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SubjectRe: [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

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