linux-kernel - Re: [PATCH 2/2] printk: use the lockless ringbuffer

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Date:   Fri, 14 Feb 2020 11:09:35 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH 2/2] printk: use the lockless ringbuffer

On (20/02/14 10:41), Sergey Senozhatsky wrote:
> On (20/02/13 23:36), John Ogness wrote:
[..]
> > We could implement it such that devkmsg_read() will skip over data-less
> > records instead of issuing an EPIPE. (That is what dmesg does.) But then
> > do we need EPIPE at all? The reader can see that is has missed records
> > by tracking the sequence number, so could we just get rid of EPIPE? Then
> > cat(1) would be a great tool to view the raw ringbuffer. Please share
> > your thoughts on this.
> 
> Looking at systemd/src/journal/journald-kmsg.c : server_read_dev_kmsg()
> -EPIPE is just one of the erronos they handle, nothing special. Could it
> be the case that some other loggers would have special handling for EPIPE?
> I'm not sure, let's look around.

rsyslog

static void
readkmsg(void)
{
	int i;
	uchar pRcv[8192+1];
	char errmsg[2048];

	for (;;) {
		dbgprintf("imkmsg waiting for kernel log line\n");

		/* every read() from the opened device node receives one record of the printk buffer */
		i = read(fklog, pRcv, 8192);

		if (i > 0) {
			/* successful read of message of nonzero length */
			pRcv[i] = '\0';
		} else if (i == -EPIPE) {
			imkmsgLogIntMsg(LOG_WARNING,
					"imkmsg: some messages in circular buffer got overwritten");
			continue;
		} else {
			/* something went wrong - error or zero length message */
			if (i < 0 && errno != EINTR && errno != EAGAIN) {
				/* error occured */
				imkmsgLogIntMsg(LOG_ERR,
				       "imkmsg: error reading kernel log - shutting down: %s",
					rs_strerror_r(errno, errmsg, sizeof(errmsg)));
				fklog = -1;
			}
			break;
		}

		submitSyslog(pRcv);
	}
}


So EPIPE errno better stay around.

	-ss