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SubjectRe: [PATCH 2/2] printk: use the lockless ringbuffer
On (20/02/13 10:42), John Ogness wrote:
> On 2020-02-13, Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> wrote:
> >> - while (user->seq == log_next_seq) {
> >> + if (!prb_read_valid(prb, user->seq, r)) {
> >> if (file->f_flags & O_NONBLOCK) {
> >> ret = -EAGAIN;
> >> logbuf_unlock_irq();
> >> @@ -890,30 +758,26 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
> >>
> >> logbuf_unlock_irq();
> >> ret = wait_event_interruptible(log_wait,
> >> - user->seq != log_next_seq);
> >> + prb_read_valid(prb, user->seq, r));
> >> if (ret)
> >> goto out;
> >> logbuf_lock_irq();
> >> }
> >>
> >> - if (user->seq < log_first_seq) {
> >> - /* our last seen message is gone, return error and reset */
> >> - user->idx = log_first_idx;
> >> - user->seq = log_first_seq;
> >> + if (user->seq < r->info->seq) {
> >> + /* the expected message is gone, return error and reset */
> >> + user->seq = r->info->seq;
> >> ret = -EPIPE;
> >> logbuf_unlock_irq();
> >> goto out;
> >> }
> >
> > Sorry, why doesn't this do something like
> >
> > if (user->seq < prb_first_seq(prb)) {
> > /* the expected message is gone, return error and reset */
> > user->seq = prb_first_seq(prb);
> > ret = -EPIPE;
> > ...
> > }
>
> Here prb_read_valid() was successful, so a record _was_ read. The
> kerneldoc for the prb_read_valid() says:

Hmm, yeah. That's true.

OK, something weird...

I ran some random printk-pressure test (mostly printks from IRQs;
+ some NMI printk-s, but they are routed through nmi printk-safe
buffers; + some limited number of printk-safe printk-s, routed
via printk-safe buffer (so, once again, IRQ); + user-space
journalctl -f syslog reader), and after the test 'cat /dev/kmsg'
is terminally broken

[..]
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
cat /dev/kmsg
cat: /dev/kmsg: Broken pipe
[..]

dmesg works. Reading from /dev/kmsg - doesn't; it did work, however,
before the test.

So I printed seq numbers from devksmg read to a seq buffer and dumped
it via procfs, just seq numbers before we adjust user->seq (set to
r->seq) and after

+ offt += snprintf(BUF + offt,
+ sizeof(BUF) - offt,
+ "%s: devkmsg_read() error %llu %llu %llu\n",
+ current->comm,
+ user->seq,
+ r->info->seq,
+ prb_first_seq(prb));


...
systemd-journal: devkmsg_read() error 1979235 1979236 1979236
systemd-journal: corrected seq 1979236 1979236
systemd-journal: devkmsg_read() error 1979237 1979243 1979243
systemd-journal: corrected seq 1979243 1979243
systemd-journal: devkmsg_read() error 1979244 1979250 1979250
systemd-journal: corrected seq 1979250 1979250
systemd-journal: devkmsg_read() error 1979251 1979257 1979257
systemd-journal: corrected seq 1979257 1979257
systemd-journal: devkmsg_read() error 1979258 1979265 1979265
systemd-journal: corrected seq 1979265 1979265
systemd-journal: devkmsg_read() error 1979266 1979272 1979272
systemd-journal: corrected seq 1979272 1979272
systemd-journal: devkmsg_read() error 1979272 1979273 1979273
systemd-journal: corrected seq 1979273 1979273
systemd-journal: devkmsg_read() error 1979274 1979280 1979280
systemd-journal: corrected seq 1979280 1979280
systemd-journal: devkmsg_read() error 1979281 1982465 1980933
systemd-journal: corrected seq 1982465 1982465
cat: devkmsg_read() error 1980987 1982531 1980987
cat: corrected seq 1982531 1982531
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981015 1982563 1981015
cat: corrected seq 1982563 1982563
cat: devkmsg_read() error 1981080 1982633 1981080
cat: corrected seq 1982633 1982633
cat: devkmsg_read() error 1981080 1982633 1981080
cat: corrected seq 1982633 1982633
cat: devkmsg_read() error 1981080 1982633 1981080
cat: corrected seq 1982633 1982633
cat: devkmsg_read() error 1981080 1982633 1981080
cat: corrected seq 1982633 1982633
cat: devkmsg_read() error 1981080 1982633 1981080
cat: corrected seq 1982633 1982633
cat: devkmsg_read() error 1981080 1982633 1981080
cat: corrected seq 1982633 1982633
cat: devkmsg_read() error 1981080 1982633 1981080
cat: corrected seq 1982633 1982633
cat: devkmsg_read() error 1981080 1982633 1981080
cat: corrected seq 1982633 1982633
cat: devkmsg_read() error 1981080 1982633 1981080
cat: corrected seq 1982633 1982633
cat: devkmsg_read() error 1981080 1982633 1981080
cat: corrected seq 1982633 1982633
cat: devkmsg_read() error 1981080 1982633 1981080
cat: corrected seq 1982633 1982633
cat: devkmsg_read() error 1981080 1982633 1981080
cat: corrected seq 1982633 1982633
cat: devkmsg_read() error 1981080 1982633 1981080
cat: corrected seq 1982633 1982633
cat: devkmsg_read() error 1981095 1982652 1981095
cat: corrected seq 1982652 1982652
cat: devkmsg_read() error 1981095 1982652 1981095
cat: corrected seq 1982652 1982652
cat: devkmsg_read() error 1981095 1982652 1981095
cat: corrected seq 1982652 1982652
cat: devkmsg_read() error 1981095 1982652 1981095
cat: corrected seq 1982652 1982652
cat: devkmsg_read() error 1981095 1982652 1981095
cat: corrected seq 1982652 1982652
cat: devkmsg_read() error 1981095 1982652 1981095
cat: corrected seq 1982652 1982652
cat: devkmsg_read() error 1981095 1982652 1981095
cat: corrected seq 1982652 1982652
...


What's up with that user->seq counter?

-ss

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