From: Peter Zijlstra <peterz@infradead.org>
To: "André Almeida" <andrealmeid@collabora.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
Darren Hart <dvhart@infradead.org>,
linux-kernel@vger.kernel.org,
Steven Rostedt <rostedt@goodmis.org>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
kernel@collabora.com, krisman@collabora.com,
pgriffais@valvesoftware.com, z.figura12@gmail.com,
joel@joelfernandes.org, malteskarupke@fastmail.fm,
linux-api@vger.kernel.org, fweimer@redhat.com,
libc-alpha@sourceware.org, linux-kselftest@vger.kernel.org,
shuah@kernel.org, acme@kernel.org, corbet@lwn.net
Subject: Re: [RFC PATCH 01/13] futex2: Implement wait and wake functions
Date: Tue, 16 Feb 2021 10:02:28 +0100 [thread overview]
Message-ID: <YCuKJEvcoXjgaNsb@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210215152404.250281-2-andrealmeid@collabora.com>
On Mon, Feb 15, 2021 at 12:23:52PM -0300, André Almeida wrote:
> +static int __futex_wait(struct futexv_head *futexv, unsigned int nr_futexes,
> + struct hrtimer_sleeper *timeout)
> +{
> + int ret;
> +
> + while (1) {
> + int awakened = -1;
> +
Might be easier to understand if the set_current_state() is here,
instead of squirreled away in futex_enqueue().
> + ret = futex_enqueue(futexv, nr_futexes, &awakened);
> +
> + if (ret) {
> + if (awakened >= 0)
> + return awakened;
> + return ret;
> + }
> +
> + /* Before sleeping, check if someone was woken */
> + if (!futexv->hint && (!timeout || timeout->task))
> + freezable_schedule();
> +
> + __set_current_state(TASK_RUNNING);
This is typically after the loop.
> +
> + /*
> + * One of those things triggered this wake:
> + *
> + * * We have been removed from the bucket. futex_wake() woke
> + * us. We just need to dequeue and return 0 to userspace.
> + *
> + * However, if no futex was dequeued by a futex_wake():
> + *
> + * * If the there's a timeout and it has expired,
> + * return -ETIMEDOUT.
> + *
> + * * If there is a signal pending, something wants to kill our
> + * thread, return -ERESTARTSYS.
> + *
> + * * If there's no signal pending, it was a spurious wake
> + * (scheduler gave us a change to do some work, even if we
chance?
> + * don't want to). We need to remove ourselves from the
> + * bucket and add again, to prevent losing wakeups in the
> + * meantime.
> + */
Anyway, doing a dequeue and enqueue for spurious wakes is a bit of an
anti-pattern that can lead to starvation. I've not actually looked at
much detail yet as this is my first read-through, but did figure I'd
mention it.
> +
> + ret = futex_dequeue_multiple(futexv, nr_futexes);
> +
> + /* Normal wake */
> + if (ret >= 0)
> + return ret;
> +
> + if (timeout && !timeout->task)
> + return -ETIMEDOUT;
> +
> + if (signal_pending(current))
> + return -ERESTARTSYS;
> +
> + /* Spurious wake, do everything again */
> + }
> +}
next prev parent reply other threads:[~2021-02-16 9:07 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-15 15:23 [RFC PATCH 00/13] Add futex2 syscalls André Almeida
2021-02-15 15:23 ` [RFC PATCH 01/13] futex2: Implement wait and wake functions André Almeida
2021-02-15 19:59 ` Gabriel Krisman Bertazi
2021-02-18 13:29 ` André Almeida
2021-02-18 15:48 ` Gabriel Krisman Bertazi
2021-02-16 9:02 ` Peter Zijlstra [this message]
2021-02-18 20:09 ` André Almeida
2021-02-16 9:35 ` Peter Zijlstra
2021-02-16 9:56 ` Peter Zijlstra
2021-02-16 10:20 ` Sebastian Andrzej Siewior
2021-02-16 12:42 ` Peter Zijlstra
2021-02-16 22:12 ` Gabriel Krisman Bertazi
2021-02-15 15:23 ` [RFC PATCH 02/13] futex2: Add support for shared futexes André Almeida
2021-02-15 15:23 ` [RFC PATCH 03/13] futex2: Implement vectorized wait André Almeida
2021-02-15 20:03 ` Gabriel Krisman Bertazi
2021-02-15 20:06 ` Zebediah Figura
2021-02-15 20:08 ` Gabriel Krisman Bertazi
2021-02-15 15:23 ` [RFC PATCH 04/13] futex2: Implement requeue operation André Almeida
2021-02-15 15:23 ` [RFC PATCH 05/13] futex2: Add compatibility entry point for x86_x32 ABI André Almeida
2021-02-15 15:23 ` [RFC PATCH 06/13] docs: locking: futex2: Add documentation André Almeida
2021-02-16 18:34 ` Randy Dunlap
2021-02-18 19:12 ` André Almeida
2021-02-15 15:23 ` [RFC PATCH 07/13] selftests: futex2: Add wake/wait test André Almeida
2021-02-15 15:23 ` [RFC PATCH 08/13] selftests: futex2: Add timeout test André Almeida
2021-02-15 15:24 ` [RFC PATCH 09/13] selftests: futex2: Add wouldblock test André Almeida
2021-02-15 15:24 ` [RFC PATCH 10/13] selftests: futex2: Add waitv test André Almeida
2021-02-15 15:24 ` [RFC PATCH 11/13] selftests: futex2: Add requeue test André Almeida
2021-02-15 15:24 ` [RFC PATCH 12/13] perf bench: Add futex2 benchmark tests André Almeida
2021-02-15 15:24 ` [RFC PATCH 13/13] kernel: Enable waitpid() for futex2 André Almeida
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