linux-kernel - Re: [PATCH] ring-buffer: Remove ring_buffer_read_prepare

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Message-ID: <2127243.1751323067@warthog.procyon.org.uk>
Date: Mon, 30 Jun 2025 23:37:47 +0100
From: David Howells <dhowells@...hat.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: dhowells@...hat.com, LKML <linux-kernel@...r.kernel.org>,
    Linux trace kernel <linux-trace-kernel@...r.kernel.org>,
    Masami Hiramatsu <mhiramat@...nel.org>,
    Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] ring-buffer: Remove ring_buffer_read_prepare_sync()

Steven Rostedt <rostedt@...dmis.org> wrote:

> From: Steven Rostedt <rostedt@...dmis.org>
> 
> When the ring buffer was first introduced, reading the non-consuming
> "trace" file required disabling the writing of the ring buffer. To make
> sure the writing was fully disabled before iterating the buffer with a
> non-consuming read, it would set the disable flag of the buffer and then
> call an RCU synchronization to make sure all the buffers were
> synchronized.
> 
> The function ring_buffer_read_start() originally  would initialize the
> iterator and call an RCU synchronization, but this was for each individual
> per CPU buffer where this would get called many times on a machine with
> many CPUs before the trace file could be read. The commit 72c9ddfd4c5bf
> ("ring-buffer: Make non-consuming read less expensive with lots of cpus.")
> separated ring_buffer_read_start into ring_buffer_read_prepare(),
> ring_buffer_read_sync() and then ring_buffer_read_start() to allow each of
> the per CPU buffers to be prepared, call the read_buffer_read_sync() once,
> and then the ring_buffer_read_start() for each of the CPUs which made
> things much faster.
> 
> The commit 1039221cc278 ("ring-buffer: Do not disable recording when there
> is an iterator") removed the requirement of disabling the recording of the
> ring buffer in order to iterate it, but it did not remove the
> synchronization that was happening that was required to wait for all the
> buffers to have no more writers. It's now OK for the buffers to have
> writers and no synchronization is needed.
> 
> Remove the synchronization and put back the interface for the ring buffer
> iterator back before commit 72c9ddfd4c5bf was applied.
> 
> Reported-by: David Howells <dhowells@...hat.com>
> Fixes: 1039221cc278 ("ring-buffer: Do not disable recording when there is an iterator")
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>

Tested-by: David Howells <dhowells@...hat.com>