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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id d84sm6867362oig.33.2020.04.15.20.05.35 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Wed, 15 Apr 2020 20:05:36 -0700 (PDT) Date: Wed, 15 Apr 2020 20:05:13 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Yang Shi cc: Hugh Dickins , syzbot , Andrew Morton , Linux Kernel Mailing List , Linux MM , syzkaller-bugs@googlegroups.com, Linus Torvalds Subject: Re: possible deadlock in shmem_uncharge In-Reply-To: Message-ID: References: <000000000000e5838c05a3152f53@google.com> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 15 Apr 2020, Yang Shi wrote: > On Wed, Apr 15, 2020 at 7:04 PM Hugh Dickins wrote: > > On Mon, 13 Apr 2020, Yang Shi wrote: > > > > > > It looks shmem_uncharge() is just called by __split_huge_page() and > > > collapse_file(). The collapse_file() has acquired xa_lock with irq > > > disabled before acquiring info->lock, so it is safe. > > > __split_huge_page() is called with holding xa_lock with irq enabled, > > > but lru_lock is acquired with irq disabled before acquiring xa_lock. > > > > > > So, it is unnecessary to acquire info->lock with irq disabled in > > > shmem_uncharge(). Can syzbot try the below patch? > > > > But I disagree with the patch below. You're right that IRQ-disabling > > here is unnecessary, given its two callers; but I'm not sure that we > > want it to look different from shmem_charge() and all other info->lock > > takers; and, more importantly, I don't see how removing the redundant > > IRQ-saving below could make it any less liable to deadlock. > > Yes, I realized the patch can't suppress the lockdep splat. But, > actually I didn't understand how this deadlock could happen because > info_lock is acquired with IRQ disabled before acquiring > user_shm_lock. So, interrupt can't come in at all if I didn't miss > anything. I think the story it's trying to tell is this (but, like most of us, I do find Mr Lockdep embarrassingly difficult to understand; and I'm not much good at drawing race diagrams either): CPU0 was in user_shm_unlock(), it's got shmlock_user_lock, then an interrupt comes in. It's an endio kind of interrupt, which goes off to test_clear_page_writeback(), which wants the xa_lock on i_pages. Meanwhile, CPU1 was doing some SysV SHM locking, it's got as far as shmem_lock(), it has acquired info->lock, and goes off to user_shm_lock() which wants shmlock_user_lock. But sadly, CPU2 is splitting a shmem THP, calling shmem_uncharge() that wants info->lock while outer level holds xa_lock on i_pages: with interrupts properly disabled, but that doesn't help. Now, that story doesn't quite hold up as a deadlock, because shmem doesn't use writeback tags; and (unless you set shmem_enabled "force") I don't think there's a way to get shmem THPs in SysV SHM (and are they hole-punchable? maybe through MADV_REMOVE); so it looks like we're talking about different inodes. But lockdep is right to report it, and more thought might arrive at a more convincing scenario. Anyway, easily fixed and best fixed. (But now I think my patch must wait until tomorrow.) Hugh