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[209.85.208.45]) by smtp.gmail.com with ESMTPSA id h18-20020aa7c952000000b0043a6fde6e7bsm1156244edt.19.2022.07.04.12.24.52 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 04 Jul 2022 12:24:52 -0700 (PDT) Received: by mail-ed1-f45.google.com with SMTP id r18so12761364edb.9 for ; Mon, 04 Jul 2022 12:24:52 -0700 (PDT) X-Received: by 2002:a5d:64e7:0:b0:21b:ad72:5401 with SMTP id g7-20020a5d64e7000000b0021bad725401mr27236387wri.442.1656962200967; Mon, 04 Jul 2022 12:16:40 -0700 (PDT) MIME-Version: 1.0 References: <20220701142310.2188015-1-glider@google.com> <20220701142310.2188015-44-glider@google.com> In-Reply-To: From: Linus Torvalds Date: Mon, 4 Jul 2022 12:16:24 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v4 43/45] namei: initialize parameters passed to step_into() To: Al Viro Cc: Alexander Potapenko , Alexei Starovoitov , Andrew Morton , Andrey Konovalov , Andy Lutomirski , Arnd Bergmann , Borislav Petkov , Christoph Hellwig , Christoph Lameter , David Rientjes , Dmitry Vyukov , Eric Dumazet , Greg Kroah-Hartman , Herbert Xu , Ilya Leoshkevich , Ingo Molnar , Jens Axboe , Joonsoo Kim , Kees Cook , Marco Elver , Mark Rutland , Matthew Wilcox , "Michael S. Tsirkin" , Pekka Enberg , Peter Zijlstra , Petr Mladek , Steven Rostedt , Thomas Gleixner , Vasily Gorbik , Vegard Nossum , Vlastimil Babka , kasan-dev , Linux-MM , linux-arch , Linux Kernel Mailing List , Evgenii Stepanov , Nathan Chancellor , Nick Desaulniers , Segher Boessenkool , Vitaly Buka , linux-toolchains Content-Type: text/plain; charset="UTF-8" ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1656962695; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=MM4ez9sq6uBjf7T2AcL2UYNl+8nxT4N2iGuYkE53QQE=; b=7S6a7UjTkX9SNq2ErfEFILhUsQ5zT7Twx/UZ/L90btWCxFClbvpQeKJnvoT7bm+cCnaVXB +ZgbKZAUAZWvQfJEPI31oLVnprX/orsN9KduWMnglLHKgVc7GgTG1Cy0FMCwokDAe/z+BO 07lJlklrbgQzfO6ypmrIqDIl3WvoM7s= ARC-Authentication-Results: i=1; imf04.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=google header.b=hM6KgRdJ; dmarc=none; spf=pass (imf04.hostedemail.com: domain of torvalds@linuxfoundation.org designates 209.85.218.44 as permitted sender) smtp.mailfrom=torvalds@linuxfoundation.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1656962695; a=rsa-sha256; cv=none; b=Qni7ScX6a70dsyZWxvJ53ziNuLkuVzzLHyxQjPuBuxo7G3MpTM3yjHZGcNoLX+9gXBqYRr +69H//zD0dX1kj2aHMx8mvlJOHuMcnq6m+vJ4NsD7XHY9vxeXcUBFurw5PDCr5ujJHK8JK AKjqFmNxF/FnMx6DdOtG4URKzvin1bI= Authentication-Results: imf04.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=google header.b=hM6KgRdJ; dmarc=none; spf=pass (imf04.hostedemail.com: domain of torvalds@linuxfoundation.org designates 209.85.218.44 as permitted sender) smtp.mailfrom=torvalds@linuxfoundation.org X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: D636D40002 X-Stat-Signature: xs3jesqy4afj3zpos1sbp98g8nc1116y X-Rspam-User: X-HE-Tag: 1656962694-732778 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: On Mon, Jul 4, 2022 at 12:03 PM Al Viro wrote: > > Anyway, I've thrown a mount_lock check in there, running xfstests to > see how it goes... So my reaction had been that it would be good to just do something like this: diff --git a/fs/namei.c b/fs/namei.c index 1f28d3f463c3..25c4bcc91142 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -1493,11 +1493,18 @@ static bool __follow_mount_rcu(struct n... if (flags & DCACHE_MOUNTED) { struct mount *mounted = __lookup_mnt(path->mnt, dentry); if (mounted) { + struct dentry *old_dentry = dentry; + unsigned old_seq = *seqp; + path->mnt = &mounted->mnt; dentry = path->dentry = mounted->mnt.mnt_root; nd->state |= ND_JUMPED; *seqp = read_seqcount_begin(&dentry->d_seq); *inode = dentry->d_inode; + + if (read_seqcount_retry(&old_dentry->d_seq, old_seq)) + return false; + /* * We don't need to re-check ->d_seq after this * ->d_inode read - there will be an RCU delay but the above is just whitespace-damaged random monkey-scribbling by yours truly. More like a "shouldn't we do something like this" than a serious patch, in other words. IOW, it has *NOT* had a lot of real thought behind it. Purely a "shouldn't we always clearly check the old sequence number after we've picked up the new one?" Linus