3.2.43-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
commit 7ea600b5314529f9d1b9d6d3c41cb26fce6a7a4a upstream.
... lest we get livelocks between path_is_under() and d_path() and friends.
The thing is, wrt fairness lglocks are more similar to rwsems than to rwlocks;
it is possible to have thread B spin on attempt to take lock shared while thread
A is already holding it shared, if B is on lower-numbered CPU than A and there's
a thread C spinning on attempt to take the same lock exclusive.
As the result, we need consistent ordering between vfsmount_lock (lglock) and
rename_lock (seq_lock), even though everything that takes both is going to take
vfsmount_lock only shared.
Spotted-by: Brad Spengler <spender@xxxxxxxxxxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.2:
- Adjust context
- s/&vfsmount_lock/vfsmount_lock/]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
fs/dcache.c | 16 +++++++++++-----
1 file changed, 11 insertions(+), 5 deletions(-)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2445,7 +2445,6 @@ static int prepend_path(const struct pat
bool slash = false;
int error = 0;
- br_read_lock(vfsmount_lock);
while (dentry != root->dentry || vfsmnt != root->mnt) {
struct dentry * parent;
@@ -2475,8 +2474,6 @@ static int prepend_path(const struct pat
if (!error && !slash)
error = prepend(buffer, buflen, "/", 1);
-out:
- br_read_unlock(vfsmount_lock);
return error;
global_root:
@@ -2493,7 +2490,7 @@ global_root:
error = prepend(buffer, buflen, "/", 1);
if (!error)
error = vfsmnt->mnt_ns ? 1 : 2;
- goto out;
+ return error;
}
/**
@@ -2520,9 +2517,11 @@ char *__d_path(const struct path *path,
int error;
prepend(&res, &buflen, "\0", 1);
+ br_read_lock(vfsmount_lock);
write_seqlock(&rename_lock);
error = prepend_path(path, root, &res, &buflen);
write_sequnlock(&rename_lock);
+ br_read_unlock(vfsmount_lock);
if (error < 0)
return ERR_PTR(error);
@@ -2539,9 +2538,11 @@ char *d_absolute_path(const struct path
int error;
prepend(&res, &buflen, "\0", 1);
+ br_read_lock(vfsmount_lock);
write_seqlock(&rename_lock);
error = prepend_path(path, &root, &res, &buflen);
write_sequnlock(&rename_lock);
+ br_read_unlock(vfsmount_lock);
if (error > 1)
error = -EINVAL;
@@ -2605,11 +2606,13 @@ char *d_path(const struct path *path, ch
return path->dentry->d_op->d_dname(path->dentry, buf, buflen);
get_fs_root(current->fs, &root);
+ br_read_lock(vfsmount_lock);
write_seqlock(&rename_lock);
error = path_with_deleted(path, &root, &res, &buflen);
+ write_sequnlock(&rename_lock);
+ br_read_unlock(vfsmount_lock);
if (error < 0)
res = ERR_PTR(error);
- write_sequnlock(&rename_lock);
path_put(&root);
return res;
}
@@ -2764,6 +2767,7 @@ SYSCALL_DEFINE2(getcwd, char __user *, b
get_fs_root_and_pwd(current->fs, &root, &pwd);
error = -ENOENT;
+ br_read_lock(vfsmount_lock);
write_seqlock(&rename_lock);
if (!d_unlinked(pwd.dentry)) {
unsigned long len;
@@ -2773,6 +2777,7 @@ SYSCALL_DEFINE2(getcwd, char __user *, b
prepend(&cwd, &buflen, "\0", 1);
error = prepend_path(&pwd, &root, &cwd, &buflen);
write_sequnlock(&rename_lock);
+ br_read_unlock(vfsmount_lock);
if (error < 0)
goto out;
@@ -2793,6 +2798,7 @@ SYSCALL_DEFINE2(getcwd, char __user *, b
}
} else {
write_sequnlock(&rename_lock);
+ br_read_unlock(vfsmount_lock);
}
out:
--
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