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d="scan'208";a="14937661" To: Juergen Gross , References: <20200326091918.12388-1-jgross@suse.com> <20200326091918.12388-4-jgross@suse.com> From: Igor Druzhinin Message-ID: <260d0f20-a424-3708-3ab7-6d8c89247a2a@citrix.com> Date: Thu, 26 Mar 2020 23:24:40 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <20200326091918.12388-4-jgross@suse.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 8bit Subject: Re: [Xen-devel] [PATCH v8 3/5] xen: don't process rcu callbacks when holding a rcu_read_lock() X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: Stefano Stabellini , Julien Grall , Wei Liu , Andrew Cooper , Ian Jackson , George Dunlap , Jan Beulich Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" 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 > Reviewed-by: Jan Beulich > --- > 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 all system hang on boot after 540d4d60378 ("cpu: sync any remaining RCU callbacks before CPU up/down"). Igor