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vfs: use READ_ONCE() to access ->i_link

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Use 'READ_ONCE(inode->i_link)' to explicitly support filesystems caching
the symlink target in ->i_link later if it was unavailable at iget()
time, or wasn't easily available.  I'll be doing this in fscrypt, to
improve the performance of encrypted symlinks on ext4, f2fs, and ubifs.

->i_link will start NULL and may later be set to a non-NULL value by a
smp_store_release() or cmpxchg_release().  READ_ONCE() is needed on the
read side.  smp_load_acquire() is unnecessary because only a data
dependency barrier is required.  (Thanks to Al for pointing this out.)

Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Eric Biggers 2019-04-10 13:21:14 -07:00 committed by Jaegeuk Kim
parent 8d4184593a
commit a73d090612
1 changed files with 4 additions and 2 deletions
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6
fs/namei.c
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@ -1039,7 +1039,7 @@ const char *get_link(struct nameidata *nd)
return ERR_PTR(error);
nd->last_type = LAST_BIND;
res = inode->i_link;
res = READ_ONCE(inode->i_link);
if (!res) {
const char * (*get)(struct dentry *, struct inode *,
struct delayed_call *);
@ -4674,9 +4674,11 @@ static int generic_readlink(struct dentry *dentry, char __user *buffer,
{
DEFINE_DELAYED_CALL(done);
struct inode *inode = d_inode(dentry);
const char *link = inode->i_link;
const char *link;
int res;
link = READ_ONCE(inode->i_link);
if (!link) {
link = inode->i_op->get_link(dentry, inode, &done);
if (IS_ERR(link))