[PATCH v2 1/1] dcache: Translating dentry into pathname without taking rename_lock

From: Waiman Long
Date: Thu Sep 05 2013 - 14:56:13 EST

When running the AIM7's short workload, Linus' lockref patch eliminated
most of the spinlock contention. However, there were still some left:

8.46% reaim [kernel.kallsyms] [k] _raw_spin_lock
|--42.21%-- d_path
| proc_pid_readlink
| SyS_readlinkat
| SyS_readlink
| system_call
| __GI___readlink
|
|--40.97%-- sys_getcwd
| system_call
| __getcwd

The big one here is the rename_lock (seqlock) contention in d_path()
and the getcwd system call. This patch will eliminate the need to take
the rename_lock while translating dentries into the full pathnames.

The need to take the rename_lock is to make sure that no rename
operation can be ongoing while the translation is in progress. However,
only one thread can take the rename_lock thus blocking all the other
threads that need it even though the translation process won't make
any change to the dentries.

This patch will replace the writer's write_seqlock/write_sequnlock
sequence of the rename_lock of the callers of the prepend_path() and
__dentry_path() functions with the reader's read_seqbegin/read_seqretry
sequence within these 2 functions. As a result, the code will have to
retry if one or more rename operations had been performed. In addition,
RCU read lock will be taken during the translation process to make sure
that no dentries will go away. To prevent live-lock from happening,
the code will switch back to take the rename_lock if read_seqretry()
fails for three times.

To further reduce spinlock contention, this patch also tries not
to take the dentry's d_lock if the dentry name is internal. For
external dentry name, it is safer to take the d_lock as racing with
d_move() may cause it to access invalid memory location leading to
segmentation fault. For safety, there is also additional check to
see if the name string is valid (no embedded null).

The following code re-factoring are also made:
1. Move prepend('/') into prepend_name().
2. Move the global root check in prepend_path() back to the top of
the while loop.

With this patch, the _raw_spin_lock will now account for only 1.2%
of the total CPU cycles for the short workload. This patch also has
the effect of reducing the effect of running perf on its profile
since the perf command itself can be a heavy user of the d_path()
function depending on the complexity of the workload.

When taking the perf profile of the high-systime workload, the amount
of spinlock contention contributed by running perf without this patch
was about 16%. With this patch, the spinlock contention caused by
the running of perf will go away and we will have a more accurate
perf profile.

Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
---
fs/dcache.c | 213 ++++++++++++++++++++++++++++++++++++++++++-----------------
1 files changed, 151 insertions(+), 62 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 96655f4..9703aa6 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2510,9 +2510,57 @@ static int prepend(char **buffer, int *buflen, const char *str, int namelen)
return 0;
}

-static int prepend_name(char **buffer, int *buflen, struct qstr *name)
+static int prepend_name(char **buffer, int *buflen, struct dentry *dentry)
{
- return prepend(buffer, buflen, name->name, name->len);
+ /*
+ * With RCU path tracing, it may race with rename. Use
+ * ACCESS_ONCE() to make sure that it is either the old or
+ * the new name pointer. The length does not really matter as
+ * the sequence number check will eventually catch any ongoing
+ * rename operation.
+ */
+ const char *dname = ACCESS_ONCE(dentry->d_name.name);
+ u32 dlen = dentry->d_name.len;
+ int error;
+
+ if (likely(dname == (const char *)dentry->d_iname)) {
+ /*
+ * Internal dname, the string memory is valid as long
+ * as its length is not over the limit.
+ */
+ if (unlikely(dlen > sizeof(dentry->d_iname)))
+ return -EINVAL;
+ } else if (!dname)
+ return -EINVAL;
+ else {
+ const char *ptr;
+ u32 len;
+
+ /*
+ * External dname, need to fetch name pointer and length
+ * again under d_lock to get a consistent set and avoid
+ * racing with d_move() which will take d_lock before
+ * acting on the dentries.
+ */
+ spin_lock(&dentry->d_lock);
+ dname = dentry->d_name.name;
+ dlen = dentry->d_name.len;
+ spin_unlock(&dentry->d_lock);
+
+ if (unlikely(!dname || !dlen))
+ return -EINVAL;
+ /*
+ * As the length and the content of the string may not be
+ * valid, need to scan the string and return EINVAL if there
+ * is embedded null byte within the length of the string.
+ */
+ for (ptr = dname, len = dlen; len; len--, ptr++) {
+ if (*ptr == '\0')
+ return -EINVAL;
+ }
+ }
+ error = prepend(buffer, buflen, dname, dlen);
+ return error ? error : prepend(buffer, buflen, "/", 1);
}

/**
@@ -2522,7 +2570,14 @@ static int prepend_name(char **buffer, int *buflen, struct qstr *name)
* @buffer: pointer to the end of the buffer
* @buflen: pointer to buffer length
*
- * Caller holds the rename_lock.
+ * The function tries to write out the pathname without taking any lock other
+ * than the RCU read lock to make sure that dentries won't go away. It only
+ * checks the sequence number of the global rename_lock as any change in the
+ * dentry's d_seq will be preceded by changes in the rename_lock sequence
+ * number. If the sequence number had been change, it will restart the whole
+ * pathname back-tracing sequence again. It performs a total of 3 trials of
+ * lockless back-tracing sequences before falling back to take the
+ * rename_lock.
*/
static int prepend_path(const struct path *path,
const struct path *root,
@@ -2531,54 +2586,80 @@ static int prepend_path(const struct path *path,
struct dentry *dentry = path->dentry;
struct vfsmount *vfsmnt = path->mnt;
struct mount *mnt = real_mount(vfsmnt);
- bool slash = false;
int error = 0;
+ unsigned seq = 0;
+ char *bptr;
+ int blen;
+ int retry_cnt = 3; /* Try lockless path conversion 3 times */

+restart:
+ bptr = *buffer;
+ blen = *buflen;
+ if (retry_cnt) {
+ seq = read_seqbegin(&rename_lock);
+ rcu_read_lock();
+ } else
+ write_seqlock(&rename_lock);
while (dentry != root->dentry || vfsmnt != root->mnt) {
struct dentry * parent;

if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
/* Global root? */
- if (!mnt_has_parent(mnt))
- goto global_root;
- dentry = mnt->mnt_mountpoint;
- mnt = mnt->mnt_parent;
- vfsmnt = &mnt->mnt;
- continue;
+ if (mnt_has_parent(mnt)) {
+ dentry = mnt->mnt_mountpoint;
+ mnt = mnt->mnt_parent;
+ vfsmnt = &mnt->mnt;
+ continue;
+ }
+ /*
+ * Filesystems needing to implement special "root names"
+ * should do so with ->d_dname()
+ */
+ if (IS_ROOT(dentry) &&
+ (dentry->d_name.len != 1 ||
+ dentry->d_name.name[0] != '/')) {
+ WARN(1, "Root dentry has weird name <%.*s>\n",
+ (int) dentry->d_name.len,
+ dentry->d_name.name);
+ }
+ if (!error)
+ error = is_mounted(vfsmnt) ? 1 : 2;
+ break;
}
parent = dentry->d_parent;
+ if (unlikely(parent == NULL)) {
+ error = -EINVAL;
+ break; /* Check sequence number */
+ }
prefetch(parent);
- spin_lock(&dentry->d_lock);
- error = prepend_name(buffer, buflen, &dentry->d_name);
- spin_unlock(&dentry->d_lock);
- if (!error)
- error = prepend(buffer, buflen, "/", 1);
+ error = prepend_name(&bptr, &blen, dentry);
if (error)
break;

- slash = true;
dentry = parent;
}
+ if (retry_cnt) {
+ retry_cnt--;
+ rcu_read_unlock();
+ if (read_seqretry(&rename_lock, seq))
+ goto restart;
+ } else
+ write_sequnlock(&rename_lock);

- if (!error && !slash)
- error = prepend(buffer, buflen, "/", 1);
-
+ if (error >= 0 && bptr == *buffer) {
+ if (--blen < 0)
+ error = -ENAMETOOLONG;
+ else
+ *--bptr = '/';
+ }
+ if (error == -EINVAL)
+ goto garbage;
+ *buffer = bptr;
+ *buflen = blen;
return error;

-global_root:
- /*
- * Filesystems needing to implement special "root names"
- * should do so with ->d_dname()
- */
- if (IS_ROOT(dentry) &&
- (dentry->d_name.len != 1 || dentry->d_name.name[0] != '/')) {
- WARN(1, "Root dentry has weird name <%.*s>\n",
- (int) dentry->d_name.len, dentry->d_name.name);
- }
- if (!slash)
- error = prepend(buffer, buflen, "/", 1);
- if (!error)
- error = is_mounted(vfsmnt) ? 1 : 2;
+garbage:
+ error = prepend(buffer, buflen, "(garbage)", 10);
return error;
}

@@ -2607,9 +2688,7 @@ char *__d_path(const struct path *path,

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)
@@ -2628,9 +2707,7 @@ char *d_absolute_path(const struct path *path,

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)
@@ -2696,9 +2773,7 @@ char *d_path(const struct path *path, char *buf, int 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);
@@ -2733,10 +2808,10 @@ char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
char *end = buffer + buflen;
/* these dentries are never renamed, so d_lock is not needed */
if (prepend(&end, &buflen, " (deleted)", 11) ||
- prepend_name(&end, &buflen, &dentry->d_name) ||
+ prepend(&end, &buflen, dentry->d_name.name, dentry->d_name.len) ||
prepend(&end, &buflen, "/", 1))
end = ERR_PTR(-ENAMETOOLONG);
- return end;
+ return end;
}

/*
@@ -2744,30 +2819,55 @@ char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
*/
static char *__dentry_path(struct dentry *dentry, char *buf, int buflen)
{
- char *end = buf + buflen;
- char *retval;
+ char *end, *retval;
+ int len, seq = 0;
+ int error = 0;
+ int retry_cnt = 3; /* Try lockless 3 times before taking lock */

- prepend(&end, &buflen, "\0", 1);
+restart:
+ end = buf + buflen;
+ len = buflen;
+ prepend(&end, &len, "\0", 1);
if (buflen < 1)
goto Elong;
/* Get '/' right */
retval = end-1;
*retval = '/';
-
+ if (retry_cnt) {
+ seq = read_seqbegin(&rename_lock);
+ rcu_read_lock();
+ } else
+ write_seqlock(&rename_lock);
while (!IS_ROOT(dentry)) {
struct dentry *parent = dentry->d_parent;
int error;

+ if (unlikely(parent == NULL)) {
+ error = -EINVAL;
+ break;
+ }
prefetch(parent);
- spin_lock(&dentry->d_lock);
- error = prepend_name(&end, &buflen, &dentry->d_name);
- spin_unlock(&dentry->d_lock);
- if (error != 0 || prepend(&end, &buflen, "/", 1) != 0)
- goto Elong;
+ error = prepend_name(&end, &len, dentry);
+ if (error)
+ break;

retval = end;
dentry = parent;
}
+ if (retry_cnt) {
+ retry_cnt--;
+ rcu_read_unlock();
+ if (read_seqretry(&rename_lock, seq))
+ goto restart;
+ } else
+ write_sequnlock(&rename_lock);
+ if (error == -EINVAL) {
+ error = prepend(&end, &len, "//garbage", 10);
+ if (!error)
+ return end;
+ }
+ if (error)
+ goto Elong;
return retval;
Elong:
return ERR_PTR(-ENAMETOOLONG);
@@ -2775,13 +2875,7 @@ Elong:

char *dentry_path_raw(struct dentry *dentry, char *buf, int buflen)
{
- char *retval;
-
- write_seqlock(&rename_lock);
- retval = __dentry_path(dentry, buf, buflen);
- write_sequnlock(&rename_lock);
-
- return retval;
+ return __dentry_path(dentry, buf, buflen);
}
EXPORT_SYMBOL(dentry_path_raw);

@@ -2790,7 +2884,6 @@ char *dentry_path(struct dentry *dentry, char *buf, int buflen)
char *p = NULL;
char *retval;

- write_seqlock(&rename_lock);
if (d_unlinked(dentry)) {
p = buf + buflen;
if (prepend(&p, &buflen, "//deleted", 10) != 0)
@@ -2798,7 +2891,6 @@ char *dentry_path(struct dentry *dentry, char *buf, int buflen)
buflen++;
}
retval = __dentry_path(dentry, buf, buflen);
- write_sequnlock(&rename_lock);
if (!IS_ERR(retval) && p)
*p = '/'; /* restore '/' overriden with '\0' */
return retval;
@@ -2837,7 +2929,6 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)

error = -ENOENT;
br_read_lock(&vfsmount_lock);
- write_seqlock(&rename_lock);
if (!d_unlinked(pwd.dentry)) {
unsigned long len;
char *cwd = page + PAGE_SIZE;
@@ -2845,7 +2936,6 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)

prepend(&cwd, &buflen, "\0", 1);
error = prepend_path(&pwd, &root, &cwd, &buflen);
- write_sequnlock(&rename_lock);
br_read_unlock(&vfsmount_lock);

if (error < 0)
@@ -2866,7 +2956,6 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
error = -EFAULT;
}
} else {
- write_sequnlock(&rename_lock);
br_read_unlock(&vfsmount_lock);
}

--
1.7.1

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