > +#endif Hmm, printk_safe.c knows about printk.c, printk.c knows about printk_safe.c. > __printf(1, 0) int vprintk_default(const char *fmt, va_list args); > __printf(1, 0) int vprintk_deferred(const char *fmt, va_list args); > __printf(1, 0) int vprintk_func(const char *fmt, va_list args); > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 247808333ba4..cf55bece43d9 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -1845,7 +1845,13 @@ asmlinkage int vprintk_emit(int facility, int level, > printk_delay(); > > /* This stops the holder of console_sem just where we want him */ > - logbuf_lock_irqsave(flags); > + printk_safe_enter_irqsave(flags); > + if (in_nmi() && !raw_spin_trylock(&logbuf_lock)) { > + printed_len = vprintk_nmi(fmt, args); > + printk_safe_exit_irqrestore(flags); > + return printed_len; > + } else > + raw_spin_lock(&logbuf_lock); OK... Can we do this in vprintk_func()? The race window should be super tiny [if matters at all], but in exchange we don't have to mix nmi, printk, printk_mni, etc. So over all I understand why you did it this way. May be I'd prefer to have less universal but shorter solution (e.g. modify only nmi_backtrace function and put there "printk_nmi_restricted_buffer"), but I won't really object your patch [unless I see some real issues with it]. -ss