From: John Ogness <john.ogness@linutronix.de> To: Andrea Parri <parri.andrea@gmail.com> Cc: linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>, Petr Mladek <pmladek@suse.com>, Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>, Steven Rostedt <rostedt@goodmis.org>, Linus Torvalds <torvalds@linux-foundation.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Thomas Gleixner <tglx@linutronix.de>, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, Brendan Higgins <brendanhiggins@google.com>, kexec@lists.infradead.org Subject: Re: [RFC PATCH v5 1/3] printk-rb: new printk ringbuffer implementation (writer) Date: Mon, 23 Dec 2019 17:01:00 +0100 [thread overview] Message-ID: <87imm7820z.fsf@linutronix.de> (raw) In-Reply-To: 20191221142235.GA7824@andrea Hi Andrea, On 2019-12-21, Andrea Parri <parri.andrea@gmail.com> wrote: >> + *desc_out = READ_ONCE(*desc); >> + >> + /* Load data before re-checking state. */ >> + smp_rmb(); /* matches LMM_REF(desc_reserve:A) */ > > I looked for a matching WRITE_ONCE() or some other type of marked write, > but I could not find it. What is the rationale? Or what did I miss? This smp_rmb() matches LMM_TAG(desc_reserve:A). >> + do { >> + next_lpos = get_next_lpos(data_ring, begin_lpos, size); >> + >> + if (!data_push_tail(rb, data_ring, >> + next_lpos - DATA_SIZE(data_ring))) { >> + /* Failed to allocate, specify a data-less block. */ >> + blk_lpos->begin = INVALID_LPOS; >> + blk_lpos->next = INVALID_LPOS; >> + return NULL; >> + } >> + } while (!atomic_long_try_cmpxchg(&data_ring->head_lpos, &begin_lpos, >> + next_lpos)); >> + >> + /* >> + * No barrier is needed here. The data validity is defined by >> + * the state of the associated descriptor. They are marked as >> + * invalid at the moment. And only the winner of the above >> + * cmpxchg() could write here. >> + */ > > The (successful) CMPXCHG provides a full barrier. This comment suggests > that that could be somehow relaxed? Or the comment could be improved? You are correct. There is no need for the full barrier here. This code is based on Petr's POC. I focussed on making sure needed barriers are in place, but did not try to eliminate excessive barriers. > (The patch introduces a number of CMPXCHG: similar questions would > apply to those other instances...) LMM_TAG(data_push_tail:A) is the only CMPXCHG that requires its full barrier. All others can be relaxed. I will make this change for the next series. Thanks for taking time for this. John Ogness
WARNING: multiple messages have this Message-ID (diff)
From: John Ogness <john.ogness@linutronix.de> To: Andrea Parri <parri.andrea@gmail.com> Cc: Petr Mladek <pmladek@suse.com>, Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>, Peter Zijlstra <peterz@infradead.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Brendan Higgins <brendanhiggins@google.com>, linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, Thomas Gleixner <tglx@linutronix.de>, Linus Torvalds <torvalds@linux-foundation.org>, kexec@lists.infradead.org Subject: Re: [RFC PATCH v5 1/3] printk-rb: new printk ringbuffer implementation (writer) Date: Mon, 23 Dec 2019 17:01:00 +0100 [thread overview] Message-ID: <87imm7820z.fsf@linutronix.de> (raw) In-Reply-To: 20191221142235.GA7824@andrea Hi Andrea, On 2019-12-21, Andrea Parri <parri.andrea@gmail.com> wrote: >> + *desc_out = READ_ONCE(*desc); >> + >> + /* Load data before re-checking state. */ >> + smp_rmb(); /* matches LMM_REF(desc_reserve:A) */ > > I looked for a matching WRITE_ONCE() or some other type of marked write, > but I could not find it. What is the rationale? Or what did I miss? This smp_rmb() matches LMM_TAG(desc_reserve:A). >> + do { >> + next_lpos = get_next_lpos(data_ring, begin_lpos, size); >> + >> + if (!data_push_tail(rb, data_ring, >> + next_lpos - DATA_SIZE(data_ring))) { >> + /* Failed to allocate, specify a data-less block. */ >> + blk_lpos->begin = INVALID_LPOS; >> + blk_lpos->next = INVALID_LPOS; >> + return NULL; >> + } >> + } while (!atomic_long_try_cmpxchg(&data_ring->head_lpos, &begin_lpos, >> + next_lpos)); >> + >> + /* >> + * No barrier is needed here. The data validity is defined by >> + * the state of the associated descriptor. They are marked as >> + * invalid at the moment. And only the winner of the above >> + * cmpxchg() could write here. >> + */ > > The (successful) CMPXCHG provides a full barrier. This comment suggests > that that could be somehow relaxed? Or the comment could be improved? You are correct. There is no need for the full barrier here. This code is based on Petr's POC. I focussed on making sure needed barriers are in place, but did not try to eliminate excessive barriers. > (The patch introduces a number of CMPXCHG: similar questions would > apply to those other instances...) LMM_TAG(data_push_tail:A) is the only CMPXCHG that requires its full barrier. All others can be relaxed. I will make this change for the next series. Thanks for taking time for this. John Ogness _______________________________________________ kexec mailing list kexec@lists.infradead.org http://lists.infradead.org/mailman/listinfo/kexec
next prev parent reply other threads:[~2019-12-23 16:01 UTC|newest] Thread overview: 65+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-11-28 1:52 [RFC PATCH v5 0/3] printk: new ringbuffer implementation John Ogness 2019-11-28 1:52 ` John Ogness 2019-11-28 1:52 ` [RFC PATCH v5 1/3] printk-rb: new printk ringbuffer implementation (writer) John Ogness 2019-11-28 1:52 ` John Ogness 2019-12-02 15:48 ` Petr Mladek 2019-12-02 15:48 ` Petr Mladek 2019-12-02 15:59 ` Petr Mladek 2019-12-02 15:59 ` Petr Mladek 2019-12-02 16:37 ` John Ogness 2019-12-02 16:37 ` John Ogness 2019-12-03 1:17 ` Sergey Senozhatsky 2019-12-03 1:17 ` Sergey Senozhatsky 2019-12-03 14:18 ` Steven Rostedt 2019-12-03 14:18 ` Steven Rostedt 2019-12-05 12:01 ` John Ogness 2019-12-05 12:01 ` John Ogness 2019-12-03 8:54 ` Petr Mladek 2019-12-03 8:54 ` Petr Mladek 2019-12-03 14:13 ` John Ogness 2019-12-03 14:13 ` John Ogness 2019-12-03 14:36 ` Petr Mladek 2019-12-03 14:36 ` Petr Mladek 2019-12-09 9:19 ` Sergey Senozhatsky 2019-12-09 9:19 ` Sergey Senozhatsky 2019-12-09 7:42 ` Sergey Senozhatsky 2019-12-09 7:42 ` Sergey Senozhatsky 2019-12-09 9:00 ` John Ogness 2019-12-09 9:00 ` John Ogness 2019-12-09 9:27 ` Sergey Senozhatsky 2019-12-09 9:27 ` Sergey Senozhatsky 2019-12-09 9:34 ` John Ogness 2019-12-09 9:34 ` John Ogness 2019-12-21 14:22 ` Andrea Parri 2019-12-21 14:22 ` Andrea Parri 2019-12-23 16:01 ` John Ogness [this message] 2019-12-23 16:01 ` John Ogness 2020-01-03 10:24 ` Petr Mladek 2020-01-04 14:33 ` Andrea Parri 2019-11-28 1:52 ` [RFC PATCH v5 2/3] printk-rb: new printk ringbuffer implementation (reader) John Ogness 2019-11-28 1:52 ` John Ogness 2019-12-03 12:06 ` Petr Mladek 2019-12-03 12:06 ` Petr Mladek 2019-12-03 13:46 ` John Ogness 2019-12-03 13:46 ` John Ogness 2019-12-04 12:54 ` Petr Mladek 2019-12-04 12:54 ` Petr Mladek 2019-12-04 13:28 ` John Ogness 2019-12-04 13:28 ` John Ogness 2019-12-09 8:43 ` Sergey Senozhatsky 2019-12-09 8:43 ` Sergey Senozhatsky 2019-12-09 9:03 ` Sergey Senozhatsky 2019-12-09 9:03 ` Sergey Senozhatsky 2019-12-09 9:09 ` John Ogness 2019-12-09 9:09 ` John Ogness 2019-11-28 1:52 ` [RFC PATCH v5 3/3] printk-rb: add test module John Ogness 2019-11-28 1:52 ` John Ogness 2019-12-09 8:44 ` Sergey Senozhatsky 2019-12-09 8:44 ` Sergey Senozhatsky 2019-12-05 13:46 ` [RFC PATCH v5 0/3] printk: new ringbuffer implementation Prarit Bhargava 2019-12-05 13:46 ` Prarit Bhargava 2019-12-05 14:05 ` John Ogness 2019-12-05 14:05 ` John Ogness 2019-12-06 14:16 ` Prarit Bhargava 2019-12-06 14:16 ` Prarit Bhargava 2020-01-27 12:20 ` Eugeniu Rosca
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