Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 5 May 2023 10:26:02 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Wander Lairson Costa <wander@redhat.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Brian Cain <bcain@quicinc.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Christian Brauner <brauner@kernel.org>,
        Andrei Vagin <avagin@gmail.com>,
        Shakeel Butt <shakeelb@google.com>,
        open list <linux-kernel@vger.kernel.org>,
        "open list:PERFORMANCE EVENTS SUBSYSTEM" 
        <linux-perf-users@vger.kernel.org>, Hu Chunyu <chuhu@redhat.com>,
        Paul McKenney <paulmck@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v7 2/3] sched/task: Add the
 put_task_struct_atomic_safe() function
Message-ID: <20230505102602.2052c83a@gandalf.local.home>
In-Reply-To: <20230505133235.GG4253@hirez.programming.kicks-ass.net>
References: <20230425114307.36889-1-wander@redhat.com>
        <20230425114307.36889-3-wander@redhat.com>
        <20230504084229.GI1734100@hirez.programming.kicks-ass.net>
        <20230504122945.GA28757@redhat.com>
        <20230504143303.GA1744142@hirez.programming.kicks-ass.net>
        <CAAq0SUmYrQbS1k9NNKGQP7hQRQJ308dk9NCiUimEiLeBJUavgA@mail.gmail.com>
        <20230504152424.GG1744258@hirez.programming.kicks-ass.net>
        <cjhvb72qrqggom5gxdjz6mtz3bntlmznx7r52adz72z2t2edzr@hwcog2pltfaq>
        <20230505133235.GG4253@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

On Fri, 5 May 2023 15:32:35 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, May 04, 2023 at 03:21:11PM -0300, Wander Lairson Costa wrote:
> > On Thu, May 04, 2023 at 05:24:24PM +0200, Peter Zijlstra wrote:  
> > > On Thu, May 04, 2023 at 11:55:15AM -0300, Wander Lairson Costa wrote:
> > >   
> > > > > Then I'm thinking something trivial like so:
> > > > >
> > > > > static inline void put_task_struct(struct task_struct *t)
> > > > > {
> > > > >         if (!refcount_dec_and_test(&t->usage))
> > > > >                 return;
> > > > >
> > > > >         if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
> > > > >                 call_rcu(&t->rcu, __put_task_struct_rcu);
> > > > >
> > > > >         __put_task_struct(t);
> > > > > }
> > > > >  
> > > > 
> > > > That's what v5 [1] does. What would be the path in this case? Should I
> > > > resend it as v8?  
> > > 
> > > It's almost what v5 does. v5 also has a !in_task() thing. v5 also
> > > violates codingstyle :-)  
> > 
> > IIRC, the in_task() is there because preemptible() doesn't check if it
> > is running in interrupt context.  
> 
> #define preemptible()   (preempt_count() == 0 && !irqs_disabled())
> 
> When in interrupt context preempt_count() will have a non-zero value in
> HARDIRQ_MASK and IRQs must be disabled, so preemptible() evaluates to
> (false && false), last time I checked that ends up being false.

Interesting, I can't find v5 anywhere in my mail folders (but I have
v4 and v6!). Anyway, from just the context of this email, and seeing
IS_ENABLED(CONFIG_PREEMPT_RT), I'm guessing that in_task() returns false if
it's running in a interrupt thread, where preemtible() does not.

-- Steve