From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f72.google.com (mail-ed1-f72.google.com [209.85.208.72]) by kanga.kvack.org (Postfix) with ESMTP id 1E6A66B0006 for ; Fri, 22 Jun 2018 04:50:39 -0400 (EDT) Received: by mail-ed1-f72.google.com with SMTP id m18-v6so281781eds.0 for ; Fri, 22 Jun 2018 01:50:39 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id i43-v6si3628087ede.243.2018.06.22.01.50.37 for (version=TLS1 cipher=AES128-SHA bits=128/128); Fri, 22 Jun 2018 01:50:38 -0700 (PDT) Date: Fri, 22 Jun 2018 10:50:35 +0200 From: Jan Kara Subject: Re: [BUG] mm: backing-dev: a possible sleep-in-atomic-context bug in cgwb_create() Message-ID: <20180622085035.2zn2voqgqxcx55f3@quack2.suse.cz> References: <626acba3-c565-7e05-6c8b-0d100ff645c5@gmail.com> <20180621033515.GA12608@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180621033515.GA12608@bombadil.infradead.org> Sender: owner-linux-mm@kvack.org List-ID: To: Matthew Wilcox Cc: Jia-Ju Bai , axboe@kernel.dk, akpm@linux-foundation.or, jack@suse.cz, zhangweiping@didichuxing.com, sergey.senozhatsky@gmail.com, andriy.shevchenko@linux.intel.com, christophe.jaillet@wanadoo.fr, aryabinin@virtuozzo.com, linux-mm@kvack.org, Linux Kernel Mailing List On Wed 20-06-18 20:35:15, Matthew Wilcox wrote: > On Thu, Jun 21, 2018 at 11:02:58AM +0800, Jia-Ju Bai wrote: > > The kernel may sleep with holding a spinlock. > > The function call path (from bottom to top) in Linux-4.16.7 is: > > > > [FUNC] schedule > > lib/percpu-refcount.c, 222: > > schedule in __percpu_ref_switch_mode > > lib/percpu-refcount.c, 339: > > __percpu_ref_switch_mode in percpu_ref_kill_and_confirm > > ./include/linux/percpu-refcount.h, 127: > > percpu_ref_kill_and_confirm in percpu_ref_kill > > mm/backing-dev.c, 545: > > percpu_ref_kill in cgwb_kill > > mm/backing-dev.c, 576: > > cgwb_kill in cgwb_create > > mm/backing-dev.c, 573: > > _raw_spin_lock_irqsave in cgwb_create > > > > This bug is found by my static analysis tool (DSAC-2) and checked by my > > code review. > > I disagree with your code review. > > * If the previous ATOMIC switching hasn't finished yet, wait for > * its completion. If the caller ensures that ATOMIC switching > * isn't in progress, this function can be called from any context. > > I believe cgwb_kill is always called under the spinlock, so we will never > sleep because the percpu ref will never be switching to atomic mode. You are right that the sleep under spinlock never happens. And the reason is that percpu_ref_kill() never results in blocking - it does call percpu_ref_kill_and_confirm() but the 'confirm' argument is NULL and thus even percpu_ref_kill_and_confirm() never blocks. Honza -- Jan Kara SUSE Labs, CR