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Tue, 25 Jun 2019 01:55:52 -0700 (PDT) Received: from localhost ([175.223.22.38]) by smtp.gmail.com with ESMTPSA id j13sm13725770pfh.13.2019.06.25.01.55.50 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Tue, 25 Jun 2019 01:55:51 -0700 (PDT) Date: Tue, 25 Jun 2019 17:55:48 +0900 From: Sergey Senozhatsky To: John Ogness Cc: linux-kernel@vger.kernel.org, Peter Zijlstra , Petr Mladek , Sergey Senozhatsky , Steven Rostedt , Linus Torvalds , Greg Kroah-Hartman , Andrea Parri , Thomas Gleixner , Sergey Senozhatsky Subject: Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation Message-ID: <20190625085548.GA532@jagdpanzerIV> References: <20190607162349.18199-1-john.ogness@linutronix.de> <20190607162349.18199-2-john.ogness@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190607162349.18199-2-john.ogness@linutronix.de> User-Agent: Mutt/1.12.1 (2019-06-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On (06/07/19 18:29), John Ogness 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); > + } 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); > + } > +} [..] > +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++. -ss