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    Subject[PATCH printk v4 5/8] printk: introduce printk_get_next_message() and printk_message
    Date
    Code for performing the console output is intermixed with code that
    is formatting the output for that console. Introduce a new helper
    function printk_get_next_message() to handle the reading and
    formatting of the printk text. The helper does not require any
    locking so that in the future it can be used for other printing
    contexts as well.

    This also introduces a new struct printk_message to wrap the struct
    printk_buffers, adding metadata about its contents. This allows
    users of printk_get_next_message() to receive all relevant
    information about the message that was read and formatted.

    Why is struct printk_message a wrapper struct?

    It is intentional that a wrapper struct is introduced instead of
    adding the metadata directly to struct printk_buffers. The upcoming
    atomic consoles support multiple printing contexts per CPU. This
    means that while a CPU is formatting a message, it can be
    interrupted and the interrupting context may also format a (possibly
    different) message. Since the printk buffers are rather large,
    there will only be one struct printk_buffers per CPU and it must be
    shared by the possible contexts of that CPU.

    If the metadata was part of struct printk_buffers, interrupting
    contexts would clobber the metadata being prepared by the
    interrupted context. This could be handled by robustifying the
    message formatting functions to cope with metadata unexpectedly
    changing. However, this would require significant amounts of extra
    data copying, also adding significant complexity to the code.

    Instead, the metadata can live on the stack of the formatting
    context and the message formatting functions do not need to be
    concerned about the metadata changing underneath them.

    Note that the message formatting functions can handle unexpected
    text buffer changes. So it is perfectly OK if a shared text buffer
    is clobbered by an interrupting context. The atomic console
    implementation will recognize the interruption and avoid printing
    the (probably garbage) text buffer.

    Signed-off-by: John Ogness <john.ogness@linutronix.de>
    ---
    kernel/printk/internal.h | 16 ++++++
    kernel/printk/printk.c | 115 +++++++++++++++++++++++++++------------
    2 files changed, 96 insertions(+), 35 deletions(-)

    diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
    index 6080d289a342..c9bb0cd86372 100644
    --- a/kernel/printk/internal.h
    +++ b/kernel/printk/internal.h
    @@ -92,3 +92,19 @@ struct printk_buffers {
    char outbuf[CONSOLE_EXT_LOG_MAX];
    char scratchbuf[LOG_LINE_MAX];
    };
    +
    +/**
    + * struct printk_message - Container for a prepared printk message.
    + * @pbufs: printk buffers used to prepare the message.
    + * @outbuf_len: The length of prepared text in @pbufs->outbuf to output. This
    + * does not count the terminator. A value of 0 means there is
    + * nothing to output and this record should be skipped.
    + * @seq: The sequence number of the record used for @pbufs->outbuf.
    + * @dropped: The number of dropped records from reading @seq.
    + */
    +struct printk_message {
    + struct printk_buffers *pbufs;
    + unsigned int outbuf_len;
    + u64 seq;
    + unsigned long dropped;
    +};
    diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
    index bc5d4574c459..6e2a6d5548e9 100644
    --- a/kernel/printk/printk.c
    +++ b/kernel/printk/printk.c
    @@ -2725,35 +2725,35 @@ static void __console_unlock(void)
    }

    /*
    - * Print one record for the given console. The record printed is whatever
    - * record is the next available record for the given console.
    + * Read and format the specified record (or a later record if the specified
    + * record is not available).
    *
    - * @handover will be set to true if a printk waiter has taken over the
    - * console_lock, in which case the caller is no longer holding both the
    - * console_lock and the SRCU read lock. Otherwise it is set to false.
    + * @pmsg will contain the formatted result. @pmsg->pbufs must point to a
    + * struct printk_buffers.
    *
    - * @cookie is the cookie from the SRCU read lock.
    + * @seq is the record to read and format. If it is not available, the next
    + * valid record is read.
    *
    - * Returns false if the given console has no next record to print, otherwise
    - * true.
    + * @is_extended specifies if the message should be formatted for extended
    + * console output.
    *
    - * Requires the console_lock and the SRCU read lock.
    + * Returns false if no record is available. Otherwise true and all fields
    + * of @pmsg are valid. (See the documentation of struct printk_message
    + * for information about the @pmsg fields.)
    */
    -static bool console_emit_next_record(struct console *con, bool *handover, int cookie)
    +static bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
    + bool is_extended)
    {
    - static char dropped_text[DROPPED_TEXT_MAX];
    - static struct printk_buffers pbufs;
    static int panic_console_dropped;

    - bool is_extended = console_srcu_read_flags(con) & CON_EXTENDED;
    - const size_t scratchbuf_sz = sizeof(pbufs.scratchbuf);
    - const size_t outbuf_sz = sizeof(pbufs.outbuf);
    - char *scratchbuf = &pbufs.scratchbuf[0];
    - char *outbuf = &pbufs.outbuf[0];
    + struct printk_buffers *pbufs = pmsg->pbufs;
    + const size_t scratchbuf_sz = sizeof(pbufs->scratchbuf);
    + const size_t outbuf_sz = sizeof(pbufs->outbuf);
    + char *scratchbuf = &pbufs->scratchbuf[0];
    + char *outbuf = &pbufs->outbuf[0];
    struct printk_info info;
    struct printk_record r;
    - unsigned long flags;
    - size_t len;
    + size_t len = 0;

    /*
    * Formatting extended messages requires a separate buffer, so use the
    @@ -2767,25 +2767,26 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co
    else
    prb_rec_init_rd(&r, &info, outbuf, outbuf_sz);

    - *handover = false;
    -
    - if (!prb_read_valid(prb, con->seq, &r))
    + if (!prb_read_valid(prb, seq, &r))
    return false;

    - if (con->seq != r.info->seq) {
    - con->dropped += r.info->seq - con->seq;
    - con->seq = r.info->seq;
    - if (panic_in_progress() && panic_console_dropped++ > 10) {
    - suppress_panic_printk = 1;
    - pr_warn_once("Too many dropped messages. Suppress messages on non-panic CPUs to prevent livelock.\n");
    - }
    + pmsg->seq = r.info->seq;
    + pmsg->dropped = r.info->seq - seq;
    +
    + /*
    + * Check for dropped messages in panic here so that printk
    + * suppression can occur as early as possible if necessary.
    + */
    + if (pmsg->dropped &&
    + panic_in_progress() &&
    + panic_console_dropped++ > 10) {
    + suppress_panic_printk = 1;
    + pr_warn_once("Too many dropped messages. Suppress messages on non-panic CPUs to prevent livelock.\n");
    }

    /* Skip record that has level above the console loglevel. */
    - if (suppress_message_printing(r.info->level)) {
    - con->seq++;
    - goto skip;
    - }
    + if (suppress_message_printing(r.info->level))
    + goto out;

    if (is_extended) {
    len = info_print_ext_header(outbuf, outbuf_sz, r.info);
    @@ -2794,6 +2795,50 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co
    } else {
    len = record_print_text(&r, console_msg_format & MSG_FORMAT_SYSLOG, printk_time);
    }
    +out:
    + pmsg->outbuf_len = len;
    + return true;
    +}
    +
    +/*
    + * Print one record for the given console. The record printed is whatever
    + * record is the next available record for the given console.
    + *
    + * @handover will be set to true if a printk waiter has taken over the
    + * console_lock, in which case the caller is no longer holding both the
    + * console_lock and the SRCU read lock. Otherwise it is set to false.
    + *
    + * @cookie is the cookie from the SRCU read lock.
    + *
    + * Returns false if the given console has no next record to print, otherwise
    + * true.
    + *
    + * Requires the console_lock and the SRCU read lock.
    + */
    +static bool console_emit_next_record(struct console *con, bool *handover, int cookie)
    +{
    + static char dropped_text[DROPPED_TEXT_MAX];
    + static struct printk_buffers pbufs;
    +
    + bool is_extended = console_srcu_read_flags(con) & CON_EXTENDED;
    + char *outbuf = &pbufs.outbuf[0];
    + struct printk_message pmsg = {
    + .pbufs = &pbufs,
    + };
    + unsigned long flags;
    +
    + *handover = false;
    +
    + if (!printk_get_next_message(&pmsg, con->seq, is_extended))
    + return false;
    +
    + con->dropped += pmsg.dropped;
    +
    + /* Skip messages of formatted length 0. */
    + if (pmsg.outbuf_len == 0) {
    + con->seq = pmsg.seq + 1;
    + goto skip;
    + }

    /*
    * While actively printing out messages, if another printk()
    @@ -2809,11 +2854,11 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co
    console_lock_spinning_enable();

    stop_critical_timings(); /* don't trace print latency */
    - call_console_driver(con, outbuf, len,
    + call_console_driver(con, outbuf, pmsg.outbuf_len,
    is_extended ? NULL : dropped_text);
    start_critical_timings();

    - con->seq++;
    + con->seq = pmsg.seq + 1;

    *handover = console_lock_spinning_disable_and_check(cookie);
    printk_safe_exit_irqrestore(flags);
    --
    2.30.2
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