linux-kernel - Re: [PATCH 0/6] tracing/ring-buffer: Fix wakeup of ring buffer waiters

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Date: Fri, 8 Mar 2024 13:39:14 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	Masami Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	joel@...lfernandes.org, linke li <lilinke99@...com>, Rabin Vincent <rabin@....in>
Subject: Re: [PATCH 0/6] tracing/ring-buffer: Fix wakeup of ring buffer waiters

On Fri, 8 Mar 2024 at 13:33, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> There's two layers:
>
> 1) the ring buffer has the above simple producer / consumer.
>    Where the wake ups can happen at the point of where the buffer has
>    the amount filled that the consumer wants to start consuming with.
>
> 2) The tracing layer; Here on close of a file, the consumers need to be
>    woken up and not wait again. And just take whatever was there to finish
>    reading.
>
>    There's also another case that the ioctl() just kicks the current
>    readers out, but doesn't care about new readers.

But that's the beauty of just using the wait_event() model.

Just add that "exit" condition to the condition.

So the above "complexity" is *literally* just changing the

                  (new = atomic_read_acquire(&my->seq)) != old

condition to

                  should_exit ||
                  (new = atomic_read_acquire(&my->seq)) != old

(replace "should_exit" with whatever that condition is, of course) and
the wait_event() logic will take care of the rest.

             Linus