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From: Hillf Danton <hdanton@sina.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Hillf Danton <hdanton@sina.com>,
	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: Sat, 24 Jul 2021 12:33:20 +0800	[thread overview]
Message-ID: <20210724043320.1654-1-hdanton@sina.com> (raw)
In-Reply-To: <41e2046f-ebbb-9fb2-3b30-1d6edceaaced@redhat.com>

On Fri, 23 Jul 2021 12:56:15 +0200 Paolo Bonzini wrote:
>On 23/07/21 11:48, Hillf Danton wrote:
>> What the comment does not cover is the cases of more-than-two-party race.
>
>But, you still haven't explained what's the bug there.

I am inclined to calling it race that has been there before and after the big
comment was added, instead of bug.


CPU1		CPU2		CPU3		CPU4
----		----		----		----
		lock WQ
		count += n
		no waiter
		unlock WQ

------------------------------- c0 = count

				lock WQ
------------------------------- c2 = count
				add waiter for EPOLLIN
				unlock WQ

						lock WQ
						count = 0
						wakeup EPOLLOUT
						unlock WQ

lock WQ
count += n
no waiter
unlock WQ
------------------------------- c1 = count


The c0 and c1 in the above chart are what I proposed, while c2 falls under the
range covered by the seep in the comment,

	 * The read _can_ therefore seep into add_wait_queue's critical
	 * section, but cannot move above it!  add_wait_queue's spin_lock acts
	 * as an acquire barrier and ensures that the read be ordered properly
	 * against the writes.

and is likely different to c1.


  reply	other threads:[~2021-07-24  4:33 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
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 [this message]
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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