From: Amol Grover <frextrite@gmail.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
"Paul E . McKenney" <paulmck@kernel.org>,
Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>,
Peter Zijlstra <peterz@infradead.org>,
linux-kernel-mentees@lists.linuxfoundation.org,
linux-kernel@vger.kernel.org,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Ingo Molnar <mingo@redhat.com>,
Namhyung Kim <namhyung@kernel.org>, Jiri Olsa <jolsa@redhat.com>
Subject: Re: [Linux-kernel-mentees] [PATCH] events: Annotate parent_ctx with __rcu
Date: Thu, 13 Feb 2020 12:14:18 +0530 [thread overview]
Message-ID: <20200213064418.GA26550@workstation-portable> (raw)
In-Reply-To: <20200210170831.GB246160@google.com>
On Mon, Feb 10, 2020 at 12:08:31PM -0500, Joel Fernandes wrote:
> On Mon, Feb 10, 2020 at 10:17:27PM +0530, Amol Grover wrote:
> > On Mon, Feb 10, 2020 at 02:34:59PM +0100, Peter Zijlstra wrote:
> > > On Mon, Feb 10, 2020 at 06:29:48PM +0530, Amol Grover wrote:
> > > > On Mon, Feb 10, 2020 at 10:36:24AM +0100, Peter Zijlstra wrote:
> > > > > On Sat, Feb 08, 2020 at 08:16:49PM +0530, Amol Grover wrote:
> > >
> > > > > > @@ -3106,26 +3106,31 @@ static void ctx_sched_out(struct perf_event_context *ctx,
> > > > > > static int context_equiv(struct perf_event_context *ctx1,
> > > > > > struct perf_event_context *ctx2)
> > > > > > {
> > > > > > + struct perf_event_context *parent_ctx1, *parent_ctx2;
> > > > > > +
> > > > > > lockdep_assert_held(&ctx1->lock);
> > > > > > lockdep_assert_held(&ctx2->lock);
> > > > > >
> > > > > > + parent_ctx1 = rcu_dereference(ctx1->parent_ctx);
> > > > > > + parent_ctx2 = rcu_dereference(ctx2->parent_ctx);
>
> You can probably remove the earlier lockdep_assert_held(s) if you're going to
> use rcu_dereference_protected() here, since that would do the checking anyway.
>
Ah yes, I was thinking this aswell.
> > > > >
> > > > > Bah.
> > > > >
> > > > > Why are you fixing all this sparse crap and making the code worse?
> > > >
> > > > Hi Peter,
> > > >
> > > > Sparse is quite noisy and we need to eliminate false-positives, right?
> > >
> > > Dunno, I've been happy just ignoring it all.
>
> FWIW some of the sparse fixes Amol made recently did uncover so existing
> "bugs" :) (Not in perf but other code).
>
> > > > __rcu will tell the developer, this pointer could change and he needs to
> > > > take the required steps to make sure the code doesn't break.
> > >
> > > I know what it does; what I don't know is why you need to make the code
> > > worse. In paricular, __rcu doesn't mandate rcu_dereference(), esp. not
> > > when you're actually holding the write side lock.
> >
> > I might've misinterpreted the code. How does replacing rcu_dereference()
> > with
> > parent_ctx1 = rcu_dereference_protected(ctx1->parent_ctx,
> > lockdep_is_held(&ctx1->lock));
> > sound?
>
> FWIW, some maintainers do hate calling RCU APIs when write side lock is held.
> Evidently it does make the code readability a bit worse and I can see Peter's
> point of view because the existing code is correct. I leave it to you guys to
> decide how you want to handle it.
>
In that case, I think the code is fine as it is. Thank you for the review both!
Thanks
Amol
> thanks!
>
> - Joel
>
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prev parent reply other threads:[~2020-02-13 6:44 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-08 14:46 [Linux-kernel-mentees] [PATCH] events: Annotate parent_ctx with __rcu Amol Grover
2020-02-10 9:36 ` Peter Zijlstra
2020-02-10 12:59 ` Amol Grover
2020-02-10 13:34 ` Peter Zijlstra
2020-02-10 16:47 ` Amol Grover
2020-02-10 17:08 ` Joel Fernandes
2020-02-13 6:44 ` Amol Grover [this message]
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