From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Dmitriy Vyukov <dvyukov@google.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Alexander Potapenko <glider@google.com>,
kbuild test robot <fengguang.wu@intel.com>,
syzkaller <syzkaller@googlegroups.com>,
LKML <linux-kernel@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] printk: inject caller information into the body of message
Date: Wed, 10 Oct 2018 06:19:11 +0900 [thread overview]
Message-ID: <a4a2ecae-3c47-db1f-8124-4d2d0ab8b91c@i-love.sakura.ne.jp> (raw)
In-Reply-To: <20181009145229.3lv5vl2ypz5i45cq@pathway.suse.cz>
On 2018/10/09 23:52, Petr Mladek wrote:
> On Tue 2018-10-09 05:48:33, Tetsuo Handa wrote:
>> On 2018/10/09 1:03, Petr Mladek wrote:
>>> On Mon 2018-10-08 19:31:58, Tetsuo Handa wrote:
>>>> A structure named "struct printk_buffer" is introduced for buffering
>>>> up to LOG_LINE_MAX bytes of printk() output which did not end with '\n'.
>>>>
>>>> A caller is allowed to allocate/free "struct printk_buffer" using
>>>> kzalloc()/kfree() if that caller is in a location where it is possible
>>>> to do so.
>>>>
>>>> A macro named "DEFINE_PRINTK_BUFFER()" is defined for allocating
>>>> "struct printk_buffer" from the stack memory or in the .bss section.
>>>>
>>>> But since sizeof("struct printk_buffer") is nearly 1KB, it might not be
>>>> preferable to allocate "struct printk_buffer" from the stack memory.
>>>> In that case, a caller can use best-effort buffering mode. Two functions
>>>> get_printk_buffer() and put_printk_buffer() are provided for that mode.
>>>>
>>>> get_printk_buffer() tries to assign a "struct printk_buffer" from
>>>> statically preallocated array. It returns NULL if all static
>>>> "struct printk_buffer" are in use.
>>>>
>>>> put_printk_buffer() flushes and releases the "struct printk_buffer".
>>>> put_printk_buffer() must match corresponding get_printk_buffer() as with
>>>> rcu_read_unlock() must match corresponding rcu_read_lock().
>>>
>>> One problem with this API is when it is used in more complicated code
>>> and put_printk_buffer() is not called in some path. I mean leaking.
>>> We might get out of buffers easily.
>>
>> Then, as an debugging config option for statically preallocated buffers,
>> we could record how get_printk_buffer() was called, like lockdep records
>> where a lock was taken.
>
> Another solution might be to store some timestamp (jiffies?) into
> struct printk_buffer when a new message is added. Then we could flush
> stalled buffers in get_printk_buffer() with some warning.
I don't think it will work. What the threshold should be? It is possible that
a thread spends very long time (many seconds for e.g. SysRq-t) between
get_printk_buffer() and put_printk_buffer(). Therefore, the threshold will
have to be very very long. As soon as we reach out of statically preallocated
buffers, we need to fallback to unbuffered printk() before such threshold
elapses.
>
> Unfortunately, it might be unsafe to put the stalled buffers.
> Well, it might be safe if there is a lock less access. I wonder
> if we could reuse the printk_safe code here.
>
> Anyway, I would like to have a solution before we add the new
> API into the kernel. We would need it sooner or later anyway.
> And I would like to be sure that the API is sane.
If we worry about get_printk_buffer() without corresponding put_printk_buffer(),
we will also need to worry about a "struct printk_buffer" returned by
get_printk_buffer() is by error shared by multiple threads. We will have to
complicate buffered_printk() by using cmpxchg() & retry logic, but the output is
after all mixed as with simply fallback to unbuffered printk() does.
Do you think that adding cmpxchg() & retry logic to this API generates better
result than simple fallback? buffered_printk() does not add a new locking dependency
is a good point of this API. Showing the backtrace (by enabling a debug kernel config
option for this API) will be sufficient.
next prev parent reply other threads:[~2018-10-09 21:19 UTC|newest]
Thread overview: 94+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <201804232233.CIC65675.OJSOMFQOFFHVtL@I-love.SAKURA.ne.jp>
[not found] ` <CACT4Y+boyw_Qy=y-iTnsKZrtTgF0Hk3nHN_xtqUdX4etgiYDQw@mail.gmail.com>
2018-04-24 1:33 ` printk feature for syzbot? Sergey Senozhatsky
2018-04-24 14:40 ` Steven Rostedt
2018-04-26 10:06 ` Petr Mladek
2018-05-10 4:22 ` Sergey Senozhatsky
2018-05-10 11:30 ` Petr Mladek
2018-05-10 12:11 ` Sergey Senozhatsky
2018-05-10 14:22 ` Steven Rostedt
2018-05-10 14:50 ` Tetsuo Handa
2018-05-11 1:45 ` Sergey Senozhatsky
[not found] ` <201805110238.w4B2cIGH079602@www262.sakura.ne.jp>
2018-05-11 6:21 ` Sergey Senozhatsky
2018-05-11 9:17 ` Dmitry Vyukov
2018-05-11 9:50 ` Sergey Senozhatsky
2018-05-11 11:58 ` [PATCH] printk: inject caller information into the body of message Tetsuo Handa
2018-05-17 11:21 ` Sergey Senozhatsky
2018-05-17 11:52 ` Sergey Senozhatsky
2018-05-18 12:15 ` Petr Mladek
2018-05-18 12:25 ` Dmitry Vyukov
2018-05-18 12:54 ` Petr Mladek
2018-05-18 13:08 ` Dmitry Vyukov
2018-05-24 2:21 ` Sergey Senozhatsky
2018-05-23 10:19 ` Tetsuo Handa
2018-05-24 2:14 ` Sergey Senozhatsky
2018-05-26 6:36 ` Dmitry Vyukov
2018-06-20 5:44 ` Dmitry Vyukov
2018-06-20 8:31 ` Sergey Senozhatsky
2018-06-20 8:45 ` Dmitry Vyukov
2018-06-20 9:06 ` Sergey Senozhatsky
2018-06-20 9:18 ` Sergey Senozhatsky
2018-06-20 9:31 ` Dmitry Vyukov
2018-06-20 11:07 ` Sergey Senozhatsky
2018-06-20 11:32 ` Dmitry Vyukov
2018-06-20 13:06 ` Sergey Senozhatsky
2018-06-22 13:06 ` Tetsuo Handa
2018-06-25 1:41 ` Sergey Senozhatsky
2018-06-25 9:36 ` Dmitry Vyukov
2018-06-27 10:29 ` Tetsuo Handa
2018-09-10 11:20 ` Alexander Potapenko
2018-09-12 6:53 ` Sergey Senozhatsky
2018-09-12 16:05 ` Steven Rostedt
2018-09-13 7:12 ` Sergey Senozhatsky
2018-09-13 12:26 ` Petr Mladek
2018-09-13 14:28 ` Sergey Senozhatsky
2018-09-14 1:22 ` Steven Rostedt
2018-09-14 2:15 ` Sergey Senozhatsky
2018-09-14 6:57 ` Sergey Senozhatsky
2018-09-14 10:37 ` Tetsuo Handa
2018-09-14 11:50 ` Sergey Senozhatsky
2018-09-14 12:03 ` Tetsuo Handa
2018-09-14 12:22 ` Sergey Senozhatsky
2018-09-19 11:02 ` Tetsuo Handa
2018-09-24 8:11 ` Tetsuo Handa
2018-09-27 16:10 ` Tetsuo Handa
2018-09-28 9:02 ` Sergey Senozhatsky
2018-09-28 9:09 ` Sergey Senozhatsky
2018-09-28 11:01 ` Tetsuo Handa
2018-09-29 10:51 ` Sergey Senozhatsky
2018-09-29 11:15 ` Tetsuo Handa
2018-10-01 2:37 ` Sergey Senozhatsky
2018-10-01 2:58 ` Sergey Senozhatsky
2018-10-01 11:21 ` Tetsuo Handa
2018-10-02 6:38 ` Sergey Senozhatsky
2018-10-08 10:31 ` Tetsuo Handa
2018-10-08 16:03 ` Petr Mladek
2018-10-08 20:48 ` Tetsuo Handa
2018-10-09 14:52 ` Petr Mladek
2018-10-09 21:19 ` Tetsuo Handa [this message]
2018-10-10 10:14 ` Tetsuo Handa
2018-10-11 10:20 ` Tetsuo Handa
2018-10-11 13:47 ` Steven Rostedt
2018-10-08 15:43 ` Petr Mladek
2018-09-28 8:56 ` Sergey Senozhatsky
2018-09-28 11:21 ` Tetsuo Handa
2018-09-29 11:13 ` Sergey Senozhatsky
2018-09-29 11:39 ` Tetsuo Handa
2018-10-01 5:52 ` Sergey Senozhatsky
2018-10-01 8:37 ` Sergey Senozhatsky
2018-10-01 18:06 ` Steven Rostedt
2018-09-14 1:12 ` Steven Rostedt
2018-09-14 1:55 ` Sergey Senozhatsky
2018-06-21 8:29 ` Sergey Senozhatsky
2018-06-20 9:30 ` Dmitry Vyukov
2018-06-20 11:19 ` Sergey Senozhatsky
2018-06-20 11:25 ` Dmitry Vyukov
2018-06-20 11:37 ` Fengguang Wu
2018-06-20 12:31 ` Dmitry Vyukov
2018-06-20 12:41 ` Fengguang Wu
2018-06-20 12:45 ` Dmitry Vyukov
2018-06-20 12:48 ` Fengguang Wu
2018-05-11 13:37 ` printk feature for syzbot? Steven Rostedt
2018-05-15 5:20 ` Sergey Senozhatsky
2018-05-15 14:39 ` Steven Rostedt
2018-05-11 11:02 ` [PATCH] printk: fix possible reuse of va_list variable Tetsuo Handa
2018-05-11 11:27 ` Sergey Senozhatsky
2018-05-17 11:57 ` Petr Mladek
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