linux-kernel - Re: [PATCH RESEND 1/3] printk: convert byte-buffer to variable-length record buffer

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Date:	Wed, 28 Nov 2012 14:33:14 +0100
From:	Michael Kerrisk <mtk.manpages@...il.com>
To:	Kay Sievers <kay@...y.org>
Cc:	Greg Kroah-Hartmann <greg@...ah.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH RESEND 1/3] printk: convert byte-buffer to variable-length
 record buffer

Hi Kay,

On Thu, May 3, 2012 at 2:29 AM, Kay Sievers <kay@...y.org> wrote:
> From: Kay Sievers <kay@...y.org>
[...]
>         case SYSLOG_ACTION_SIZE_UNREAD:
> -               error = log_end - log_start;
> +               raw_spin_lock_irq(&logbuf_lock);
> +               if (syslog_seq < log_first_seq) {
> +                       /* messages are gone, move to first one */
> +                       syslog_seq = log_first_seq;
> +                       syslog_idx = log_first_idx;
> +               }
> +               if (from_file) {
> +                       /*
> +                        * Short-cut for poll(/"proc/kmsg") which simply checks
> +                        * for pending data, not the size; return the count of
> +                        * records, not the length.
> +                        */
> +                       error = log_next_idx - syslog_idx;
> +               } else {
> +                       u64 seq;
> +                       u32 idx;
> +
> +                       error = 0;
> +                       seq = syslog_seq;
> +                       idx = syslog_idx;
> +                       while (seq < log_next_seq) {
> +                               error += syslog_print_line(idx, NULL, 0);
> +                               idx = log_next(idx);
> +                               seq++;
> +                       }
> +               }
> +               raw_spin_unlock_irq(&logbuf_lock);
>                 break;
[...]

It looks as though the changes here have broken SYSLOG_ACTION_SIZE_UNREAD.

On a 2.6.31 system, immediately after SYSLOG_ACTION_READ_CLEAR, a
SYSLOG_ACTION_SIZE_UNREAD returns 0.

On 3.5, immediately after SYSLOG_ACTION_READ_CLEAR, the value returned
by SYSLOG_ACTION_SIZE_UNREAD is unchanged (i.e., assuming that the
value returned was non-zero before SYSLOG_ACTION_SIZE_UNREAD, it is
still nonzero afterward), even though a subsequent
SYSLOG_ACTION_READ_CLEAR indicates that there are zero bytes to read.

(All tests conducted with (r)syslogd stopped.)

Cheers,

Michael


-- 
Michael Kerrisk Linux man-pages maintainer;
http://www.kernel.org/doc/man-pages/
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