linux-kernel - Re: [PATCH 1/3] perf: convert perf_event_context.refcount to refcount

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Message-ID: <20190201103254.GB31409@lakrids.cambridge.arm.com>
Date:   Fri, 1 Feb 2019 10:32:54 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [PATCH 1/3] perf: convert perf_event_context.refcount to
 refcount_t

On Tue, Jan 29, 2019 at 01:55:32PM +0000, Reshetova, Elena wrote:
> > On Mon, Jan 28, 2019 at 02:27:26PM +0200, Elena Reshetova wrote:
> > > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > > index 3cd13a3..a1e87d2 100644
> > > --- a/kernel/events/core.c
> > > +++ b/kernel/events/core.c
> > > @@ -1171,7 +1171,7 @@ static void perf_event_ctx_deactivate(struct
> > perf_event_context *ctx)
> > >
> > >  static void get_ctx(struct perf_event_context *ctx)
> > >  {
> > > -	WARN_ON(!atomic_inc_not_zero(&ctx->refcount));
> > > +	WARN_ON(!refcount_inc_not_zero(&ctx->refcount));
> > 
> > This could be refcount_inc(), remember how that already produces a WARN
> > when we try and increment 0.
> 
> But is this true for the x86 arch-specific implementation also? 

If you use recount_inc_checked(), it will always produce the WARN(),
even when using the x86-specific refcount implementation.

(this was one place I had intended to use the *_checked() forms of the
refcount ops).

Thanks,
Mark.