From: Paolo Bonzini <pbonzini@redhat.com>
To: Hillf Danton <hdanton@sina.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
"Michael S. Tsirkin" <mst@redhat.com>,
linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()
Date: Mon, 19 Jul 2021 17:38:45 +0200 [thread overview]
Message-ID: <724e7951-59c8-8b2b-37b8-6b0bf696ab04@redhat.com> (raw)
In-Reply-To: <20210718124219.1521-1-hdanton@sina.com>
On 18/07/21 14:42, Hillf Danton wrote:
>> It's caused by the missing wakeup, i.e. eventfd_signal not really
>> signaling anything.
>
> Can you please point me to the waiters in the mainline?
It's irqfd_wakeup.
> There are two cases of write_seqcount_begin in x/virt/kvm/eventfd.c, and
> in kvm_irqfd_deassign() it is surrounded by spin_lock_irq(&kvm->irqfds.lock)
> that also protects irqfd_update().
>
> What isnt clear is if the risk is zero that either case can be preempted by
> seqcount reader. That risk may end up with the livelock described in
> x/Documentation/locking/seqlock.rst.
Since the introduction of seqcount_spinlock_t, the writers automatically
disable preemption. This is definitely the right thing in this case
where the seqcount writers are small enough, and the readers are hot
enough, that using a local lock would be too heavyweight.
Without that, the livelock would be possible, though very unlikely. In
practice seqcount updates should only happen while the producer is
quiescent; and also the seqcount readers and writers will often be
pinned to separate CPUs.
Paolo
> +A sequence counter write side critical section must never be preempted
> +or interrupted by read side sections. Otherwise the reader will spin for
> +the entire scheduler tick due to the odd sequence count value and the
> +interrupted writer. If that reader belongs to a real-time scheduling
> +class, it can spin forever and the kernel will livelock.
>
next prev parent reply other threads:[~2021-07-19 16:17 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-14 8:01 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal() Daniel Bristot de Oliveira
2021-07-14 8:10 ` Paolo Bonzini
2021-07-14 9:23 ` Jason Wang
2021-07-14 10:35 ` Paolo Bonzini
2021-07-14 10:41 ` Michael S. Tsirkin
2021-07-14 10:44 ` Paolo Bonzini
2021-07-14 12:20 ` Daniel Bristot de Oliveira
2021-07-15 4:14 ` Jason Wang
2021-07-15 5:58 ` Paolo Bonzini
2021-07-15 6:45 ` Jason Wang
2021-07-15 8:22 ` Daniel Bristot de Oliveira
2021-07-15 8:44 ` He Zhe
2021-07-15 9:51 ` Paolo Bonzini
2021-07-15 10:10 ` He Zhe
2021-07-15 11:05 ` Paolo Bonzini
2021-07-16 2:26 ` Jason Wang
2021-07-16 2:43 ` He Zhe
2021-07-16 2:46 ` Jason Wang
2021-07-15 9:46 ` Paolo Bonzini
2021-07-15 12:34 ` Daniel Bristot de Oliveira
[not found] ` <20210715102249.2205-1-hdanton@sina.com>
2021-07-15 12:31 ` Daniel Bristot de Oliveira
[not found] ` <20210716020611.2288-1-hdanton@sina.com>
2021-07-16 6:54 ` Paolo Bonzini
[not found] ` <20210716075539.2376-1-hdanton@sina.com>
2021-07-16 7:59 ` Paolo Bonzini
[not found] ` <20210716093725.2438-1-hdanton@sina.com>
2021-07-16 11:55 ` Paolo Bonzini
2021-07-18 12:42 ` Hillf Danton
2021-07-19 15:38 ` Paolo Bonzini [this message]
2021-07-21 7:04 ` Hillf Danton
2021-07-21 7:25 ` Thomas Gleixner
2021-07-21 10:11 ` Hillf Danton
2021-07-21 10:59 ` Paolo Bonzini
2021-07-22 5:58 ` Hillf Danton
2021-07-23 2:23 ` Hillf Danton
2021-07-23 7:59 ` Paolo Bonzini
2021-07-23 9:48 ` Hillf Danton
2021-07-23 10:56 ` Paolo Bonzini
2021-07-24 4:33 ` Hillf Danton
2021-07-26 11:03 ` Paolo Bonzini
2021-07-28 8:06 ` Thomas Gleixner
2021-07-28 10:21 ` Paolo Bonzini
2021-07-28 19:07 ` Thomas Gleixner
2021-07-29 11:01 ` [PATCH] eventfd: Make signal recursion protection a task bit Thomas Gleixner
2021-07-29 14:32 ` Daniel Bristot de Oliveira
2021-07-29 19:23 ` Daniel Bristot de Oliveira
2021-08-26 7:03 ` Jason Wang
2021-08-27 23:41 ` [tip: sched/core] " tip-bot2 for Thomas Gleixner
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