From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
To: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Petr Mladek <pmladek@suse.com>,
Sumit Garg <sumit.garg@linaro.org>,
Jason Wessel <jason.wessel@windriver.com>,
Douglas Anderson <dianders@chromium.org>,
kgdb-bugreport@lists.sourceforge.net,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Arnd Bergmann <arnd@arndb.de>,
Andrew Morton <akpm@linux-foundation.org>,
Peter Zijlstra <peterz@infradead.org>,
Steven Rostedt <rostedt@goodmis.org>,
John Ogness <john.ogness@linutronix.de>
Subject: Re: [PATCH] printk/kdb: Redirect printk messages into kdb in any context
Date: Sat, 16 May 2020 01:24:11 +0900 [thread overview]
Message-ID: <20200515162411.GG42471@jagdpanzerIV.localdomain> (raw)
In-Reply-To: <20200515134806.5cw4xxnxw7k3223l@holly.lan>
On (20/05/15 14:48), Daniel Thompson wrote:
> On Fri, May 15, 2020 at 07:33:08PM +0900, Sergey Senozhatsky wrote:
> > On (20/05/15 10:50), Petr Mladek wrote:
[..]
> > Is this guaranteed that we never execute this path from NMI?
>
> Absolutely not.
>
> The execution context for kdb is pretty much unique...
OK, that was what I expected.
> we are running a debug mode with all CPUs parked in a holding loop with
> interrupts disabled. One CPU is at an unknown exception state and the
> others are either handling an IRQ or NMI depending on architecture[1].
Can a CPU be parked while holding the console driver port lock?
Hmm, a side note - this also means that we cannot handle it from
poll-ing console drivers and need to switch to direct console writes.
> However there are a number of factors that IMHO weigh in favour of
> allowing kdb to intercept here.
>
> 1. kgdb/kdb are designed to work from NMI, modulo the bugs that are
> undoubtedly present.
>
> 2. A synchronous breakpoint (including an implicit breakpoint-on-oops)
> from any code that executes with irqs disabled will exhibit most of
> the same problems as an NMI but without waking up all the NMI logic.
>
> 3. kdb_trap_printk is only set for *very* narrow time intervals by the
> debug master (the single CPU in the system that is *not* in a
> holding loop). Thus in all cases the system has already successfully
> executed kdb_printf() several times before we ever call the printk()
> interception code.
>
> Or put another way, even if we did tickle a bug speculated about in
> #1, it won't be the call to printk() that triggers it; we'd never
> get that far!
OK. I would appreciate a more detailed commit message:
- what do we fix, and what risks do we take. Just for the record.
+ a small nit: looking at for_each_console() loop -- not all consoles
can be invoked at any time and not all consoles are enabled at any time.
You _probably_ might want to do what printk does in call_console_drivers()
loop. printk also had problems with console callbacks being placed in
sections that get discarded, but that's way too niche.
-ss
next prev parent reply other threads:[~2020-05-15 16:24 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-12 8:48 [PATCH] kgdb: Fix broken handling of printk() in NMI context Sumit Garg
2020-05-12 14:25 ` Daniel Thompson
2020-05-13 13:34 ` Sumit Garg
2020-05-14 8:42 ` Petr Mladek
2020-05-15 5:46 ` Sumit Garg
2020-05-15 8:50 ` [PATCH] printk/kdb: Redirect printk messages into kdb in any context Petr Mladek
2020-05-15 10:33 ` Sergey Senozhatsky
2020-05-15 12:02 ` Sumit Garg
2020-05-15 16:36 ` Sergey Senozhatsky
2020-05-15 16:52 ` Doug Anderson
2020-05-18 6:19 ` Sumit Garg
2020-06-10 16:41 ` Daniel Thompson
2020-06-11 7:27 ` Sergey Senozhatsky
2020-06-11 7:58 ` Petr Mladek
2020-05-15 13:48 ` Daniel Thompson
2020-05-15 16:24 ` Sergey Senozhatsky [this message]
2020-05-18 9:21 ` Petr Mladek
2020-05-20 4:21 ` Sergey Senozhatsky
2020-05-20 9:35 ` Daniel Thompson
2020-05-20 10:22 ` [PATCH v2] " Petr Mladek
2020-05-20 11:17 ` Sergey Senozhatsky
2020-05-20 16:07 ` Daniel Thompson
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