linux-kernel - Re: linux.git: printk() problem

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Date:   Mon, 24 Oct 2016 19:22:59 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Joe Perches <joe@...ches.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Petr Mladek <pmladek@...e.cz>, Tejun Heo <tj@...nel.org>,
        Calvin Owens <calvinowens@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: linux.git: printk() problem

On Mon, Oct 24, 2016 at 7:06 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Mon, Oct 24, 2016 at 6:55 PM, Sergey Senozhatsky
> <sergey.senozhatsky.work@...il.com> wrote:
>>
>> I think cont_flush() should grab the logbuf_lock lock, because
>> it does log_store() and touches the cont.len. so something like
>> this perhaps
>
> Absolutely. Good catch.

Actually, you can't do it the way you did (inside cont_flush), because
"cont_flush()" is already called with logbuf_lock held in most cases
(see "cont_add()").

So it's really just the timer function that needs to take the
logbuf_lock before it calls cont_flush().

So here's a new version. How does this look to you?

Again, this still tests "cont.len" outside the lock (not just in
console_unlock(), but also in deferred_cont_flush()). And it's fine:
even if it sees the "wrong" value due to some race, it does so either
because cont.len was just set to non-zero (and whoever set it will
force the re-check anyway), or it got cleared just as it was tested
(and at worst you end up with an extra timer invocation).

              Linus

View attachment "patch.diff" of type "text/plain" (19844 bytes)