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[2003:d9:9705:f800:6fad:16d2:8cc8:b8e9]) by smtp.googlemail.com with ESMTPSA id 91sm30533410wra.37.2020.04.28.22.22.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 28 Apr 2020 22:22:14 -0700 (PDT) Subject: Re: [PATCH -next] ipc: use GFP_ATOMIC under spin lock To: Matthew Wilcox , Wei Yongjun Cc: Pankaj Bharadiya , Andrew Morton , Waiman Long , Stephen Rothwell , Alexey Dobriyan , linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org References: <20200428034736.27850-1-weiyongjun1@huawei.com> <20200428111403.GJ29705@bombadil.infradead.org> From: Manfred Spraul Message-ID: Date: Wed, 29 Apr 2020 07:22:13 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200428111403.GJ29705@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello together, On 4/28/20 1:14 PM, Matthew Wilcox wrote: > On Tue, Apr 28, 2020 at 03:47:36AM +0000, Wei Yongjun wrote: >> The function ipc_id_alloc() is called from ipc_addid(), in which >> a spin lock is held, so we should use GFP_ATOMIC instead. >> >> Fixes: de5738d1c364 ("ipc: convert ipcs_idr to XArray") >> Signed-off-by: Wei Yongjun > I see why you think that, but it's not true. Yes, we hold a spinlock, but > the spinlock is in an object which is not reachable from any other CPU. Is it really allowed that spin_lock()/spin_unlock may happen on different cpus? CPU1: spin_lock() CPU1: schedule() -> sleeps CPU2: -> schedule() returns CPU2: spin_unlock(). > Converting to GFP_ATOMIC is completely wrong. What is your solution proposal? xa_store() also gets a gfp_ flag. Thus even splitting _alloc() and _store() won't help     xa_alloc(,entry=NULL,)     new->seq = ...     spin_lock();     xa_store(,entry=new,GFP_KERNEL); --     Manfred