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From: "Jürgen Groß" <jgross@suse.com>
To: Igor Druzhinin <igor.druzhinin@citrix.com>,
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Julien Grall <julien@xen.org>, Wei Liu <wl@xen.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	George Dunlap <george.dunlap@citrix.com>,
	Jan Beulich <jbeulich@suse.com>
Subject: Re: [Xen-devel] [PATCH v8 3/5] xen: don't process rcu callbacks when holding a rcu_read_lock()
Date: Fri, 27 Mar 2020 09:10:54 +0100
Message-ID: <7a9cff0b-4c8f-899a-3fae-8a703bc90125@suse.com> (raw)
In-Reply-To: <260d0f20-a424-3708-3ab7-6d8c89247a2a@citrix.com>

On 27.03.20 00:24, Igor Druzhinin wrote:
> On 26/03/2020 09:19, Juergen Gross wrote:
>> Some keyhandlers are calling process_pending_softirqs() while holding
>> a rcu_read_lock(). This is wrong, as process_pending_softirqs() might
>> activate rcu calls which should not happen inside a rcu_read_lock().
>> For that purpose modify process_pending_softirqs() to not allow rcu
>> callback processing when a rcu_read_lock() is being held.
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> Reviewed-by: Jan Beulich <jbeulich@suse.com>
>> ---
>> V3:
>> - add RCU_SOFTIRQ to ignore in process_pending_softirqs_norcu()
>>    (Roger Pau Monné)
>> V5:
>> - block rcu processing depending on rch_read_lock() being held or not
>>    (Jan Beulich)
> Juergen,
> Our BVT revealed a likely problem with this commit in that form.
> Since 12509bbeb9e ("rwlocks: call preempt_disable() when taking a rwlock")
> preemption is disabled after taking cpu_maps which will block RCU
> callback processing inside rcu_barrier itself. This will result in

Why would that block RCU callback processing?

RCU callbacks should be blocked only if a rcu lock is being held.

Did I miss something in my patches?

> all system hang on boot after 540d4d60378 ("cpu: sync any remaining
> RCU callbacks before CPU up/down").


  reply index

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-26  9:19 [Xen-devel] [PATCH v8 0/5] xen/rcu: let rcu work better with core scheduling Juergen Gross
2020-03-26  9:19 ` [Xen-devel] [PATCH v8 1/5] xen: introduce smp_mb__[after|before]_atomic() barriers Juergen Gross
2020-03-26  9:19 ` [Xen-devel] [PATCH v8 2/5] xen/rcu: don't use stop_machine_run() for rcu_barrier() Juergen Gross
2020-03-26  9:19 ` [Xen-devel] [PATCH v8 3/5] xen: don't process rcu callbacks when holding a rcu_read_lock() Juergen Gross
2020-03-26 23:24   ` Igor Druzhinin
2020-03-27  8:10     ` Jürgen Groß [this message]
2020-03-27  8:35       ` Jan Beulich
2020-03-27  8:39         ` Jürgen Groß
2020-03-27  9:56         ` Igor Druzhinin
2020-03-26  9:19 ` [Xen-devel] [PATCH v8 4/5] xen/rcu: add assertions to debug build Juergen Gross
2020-03-26  9:19 ` [Xen-devel] [PATCH v8 5/5] xen/rcu: add per-lock counter in debug builds Juergen Gross

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