linux-kernel - Re: [PATCH v4 01/13] ring-buffer: Allow mapped field to be set without mapping

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Date: Tue, 11 Jun 2024 21:57:18 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Guenter Roeck <linux@...ck-us.net>
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>, Vincent Donnefort <vdonnefort@...gle.com>,
 Joel Fernandes <joel@...lfernandes.org>, Daniel Bristot de Oliveira
 <bristot@...hat.com>, Ingo Molnar <mingo@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>, suleiman@...gle.com, Thomas Gleixner
 <tglx@...utronix.de>, Vineeth Pillai <vineeth@...byteword.org>, Youssef
 Esmat <youssefesmat@...gle.com>, Beau Belgrave <beaub@...ux.microsoft.com>,
 Alexander Graf <graf@...zon.com>, Baoquan He <bhe@...hat.com>, Borislav
 Petkov <bp@...en8.de>, "Paul E. McKenney" <paulmck@...nel.org>, David
 Howells <dhowells@...hat.com>, Mike Rapoport <rppt@...nel.org>, Dave Hansen
 <dave.hansen@...ux.intel.com>, Tony Luck <tony.luck@...el.com>, Ross
 Zwisler <zwisler@...gle.com>, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v4 01/13] ring-buffer: Allow mapped field to be set
 without mapping

On Tue, 11 Jun 2024 21:39:37 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> > 
> > Maybe explain why sometimes __rb_inc_dec_mapped() is called to
> > increment or decrement ->mapped, and sometimes it id done directly ?
> > I can see that the function also acquires the buffer mutex, which
> > isn't needed at the places where mapped is incremented/decremented
> > directly, but common code would still be nice, and it is odd to see
> > over/underflows handled sometimes but not always.  
> 
> Sure. I'll add comments explaining more.

And I found a bug with this code. It assumes that mapped will be equal
to 1 if it's the last mapping. That will no longer be the case.

-- Steve