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    SubjectRe: [RFC PATCH v1 10/25] printk: redirect emit/store to new ringbuffer
    On Mon 2019-02-25 17:41:50, John Ogness wrote:
    > On 2019-02-25, Petr Mladek <pmladek@suse.com> wrote:
    > >> >> vprintk_emit and vprintk_store are the main functions that all printk
    > >> >> variants eventually go through. Change these to store the message in
    > >> >> the new printk ring buffer that the printk kthread is reading.
    > >> >
    > >> > Please, are there any candidates or plans to reuse the new ring
    > >> > buffer implementation?
    > >>
    > >> As you pointed out below, this patch already uses the ring buffer
    > >> implementation for a totally different purpose: NMI safe dynamic memory
    > >> allocation.
    > >
    > > I have found an alternative solution. We could calculate the length
    > > of the formatted string without any buffer:
    > >
    > > va_list args_copy;
    > >
    > > va_copy(args_copy, args);
    > > len = vscprintf(NULL, fmt, args_copy);
    > > va_end(args_copy);
    > >
    > > This vsprintf() mode was implemented for exactly this purpose.
    >
    > For vprintk_emit() that would work. As you will see in later (patch 23),
    > the sprint_rb ringbuffer is used for dynamic memory allocation for
    > kmsg_dump functions as well.

    It looks dangerous to share a limited buffer between core kernel
    functionality and user-space triggered one. I mean that an unlimited
    number of devkmsg operations must not cause loosing printk() messages.

    > The current printk implementation allows readers to read directly from
    > the ringbuffer. The proposed ringbuffer requires the reader (printk) to
    > have its own buffers.
    >
    > We may be able to find an alternate solution here as well if that is
    > desired.

    I hope that we will be able to find one. The previous implementation
    needed some buffers as well. We should be able to use the same
    approach.

    I guess that one problem is that the new ringbuffer API is not
    able to copy just the text directly into the user-provided buffer.
    It might get solved by extending the API.

    Anyway, I still have to look at the remaining patches.

    Best Regards,
    Petr

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