linux-kernel - Re: [PATCH v2 5/7][next] printk: ringbuffer: add finalization/extension support

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Date:   Wed, 26 Aug 2020 19:01:13 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Paul McKenney <paulmck@...nel.org>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/7][next] printk: ringbuffer: add
 finalization/extension support

On (20/08/26 10:45), John Ogness wrote:
> On 2020-08-24, John Ogness <john.ogness@...utronix.de> wrote:
> > @@ -1157,6 +1431,14 @@ bool prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
> >  		goto fail;
> >  	}
> >  
> > +	/*
> > +	 * New data is about to be reserved. Once that happens, previous
> > +	 * descriptors are no longer able to be extended. Finalize the
> > +	 * previous descriptor now so that it can be made available to
> > +	 * readers (when committed).
> > +	 */
> > +	desc_finalize(desc_ring, DESC_ID(id - 1));
> > +
> >  	d = to_desc(desc_ring, id);
> >  
> >  	/*
> 
> Apparently this is not enough to guarantee that past descriptors are
> finalized. I am able to reproduce a scenario where the finalization of a
> certain descriptor never happens. That leaves the descriptor permanently
> in the reserved queried state, which prevents any new records from being
> created. I am investigating.

Good to know. I also run into problems:
- broken dmesg (and broken journalctl -f /dev/kmsg poll) and broken
  syslog read

$ strace dmesg

...
openat(AT_FDCWD, "/dev/kmsg", O_RDONLY|O_NONBLOCK) = 3
lseek(3, 0, SEEK_DATA)                  = 0
read(3, 0x55dda8c240a8, 8191)           = -1 EAGAIN (Resource temporarily unavailable)
close(3)                                = 0
syslog(10 /* SYSLOG_ACTION_SIZE_BUFFER */) = 524288
mmap(NULL, 528384, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f43ea847000
syslog(3 /* SYSLOG_ACTION_READ_ALL */, "", 524296) = 0
munmap(0x7f43ea847000, 528384)          = 0
...

	-ss