From: Petr Mladek <pmladek@suse.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: linux-kernel@vger.kernel.org,
Andrea Parri <andrea.parri@amarulasolutions.com>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Brendan Higgins <brendanhiggins@google.com>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Linus Torvalds <torvalds@linux-foundation.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: assign_desc() barriers: Re: [RFC PATCH v4 1/9] printk-rb: add a new printk ringbuffer implementation
Date: Tue, 20 Aug 2019 16:14:29 +0200 [thread overview]
Message-ID: <20190820141429.hkrnynmr5ou4lem2@pathway.suse.cz> (raw)
In-Reply-To: <20190820082253.ybys4fsakxxdvahx@pathway.suse.cz>
On Tue 2019-08-20 10:22:53, Petr Mladek wrote:
> On Thu 2019-08-08 00:32:26, John Ogness wrote:
> > --- /dev/null
> > +++ b/kernel/printk/ringbuffer.c
> > +/**
> > + * assign_desc() - Assign a descriptor to the caller.
> > + *
> > + * @e: The entry structure to store the assigned descriptor to.
> > + *
> > + * Find an available descriptor to assign to the caller. First it is checked
> > + * if the tail descriptor from the committed list can be recycled. If not,
> > + * perhaps a never-used descriptor is available. Otherwise, data blocks will
> > + * be invalidated until the tail descriptor from the committed list can be
> > + * recycled.
> > + *
> > + * Assigned descriptors are invalid until data has been reserved for them.
> > + *
> > + * Return: true if a descriptor was assigned, otherwise false.
> > + *
> > + * This will only fail if it was not possible to invalidate data blocks in
> > + * order to recycle a descriptor. This can happen if a writer has reserved but
> > + * not yet committed data and that reserved data is currently the oldest data.
> > + */
> > +static bool assign_desc(struct prb_reserved_entry *e)
> > +{
> > + struct printk_ringbuffer *rb = e->rb;
> > + struct prb_desc *d;
> > + struct nl_node *n;
> > + unsigned long i;
> > +
> > + for (;;) {
> > + /*
> > + * jA:
> > + *
> > + * Try to recycle a descriptor on the committed list.
> > + */
> > + n = numlist_pop(&rb->nl);
> > + if (n) {
> > + d = container_of(n, struct prb_desc, list);
> > + break;
> > + }
> > +
> > + /* Fallback to static never-used descriptors. */
> > + if (atomic_read(&rb->desc_next_unused) < DESCS_COUNT(rb)) {
> > + i = atomic_fetch_inc(&rb->desc_next_unused);
> > + if (i < DESCS_COUNT(rb)) {
> > + d = &rb->descs[i];
> > + atomic_long_set(&d->id, i);
> > + break;
> > + }
> > + }
> > +
> > + /*
> > + * No descriptor available. Make one available for recycling
> > + * by invalidating data (which some descriptor will be
> > + * referencing).
> > + */
> > + if (!dataring_pop(&rb->dr))
> > + return false;
> > + }
> > +
> > + /*
> > + * jB:
> > + *
> > + * Modify the descriptor ID so that users of the descriptor see that
> > + * it has been recycled. A _release() is used so that prb_getdesc()
> > + * callers can see all data ringbuffer updates after issuing a
> > + * pairing smb_rmb(). See iA for details.
> > + *
> > + * Memory barrier involvement:
> > + *
> > + * If dB->iA reads from jB, then dI reads the same value as
> > + * jA->cD->hA.
> > + *
> > + * Relies on:
> > + *
> > + * RELEASE from jA->cD->hA to jB
> > + * matching
> > + * RMB between dB->iA and dI
> > + */
> > + atomic_long_set_release(&d->id, atomic_long_read(&d->id) +
> > + DESCS_COUNT(rb));
>
> atomic_long_set_release() might be a bit confusing here.
> There is no related acquire.
>
> In fact, d->id manipulation has barriers from both sides:
>
> + smp_rmb() before so that all reads are finished before
> the id is updated (release)
Uh, this statement does not make sense. The read barrier is not
needed here. Instead the readers need it.
Well, we might need a write barrier before d->id manipulation.
It should be in numlist_pop() after successfully updating nl->tail_id.
It will allow readers to detect that the desriptor is being reused
(not in valid tail_id..head_id range) before we start manipulating it.
> + smp_wmb() after so that the new ID is written before other
> related values are modified (acquire).
>
> The smp_wmb() barrier is in prb_reserve(). I would move it here.
This still makes sense. I would move the write barrier from
prb_reserve() here.
Sigh, I have to admit that I am not familiar with the _acquire(),
_release(), and _relaxed() variants of the atomic operations.
They probably make it easier to implement some locking API.
I am not sure how to use it here. This code implements a complex
interlock between several variables. I mean that several variables
lock each other in a cycle, like a state machine? In each case,
it is not a simple locking where we check state of a single
variable.
Best Regards,
Petr
next prev parent reply other threads:[~2019-08-20 14:14 UTC|newest]
Thread overview: 131+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-07 22:26 [RFC PATCH v4 0/9] printk: new ringbuffer implementation John Ogness
2019-08-07 22:26 ` [RFC PATCH v4 1/9] printk-rb: add a new printk " John Ogness
2019-08-20 8:15 ` numlist_pop(): " Petr Mladek
2019-08-21 5:41 ` John Ogness
2019-09-04 12:19 ` Peter Zijlstra
2019-08-20 8:22 ` assign_desc() barriers: " Petr Mladek
2019-08-20 14:14 ` Petr Mladek [this message]
2019-08-21 5:52 ` John Ogness
2019-08-22 11:53 ` Petr Mladek
2019-08-25 2:06 ` John Ogness
2019-08-26 8:21 ` John Ogness
2019-08-20 8:55 ` comments style: " Petr Mladek
2019-08-20 9:27 ` Sergey Senozhatsky
2019-08-21 5:46 ` John Ogness
2019-08-22 13:50 ` Petr Mladek
2019-08-22 17:38 ` Andrea Parri
2019-08-23 10:47 ` Petr Mladek
2019-08-23 14:27 ` Andrea Parri
2019-08-23 9:49 ` Sergey Senozhatsky
2019-08-23 5:54 ` Sergey Senozhatsky
2019-08-23 10:29 ` Petr Mladek
2019-08-21 5:42 ` John Ogness
2019-08-22 12:44 ` Petr Mladek
2019-08-20 13:50 ` dataring_push() barriers " Petr Mladek
2019-08-25 2:42 ` John Ogness
2019-08-27 14:36 ` Petr Mladek
2019-08-28 13:43 ` John Ogness
2019-08-20 15:12 ` datablock reuse races " Petr Mladek
2019-08-23 9:21 ` numlist_push() barriers " Petr Mladek
2019-08-26 8:34 ` Andrea Parri
2019-08-26 8:43 ` Andrea Parri
2019-08-26 14:10 ` Petr Mladek
2019-08-26 16:01 ` Andrea Parri
2019-08-26 22:36 ` John Ogness
2019-08-27 7:40 ` Petr Mladek
2019-08-27 14:28 ` John Ogness
2019-08-27 15:07 ` Petr Mladek
2019-08-28 10:24 ` John Ogness
2019-08-23 17:18 ` numlist API " Petr Mladek
2019-08-26 23:57 ` John Ogness
2019-08-27 13:03 ` Petr Mladek
2019-08-28 7:13 ` John Ogness
2019-08-28 8:58 ` Petr Mladek
2019-08-28 14:03 ` John Ogness
2019-08-29 11:28 ` Petr Mladek
2019-09-03 7:58 ` Sergey Senozhatsky
2019-08-30 14:48 ` dataring " Petr Mladek
2019-08-07 22:26 ` [RFC PATCH v4 2/9] printk-rb: add test module John Ogness
2019-08-07 22:26 ` [RFC PATCH v4 3/9] printk-rb: fix missing includes/exports John Ogness
2019-08-07 22:26 ` [RFC PATCH v4 4/9] printk-rb: initialize new descriptors as invalid John Ogness
2019-08-20 9:23 ` Petr Mladek
2019-08-20 10:16 ` Sergey Senozhatsky
2019-08-21 5:56 ` John Ogness
2019-08-07 22:26 ` [RFC PATCH v4 5/9] printk-rb: remove extra data buffer size allocation John Ogness
2019-08-07 22:26 ` [RFC PATCH v4 6/9] printk-rb: adjust test module ringbuffer sizes John Ogness
2019-08-19 21:29 ` [PATCH] printk-rb: fix test module macro usage John Ogness
2019-08-07 22:26 ` [RFC PATCH v4 7/9] printk-rb: increase size of seq and size variables John Ogness
2019-08-07 22:26 ` [RFC PATCH v4 8/9] printk-rb: new functionality to support printk John Ogness
2019-08-20 9:59 ` Sergey Senozhatsky
2019-08-21 5:47 ` John Ogness
2019-08-07 22:26 ` [RFC PATCH v4 9/9] printk: use a new ringbuffer implementation John Ogness
2019-08-08 19:07 ` Linus Torvalds
2019-08-08 22:55 ` John Ogness
2019-08-08 23:33 ` Linus Torvalds
2019-08-08 23:45 ` Steven Rostedt
2019-08-09 0:21 ` Linus Torvalds
2019-08-09 0:48 ` Steven Rostedt
2019-08-09 1:15 ` Linus Torvalds
2019-08-09 11:15 ` Thomas Gleixner
2019-08-09 16:00 ` Linus Torvalds
2019-08-09 20:07 ` Thomas Gleixner
2019-08-09 20:20 ` Linus Torvalds
2019-08-09 6:14 ` Peter Zijlstra
2019-08-09 7:08 ` John Ogness
2019-08-09 15:57 ` Linus Torvalds
2019-08-10 5:53 ` Thomas Gleixner
2019-09-10 3:19 ` Sergey Senozhatsky
2019-08-12 9:54 ` Geert Uytterhoeven
2019-08-16 5:46 ` Dave Young
2019-08-16 5:54 ` Dave Young
2019-08-16 9:40 ` John Ogness
2019-09-04 12:35 ` [RFC PATCH v4 0/9] printk: " Peter Zijlstra
2019-09-05 13:05 ` Petr Mladek
2019-09-05 14:31 ` Peter Zijlstra
2019-09-05 15:38 ` Thomas Gleixner
2019-09-05 16:11 ` Steven Rostedt
2019-09-05 21:10 ` John Ogness
2019-09-06 9:39 ` Petr Mladek
2019-09-09 14:11 ` printk meeting at LPC Thomas Gleixner
2019-09-13 13:26 ` John Ogness
2019-09-13 14:48 ` Daniel Vetter
2019-09-15 13:47 ` John Ogness
2019-09-16 8:44 ` Daniel Vetter
2019-09-16 4:30 ` Tetsuo Handa
2019-09-16 10:46 ` Petr Mladek
2019-09-16 13:43 ` Steven Rostedt
2019-09-16 14:28 ` John Ogness
2019-09-17 8:11 ` Petr Mladek
2019-09-17 7:52 ` Petr Mladek
2019-09-17 13:02 ` Steven Rostedt
2019-09-17 13:12 ` Greg Kroah-Hartman
2019-09-17 13:37 ` Steven Rostedt
2019-09-17 14:08 ` Tetsuo Handa
2019-09-17 7:51 ` Sergey Senozhatsky
2019-09-18 1:25 ` Sergey Senozhatsky
2019-09-18 2:08 ` Steven Rostedt
2019-09-18 2:36 ` Sergey Senozhatsky
2019-09-18 5:19 ` Sergey Senozhatsky
2019-09-18 7:42 ` John Ogness
2019-09-18 8:10 ` Sergey Senozhatsky
2019-09-18 9:05 ` John Ogness
2019-09-18 9:11 ` Sergey Senozhatsky
2019-09-18 16:41 ` Petr Mladek
2019-09-18 16:48 ` Steven Rostedt
2019-09-24 14:24 ` Petr Mladek
2019-09-19 8:06 ` Daniel Vetter
2019-09-18 7:33 ` John Ogness
2019-09-18 8:08 ` Sergey Senozhatsky
2019-10-04 14:48 ` Tony Asleson
2019-10-07 12:01 ` Petr Mladek
2019-09-06 9:06 ` [RFC PATCH v4 0/9] printk: new ringbuffer implementation Peter Zijlstra
2019-09-06 10:09 ` Sergey Senozhatsky
2019-09-06 10:49 ` Peter Zijlstra
2019-09-06 13:44 ` Sergey Senozhatsky
2019-09-06 12:42 ` Petr Mladek
2019-09-06 14:01 ` Peter Zijlstra
2019-09-06 14:22 ` Peter Zijlstra
2019-09-06 19:53 ` Sergey Senozhatsky
2019-09-06 22:47 ` John Ogness
2019-09-08 22:18 ` Peter Zijlstra
2019-09-10 3:22 ` Sergey Senozhatsky
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