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Subject[PATCH 3/5 resend] VFS: Fix s_umount thaw/write deadlock
Date
From: Valerie Aurora <val@vaaconsulting.com>

File system freeze/thaw require the superblock's s_umount lock.
However, we write to the file system while holding the s_umount lock
in several places (e.g., writeback and quotas). Any of these can
block on a frozen file system while holding s_umount, preventing any
later thaw from occurring, since thaw requires s_umount.

The solution is to add a check, vfs_is_frozen(), to all code paths
that write while holding sb->s_umount and return without writing if it
is true.

BugLink: https://bugs.launchpad.net/bugs/897421
Signed-off-by: Valerie Aurora <val@vaaconsulting.com>
Cc: Kamal Mostafa <kamal@canonical.com>
Tested-by: Peter M. Petrakis <peter.petrakis@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
---
fs/drop_caches.c | 8 ++++++++
fs/fs-writeback.c | 50 +++++++++++++++++++++++++++++++++-----------------
fs/quota/quota.c | 12 +++++++++++-
fs/super.c | 26 ++++++++++++++++++++++++++
fs/sync.c | 9 +++++----
include/linux/fs.h | 6 +++++-
6 files changed, 88 insertions(+), 23 deletions(-)
diff --git a/fs/drop_caches.c b/fs/drop_caches.c
index c00e055..90f8f7e 100644
--- a/fs/drop_caches.c
+++ b/fs/drop_caches.c
@@ -57,6 +57,14 @@ int drop_caches_sysctl_handler(ctl_table *table, int write,
ret = proc_dointvec_minmax(table, write, buffer, length, ppos);
if (ret)
return ret;
+
+ /*
+ * Any file-system specific routines eventually called by
+ * drop_pagecache_sb() and drop_slab() ought to check for a
+ * frozen file system before writing to the disk. Most file
+ * systems won't write in this case but XFS is a notable
+ * exception in certain cases.
+ */
if (write) {
if (sysctl_drop_caches & 1)
iterate_supers(drop_pagecache_sb, NULL);
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 73c3992..dbeaede 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -554,6 +554,14 @@ static long writeback_sb_inodes(struct super_block *sb,
while (!list_empty(&wb->b_io)) {
struct inode *inode = wb_inode(wb->b_io.prev);

+ if (inode->i_sb == sb && vfs_is_frozen(sb)) {
+ /*
+ * Inode is on a frozen superblock; skip it for now.
+ */
+ redirty_tail(inode, wb);
+ continue;
+ }
+
if (inode->i_sb != sb) {
if (work->sb) {
/*
@@ -1266,40 +1274,48 @@ EXPORT_SYMBOL(writeback_inodes_sb);
* writeback_inodes_sb_if_idle - start writeback if none underway
* @sb: the superblock
*
- * Invoke writeback_inodes_sb if no writeback is currently underway.
- * Returns 1 if writeback was started, 0 if not.
+ * Invoke writeback_inodes_sb if no writeback is currently underway
+ * and no one else holds the s_umount lock. Returns 1 if writeback
+ * was started, 0 if not.
*/
int writeback_inodes_sb_if_idle(struct super_block *sb, enum wb_reason reason)
{
if (!writeback_in_progress(sb->s_bdi)) {
- down_read(&sb->s_umount);
- writeback_inodes_sb(sb, reason);
- up_read(&sb->s_umount);
- return 1;
- } else
- return 0;
+ /*
+ * Trylock also avoids read-write deadlocks that could be
+ * triggered by freeze.
+ */
+ if (down_read_trylock(&sb->s_umount)) {
+ writeback_inodes_sb(sb, reason);
+ up_read(&sb->s_umount);
+ return 1;
+ }
+ }
+ return 0;
}
EXPORT_SYMBOL(writeback_inodes_sb_if_idle);

/**
- * writeback_inodes_sb_if_idle - start writeback if none underway
+ * writeback_inodes_sb_nr_if_idle - start writeback if none underway
* @sb: the superblock
* @nr: the number of pages to write
*
- * Invoke writeback_inodes_sb if no writeback is currently underway.
- * Returns 1 if writeback was started, 0 if not.
+ * Invoke writeback_inodes_sb_nr if no writeback is currently underway
+ * and no one else holds the s_umount lock. Returns 1 if writeback
+ * was started, 0 if not.
*/
int writeback_inodes_sb_nr_if_idle(struct super_block *sb,
unsigned long nr,
enum wb_reason reason)
{
if (!writeback_in_progress(sb->s_bdi)) {
- down_read(&sb->s_umount);
- writeback_inodes_sb_nr(sb, nr, reason);
- up_read(&sb->s_umount);
- return 1;
- } else
- return 0;
+ if (down_read_trylock(&sb->s_umount)) {
+ writeback_inodes_sb_nr(sb, nr, reason);
+ up_read(&sb->s_umount);
+ return 1;
+ }
+ }
+ return 0;
}
EXPORT_SYMBOL(writeback_inodes_sb_nr_if_idle);

diff --git a/fs/quota/quota.c b/fs/quota/quota.c
index 35f4b0e..47983d9 100644
--- a/fs/quota/quota.c
+++ b/fs/quota/quota.c
@@ -47,7 +47,7 @@ static int check_quotactl_permission(struct super_block *sb, int type, int cmd,

static void quota_sync_one(struct super_block *sb, void *arg)
{
- if (sb->s_qcop && sb->s_qcop->quota_sync)
+ if (sb->s_qcop && sb->s_qcop->quota_sync && !vfs_is_frozen(sb))
sb->s_qcop->quota_sync(sb, *(int *)arg, 1);
}

@@ -368,8 +368,18 @@ SYSCALL_DEFINE4(quotactl, unsigned int, cmd, const char __user *, special,
goto out;
}

+ /*
+ * It's not clear which quota functions may write to the file
+ * system (all?). Check for a frozen fs and bail out now.
+ */
+ if (vfs_is_frozen(sb)) {
+ ret = -EBUSY;
+ goto out_drop_super;
+ }
+
ret = do_quotactl(sb, type, cmds, id, addr, pathp);

+out_drop_super:
drop_super(sb);
out:
if (pathp && !IS_ERR(pathp))
diff --git a/fs/super.c b/fs/super.c
index afd0f1a..a56696b 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -526,6 +526,10 @@ void sync_supers(void)
*
* Scans the superblock list and calls given function, passing it
* locked superblock and given argument.
+ *
+ * Note: If @f is going to write to the file system, it must first
+ * check if the file system is frozen (via vfs_is_frozen(sb)) and
+ * refuse to write if so.
*/
void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
{
@@ -595,6 +599,10 @@ EXPORT_SYMBOL(iterate_supers_type);
*
* Scans the superblock list and finds the superblock of the file system
* mounted on the device given. %NULL is returned if no match is found.
+ *
+ * Note: If caller is going to write to the superblock, it must first
+ * check if the file system is frozen (via vfs_is_frozen(sb)) and
+ * refuse to write if so.
*/

struct super_block *get_super(struct block_device *bdev)
@@ -1132,6 +1140,24 @@ out:
* Syncs the super to make sure the filesystem is consistent and calls the fs's
* freeze_fs. Subsequent calls to this without first thawing the fs will return
* -EBUSY.
+ *
+ * During this function, sb->s_frozen goes through these values:
+ *
+ * SB_UNFROZEN: File system is normal, all writes progress as usual.
+ *
+ * SB_FREEZE_WRITE: The file system is in the process of being frozen
+ * and any remaining out-standing writes are being synced. Writes
+ * that complete in-process writes should be permitted but new ones
+ * should be blocked.
+ *
+ * SB_FREEZE_TRANS: The file system is frozen. The ->freeze_fs and
+ * ->unfreeze_fs ops are the only operations permitted to write to the
+ * file system in this state.
+ *
+ * sb->s_frozen is protected by sb->s_umount. Additionally,
+ * SB_FREEZE_WRITE is only temporarily set during freeze/thaw while
+ * holding sb->s_umount for writing, so any other callers holding
+ * sb->s_umount will never see this state.
*/
int freeze_super(struct super_block *sb)
{
diff --git a/fs/sync.c b/fs/sync.c
index 101b8ef..f497be8 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -68,7 +68,7 @@ int sync_filesystem(struct super_block *sb)
/*
* No point in syncing out anything if the filesystem is read-only.
*/
- if (sb->s_flags & MS_RDONLY)
+ if ((sb->s_flags & MS_RDONLY) || vfs_is_frozen(sb))
return 0;

ret = __sync_filesystem(sb, 0);
@@ -80,7 +80,7 @@ EXPORT_SYMBOL_GPL(sync_filesystem);

static void sync_one_sb(struct super_block *sb, void *arg)
{
- if (!(sb->s_flags & MS_RDONLY))
+ if (!(sb->s_flags & MS_RDONLY) && !vfs_is_frozen(sb))
__sync_filesystem(sb, *(int *)arg);
}
/*
@@ -136,7 +136,7 @@ SYSCALL_DEFINE1(syncfs, int, fd)
{
struct file *file;
struct super_block *sb;
- int ret;
+ int ret = 0;
int fput_needed;

file = fget_light(fd, &fput_needed);
@@ -145,7 +145,8 @@ SYSCALL_DEFINE1(syncfs, int, fd)
sb = file->f_dentry->d_sb;

down_read(&sb->s_umount);
- ret = sync_filesystem(sb);
+ if (!(sb->s_flags & MS_RDONLY) && !vfs_is_frozen(sb))
+ ret = sync_filesystem(sb);
up_read(&sb->s_umount);

fput_light(file, fput_needed);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e313022..ac7a495 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1490,7 +1490,7 @@ extern void prune_dcache_sb(struct super_block *sb, int nr_to_scan);
extern struct timespec current_fs_time(struct super_block *sb);

/*
- * Snapshotting support.
+ * Snapshotting support. See freeze_super() for documentation.
*/
enum {
SB_UNFROZEN = 0,
@@ -1501,6 +1501,10 @@ enum {
#define vfs_check_frozen(sb, level) \
wait_event((sb)->s_wait_unfrozen, ((sb)->s_frozen < (level)))

+static inline int vfs_is_frozen(struct super_block *sb) {
+ return sb->s_frozen == SB_FREEZE_TRANS;
+}
+
/*
* until VFS tracks user namespaces for inodes, just make all files
* belong to init_user_ns
--
1.7.5.4


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