From: Jan Beulich <jbeulich@suse.com>
To: Julien Grall <julien@xen.org>, Roger Pau Monne <roger.pau@citrix.com>
Cc: xen-devel@lists.xenproject.org, Wei Liu <wl@xen.org>,
Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [Xen-devel] [PATCH 0/3] x86: fixes/improvements for scratch cpumask
Date: Thu, 13 Feb 2020 10:53:43 +0100 [thread overview]
Message-ID: <c09e9d06-af6b-014e-9d0c-027cdfe753d4@suse.com> (raw)
In-Reply-To: <72a1c94b-fd3b-3d47-4963-d92c6293000c@xen.org>
On 12.02.2020 22:05, Julien Grall wrote:
> On 12/02/2020 17:49, Roger Pau Monne wrote:
>> Commit:
>>
>> 5500d265a2a8fa63d60c08beb549de8ec82ff7a5
>> x86/smp: use APIC ALLBUT destination shorthand when possible
>
> There is a more subtle problem introduced by this patch. I thought I
> would mention it here just in case this affect the approach you have
> chosen in this series.
>
> get_cpu_maps() is used by stop_machine_run() to serialize the callers.
> If the latter fails to acquire the lock, it will bail out.
> Unfortunately, rcu_barrier() is implemented using stop_machine_run() and
> will be turned to pretty much a NOP if the latter fails (e.g the lock
> cannot be acquired).
>
> This means that the rcu_barrier() will not do the expected job and
> potentially introduce unknown issues (e.g use-after-free...).
>
> Before your patch, it would have been pretty hard to hit the problem
> above. After, you can race more easily with rcu_barrier() as sending IPI
> is pretty common.
>
> Sadly, I don't have a suggestion yet how to fix this problem.
A frequent use like on the IPI sending path suggests the lock may
want to become an r/w one, where both parties you mention want to
acquire it in read mode.
Jan
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next prev parent reply other threads:[~2020-02-13 9:54 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-12 16:49 [Xen-devel] [PATCH 0/3] x86: fixes/improvements for scratch cpumask Roger Pau Monne
2020-02-12 16:49 ` [Xen-devel] [PATCH 1/3] x86/smp: unify header includes in smp.h Roger Pau Monne
2020-02-12 23:05 ` Wei Liu
2020-02-13 9:54 ` Jan Beulich
2020-02-12 16:49 ` [Xen-devel] [PATCH 2/3] x86/smp: use a dedicated scratch cpumask in send_IPI_mask Roger Pau Monne
2020-02-13 9:59 ` Jan Beulich
2020-02-13 10:03 ` Roger Pau Monné
2020-02-13 10:19 ` Jan Beulich
2020-02-13 11:41 ` Roger Pau Monné
2020-02-13 13:35 ` Jan Beulich
2020-02-12 16:49 ` [Xen-devel] [PATCH 3/3] x86: add accessors for scratch cpu mask Roger Pau Monne
2020-02-13 10:12 ` Jan Beulich
2020-02-13 15:15 ` Roger Pau Monné
2020-02-13 15:43 ` Jan Beulich
2020-02-12 16:53 ` [Xen-devel] [PATCH 0/3] x86: fixes/improvements for scratch cpumask Sander Eikelenboom
2020-02-12 17:01 ` Roger Pau Monné
2020-02-12 17:13 ` Sander Eikelenboom
2020-02-13 9:31 ` Sander Eikelenboom
2020-02-12 21:05 ` Julien Grall
2020-02-13 9:53 ` Jan Beulich [this message]
2020-02-13 10:05 ` Roger Pau Monné
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