linux-kernel - Re: [PATCH v8 01/11] timekeeping: move multigrain timestamp floor handling into timekeeper

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Message-ID: <874j6f99dg.ffs@tglx>
Date: Mon, 16 Sep 2024 12:32:43 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Jeff Layton <jlayton@...nel.org>, John Stultz <jstultz@...gle.com>,
 Stephen Boyd <sboyd@...nel.org>, Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, Steven
 Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Jonathan Corbet
 <corbet@....net>, Chandan Babu R <chandan.babu@...cle.com>, "Darrick J.
 Wong" <djwong@...nel.org>, Theodore Ts'o <tytso@....edu>, Andreas Dilger
 <adilger.kernel@...ger.ca>, Chris Mason <clm@...com>, Josef Bacik
 <josef@...icpanda.com>, David Sterba <dsterba@...e.com>, Hugh Dickins
 <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, Chuck Lever
 <chuck.lever@...cle.com>, Vadim Fedorenko <vadim.fedorenko@...ux.dev>
Cc: Randy Dunlap <rdunlap@...radead.org>, linux-kernel@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-xfs@...r.kernel.org,
 linux-ext4@...r.kernel.org, linux-btrfs@...r.kernel.org,
 linux-nfs@...r.kernel.org, linux-mm@...ck.org, Jeff Layton
 <jlayton@...nel.org>
Subject: Re: [PATCH v8 01/11] timekeeping: move multigrain timestamp floor
 handling into timekeeper

On Mon, Sep 16 2024 at 12:12, Thomas Gleixner wrote:
> On Sat, Sep 14 2024 at 13:07, Jeff Layton wrote:
>> +	do {
>> +		seq = read_seqcount_begin(&tk_core.seq);
>> +
>> +		ts->tv_sec = tk->xtime_sec;
>> +		mono = tk->tkr_mono.base;
>> +		nsecs = timekeeping_get_ns(&tk->tkr_mono);
>> +		offset = *offsets[TK_OFFS_REAL];
>> +	} while (read_seqcount_retry(&tk_core.seq, seq));
>> +
>> +	mono = ktime_add_ns(mono, nsecs);
>> +
>> +	if (atomic64_try_cmpxchg(&mg_floor, &old, mono)) {
>> +		ts->tv_nsec = 0;
>> +		timespec64_add_ns(ts, nsecs);
>> +	} else {
>> +		/*
>> +		 * Something has changed mg_floor since "old" was
>> +		 * fetched. "old" has now been updated with the
>> +		 * current value of mg_floor, so use that to return
>> +		 * the current coarse floor value.
>
> 'Something has changed' is a truly understandable technical
> explanation.

     old = mg_floor
                                mono = T1;
                                mg_floor = mono
preemption

     do {
        mono = T2;
     }

     cmpxchg fails and the function returns a value based on T1

No?

Thanks,

        tglx