From: John Ogness <john.ogness@linutronix.de>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.org>,
Petr Mladek <pmladek@suse.com>,
Steven Rostedt <rostedt@goodmis.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Andrea Parri <andrea.parri@amarulasolutions.com>,
Thomas Gleixner <tglx@linutronix.de>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation
Date: Tue, 25 Jun 2019 11:19:02 +0200 [thread overview]
Message-ID: <87blymhvjt.fsf@linutronix.de> (raw)
In-Reply-To: <20190625085548.GA532@jagdpanzerIV> (Sergey Senozhatsky's message of "Tue, 25 Jun 2019 17:55:48 +0900")
On 2019-06-25, Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> wrote:
> [..]
>> +static void add_descr_list(struct prb_reserved_entry *e)
>> +{
>> + struct printk_ringbuffer *rb = e->rb;
>> + struct prb_list *l = &rb->descr_list;
>> + struct prb_descr *d = e->descr;
>> + struct prb_descr *newest_d;
>> + unsigned long newest_id;
>> +
>> + /* set as newest */
>> + do {
>> + /* MB5: synchronize add descr */
>> + newest_id = smp_load_acquire(&l->newest);
>> + newest_d = TO_DESCR(rb, newest_id);
>> +
>> + if (newest_id == EOL)
>> + WRITE_ONCE(d->seq, 1);
>> + else
>> + WRITE_ONCE(d->seq, READ_ONCE(newest_d->seq) + 1);
>> + /*
>> + * MB5: synchronize add descr
>> + *
>> + * In particular: next written before cmpxchg
>> + */
>> + } while (cmpxchg_release(&l->newest, newest_id, e->id) != newest_id);
>> +
>> + if (unlikely(newest_id == EOL)) {
>> + /* no previous newest means we *are* the list, set oldest */
>> +
>> + /*
>> + * MB UNPAIRED
>> + *
>> + * In particular: Force cmpxchg _after_ cmpxchg on newest.
>> + */
>> + WARN_ON_ONCE(cmpxchg_release(&l->oldest, EOL, e->id) != EOL);
This WARN_ON_ONCE...
>> + } else {
>> + /* link to previous chain */
>> +
>> + /*
>> + * MB6: synchronize link descr
>> + *
>> + * In particular: Force cmpxchg _after_ cmpxchg on newest.
>> + */
>> + WARN_ON_ONCE(cmpxchg_release(&newest_d->next,
>> + EOL, e->id) != EOL);
... and this WARN_ON_ONCE should both really be BUG_ON. These situations
will not happen. Actually, they should both be xchg_release(). But until
everyone is happy with the memory barriers, I wanted to leave this bug
checking in place.
>> + }
>> +}
>
> [..]
>
>> +char *prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
>> + unsigned int size)
>> +{
>> + struct prb_datablock *b;
>> + struct prb_descr *d;
>> + char *buf;
>> +
>> + if (size == 0)
>> + return NULL;
>> +
>> + size += sizeof(struct prb_datablock);
>> + size = DATA_ALIGN_SIZE(size);
>> + if (size > DATAARRAY_SIZE(rb))
>> + return NULL;
>> +
>> + e->rb = rb;
>> +
>> + local_irq_save(e->irqflags);
>> +
>> + if (!assign_descr(e))
>> + goto err_out;
>> +
>> + d = e->descr;
>> + WRITE_ONCE(d->id, e->id);
>> +
>> + if (!data_reserve(e, size)) {
>> + /* put invalid descriptor on list, can still be traversed */
>> + WRITE_ONCE(d->next, EOL);
>> + add_descr_list(e);
>> + goto err_out;
>> + }
>
> I'm wondering if prb can always report about its problems. Including the
> cases when things "go rather bad".
>
> Suppose we have
>
> printk()
> prb_reserve()
> !data_reserve()
> add_descr_list()
> WARN_ON_ONCE()
> printk()
> prb_reserve()
> !assign_descr(e) << lost WARN_ON's "printk" or "printks"?
>
> In general, assuming that there might be more error printk-s either
> called directly directly from prb->printk on indirectly, from
> prb->ABC->printk.
>
> Also note,
> Lost printk-s are not going to be accounted as 'lost' automatically.
> It seems that for printk() there is no way to find out that it has
> recursed from printk->prb_commit but hasn't succeeded in storing
> recursive messages. I'd say that prb_reserve() err_out should probably
> &rb->lost++.
This is a good point. I have no problems with that. In that case, it
should probably be called "fail" instead of "lost".
John Ogness
next prev parent reply other threads:[~2019-06-25 9:19 UTC|newest]
Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-07 16:23 [RFC PATCH v2 0/2] printk: new ringbuffer implementation John Ogness
2019-06-07 16:23 ` [RFC PATCH v2 1/2] printk-rb: add a new printk " John Ogness
2019-06-18 4:51 ` Sergey Senozhatsky
2019-06-18 22:12 ` John Ogness
2019-06-25 6:45 ` Sergey Senozhatsky
2019-06-25 7:15 ` Sergey Senozhatsky
2019-06-25 8:44 ` John Ogness
2019-06-25 9:06 ` Petr Mladek
2019-06-25 10:03 ` Sergey Senozhatsky
2019-06-25 12:03 ` John Ogness
2019-06-26 2:08 ` Sergey Senozhatsky
2019-06-26 7:16 ` John Ogness
2019-06-26 7:45 ` Sergey Senozhatsky
2019-06-26 7:47 ` Petr Mladek
2019-06-26 7:59 ` Sergey Senozhatsky
2019-06-25 9:09 ` Sergey Senozhatsky
2019-06-18 11:12 ` Peter Zijlstra
2019-06-18 22:18 ` John Ogness
2019-06-18 11:22 ` Peter Zijlstra
2019-06-18 22:30 ` John Ogness
2019-06-19 10:46 ` Andrea Parri
2019-06-20 22:50 ` John Ogness
2019-06-21 12:16 ` Andrea Parri
2019-06-19 11:08 ` Peter Zijlstra
2019-06-18 11:47 ` Peter Zijlstra
2019-06-20 22:23 ` John Ogness
2019-06-26 22:40 ` Peter Zijlstra
2019-06-26 22:53 ` Peter Zijlstra
2019-06-28 9:50 ` John Ogness
2019-06-28 15:44 ` Peter Zijlstra
2019-06-28 16:07 ` Peter Zijlstra
2019-07-01 10:39 ` John Ogness
2019-07-01 14:10 ` Peter Zijlstra
2019-07-01 14:11 ` Peter Zijlstra
2019-06-29 21:05 ` Andrea Parri
2019-06-30 2:03 ` John Ogness
2019-06-30 14:08 ` Andrea Parri
2019-07-02 14:13 ` John Ogness
2019-06-26 22:47 ` Peter Zijlstra
2019-06-21 14:05 ` Petr Mladek
2019-06-24 8:33 ` John Ogness
2019-06-24 14:09 ` Petr Mladek
2019-06-25 13:29 ` John Ogness
2019-06-26 8:29 ` Petr Mladek
2019-06-26 9:09 ` John Ogness
2019-06-26 21:16 ` Peter Zijlstra
2019-06-26 21:43 ` John Ogness
2019-06-27 8:28 ` Petr Mladek
2019-07-04 10:33 ` [PATCH POC] printk_ringbuffer: Alternative implementation of lockless printk ringbuffer Petr Mladek
2019-07-04 14:59 ` John Ogness
2019-07-08 15:23 ` Petr Mladek
2019-07-09 1:34 ` John Ogness
2019-07-09 9:06 ` Petr Mladek
2019-07-09 10:21 ` John Ogness
2019-07-09 11:58 ` Petr Mladek
2019-08-14 3:46 ` John Ogness
2019-06-24 13:55 ` [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation John Ogness
2019-06-25 8:55 ` Sergey Senozhatsky
2019-06-25 9:19 ` John Ogness [this message]
2019-06-07 16:23 ` [RFC PATCH v2 2/2] printk-rb: add test module John Ogness
2019-06-17 21:09 ` [RFC PATCH v2 0/2] printk: new ringbuffer implementation Thomas Gleixner
2019-06-18 7:15 ` Petr Mladek
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