linux-ext4 - [PATCH v5 04/42] fs: split out functions to hold writers

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Date:   Tue, 12 Jan 2021 23:00:46 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org
Cc:     John Johansen <john.johansen@...onical.com>,
        James Morris <jmorris@...ei.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Geoffrey Thomas <geofft@...reload.com>,
        Mrunal Patel <mpatel@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Andy Lutomirski <luto@...nel.org>,
        Theodore Tso <tytso@....edu>, Alban Crequy <alban@...volk.io>,
        Tycho Andersen <tycho@...ho.ws>,
        David Howells <dhowells@...hat.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Seth Forshee <seth.forshee@...onical.com>,
        Stéphane Graber <stgraber@...ntu.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Lennart Poettering <lennart@...ttering.net>,
        "Eric W. Biederman" <ebiederm@...ssion.com>, smbarber@...omium.org,
        Phil Estes <estesp@...il.com>, Serge Hallyn <serge@...lyn.com>,
        Kees Cook <keescook@...omium.org>,
        Todd Kjos <tkjos@...gle.com>, Paul Moore <paul@...l-moore.com>,
        Jonathan Corbet <corbet@....net>,
        containers@...ts.linux-foundation.org,
        linux-security-module@...r.kernel.org, linux-api@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-integrity@...r.kernel.org, selinux@...r.kernel.org,
        Christian Brauner <christian.brauner@...ntu.com>,
        Christoph Hellwig <hch@....de>
Subject: [PATCH v5 04/42] fs: split out functions to hold writers

When a mount is marked read-only we set MNT_WRITE_HOLD on it if there
aren't currently any active writers. Split this logic out into simple
helpers that we can use in follow-up patches.

Suggested-by: Christoph Hellwig <hch@....de>
Cc: David Howells <dhowells@...hat.com>
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: linux-fsdevel@...r.kernel.org
Reviewed-by: Christoph Hellwig <hch@....de>
Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
---
/* v2 */
patch not present

/* v3 */
patch not present

/* v4 */
patch introduced

/* v5 */
base-commit: 7c53f6b671f4aba70ff15e1b05148b10d58c2837
---
 fs/namespace.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 5177bf6d810e..4aab396e5f21 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -469,10 +469,8 @@ void mnt_drop_write_file(struct file *file)
 }
 EXPORT_SYMBOL(mnt_drop_write_file);
 
-static int mnt_make_readonly(struct mount *mnt)
+static inline int mnt_hold_writers(struct mount *mnt)
 {
-	int ret = 0;
-
 	mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
 	/*
 	 * After storing MNT_WRITE_HOLD, we'll read the counters. This store
@@ -497,15 +495,29 @@ static int mnt_make_readonly(struct mount *mnt)
 	 * we're counting up here.
 	 */
 	if (mnt_get_writers(mnt) > 0)
-		ret = -EBUSY;
-	else
-		mnt->mnt.mnt_flags |= MNT_READONLY;
+		return -EBUSY;
+
+	return 0;
+}
+
+static inline void mnt_unhold_writers(struct mount *mnt)
+{
 	/*
 	 * MNT_READONLY must become visible before ~MNT_WRITE_HOLD, so writers
 	 * that become unheld will see MNT_READONLY.
 	 */
 	smp_wmb();
 	mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
+}
+
+static int mnt_make_readonly(struct mount *mnt)
+{
+	int ret;
+
+	ret = mnt_hold_writers(mnt);
+	if (!ret)
+		mnt->mnt.mnt_flags |= MNT_READONLY;
+	mnt_unhold_writers(mnt);
 	return ret;
 }
 
-- 
2.30.0