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Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1uN88y-000000048wo-1dn9; Thu, 05 Jun 2025 10:48:20 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id D35803005AF; Thu, 5 Jun 2025 12:48:19 +0200 (CEST) Date: Thu, 5 Jun 2025 12:48:19 +0200 From: Peter Zijlstra To: Petr Mladek Cc: Vlastimil Babka , syzbot , Liam.Howlett@oracle.com, akpm@linux-foundation.org, jannh@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, pfalcato@suse.de, syzkaller-bugs@googlegroups.com, Suren Baghdasaryan , John Ogness Subject: Re: [syzbot] [mm?] possible deadlock in __vma_start_write Message-ID: <20250605104819.GP39944@noisy.programming.kicks-ass.net> References: <68387feb.a70a0220.29d4a0.0830.GAE@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 0171A14000A X-Stat-Signature: c74s3g5pr6z6sqidi6ee96k86roog334 X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1749120504-2158 X-HE-Meta: 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 X4GKx1fS od9RGYGmYlhVoVMZuXVL0n++ko9kdVE9B6+0+BkSulSG5SL3dtH+n15mqNW8s0zB1KQLZjqRAA48pRhec36A3ybUcW6OGZeDsj9D3H/fVQtHXb7HmI4DSiKcftInVkZI7UHcxrsVowUh+xDDAGsLf0PsxVRBkJHFPLMHHExPrayG+JZR2AYoOCDStllJn7vjbtsLgozhF0SfvjFQitzVhwPADadj66qSozjTvKWYLxRqgdF/eBPu3RmaDr1HQaG+OMTVaeacXsQhE0tFLDH2emsudxN6oQmWwh+ejJLs1TdASJNkI04bIxhqfqvaj9PMqT+O/xgKxAZ+AV/l0DVUWU+Ce/BXSV9siIxvtOLIiTjsXik4YdT1R9OW7uIS/nNgAZBnC X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Jun 05, 2025 at 11:40:04AM +0200, Petr Mladek wrote: > I just wanted to better understand what was going on here ;-) > Let me share my thoughts. > > First, the deadlock might happen only when all these (or similar) code > paths might be called in parallel. I've found that this is often easier than you think. > The two backtraces, right above, are from initcalls. It is possible > that the code path in the 1st backtrace can't be called before > the device is initialized. So, it is possible that the deadlock > can't happen in the real life. This is a very shaky assumption. Lockdep only records the first time it sees a particular order. So seeing an initcall in the stack trace only means it happens first during an initcall. It does *NOT* mean it only happens during initcall. The only conclusion that can be had is that nobody thereafter uses those locks in the reverse order -- because that would result in a splat.