I am currently tracking a hotlock reported by a customer on a large system,
512 cores. I am currently running 3.8-rc7 but the issue looks like it has been
this way for a very long time.
The offending lock is proc_dir_entry->pde_unload_lock.
This patch converts the lock to use the rcu. However the pde_openers list
still is controlled by a spin lock. I tested on a 4096 machine and the lock
doesn't seem hot at least according to perf.
This is a refresh of what was orignally suggested by Eric Dumazet some time
ago. I have also taken in some comments from Andrew and several other people
whose names escape me but I am quite grateful too.
Supporting numbers, lower is better, they are from the test I posted earlier.
cpuinfo baseline Rcu
tasks read-sec read-sec
1 0.0141 0.0141
2 0.0140 0.0142
4 0.0140 0.0141
8 0.0145 0.0140
16 0.0553 0.0168
32 0.1688 0.0549
64 0.5017 0.1690
128 1.7005 0.5038
256 5.2513 2.0804
512 8.0529 3.0162
v4: Corrected some comments and warnings.
v5: Corrected some warnings from sparce including the supplied by Sasha.
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Nathan Zimmer <nzimmer@xxxxxxx>
---
fs/proc/generic.c | 62 ++++++++---------
fs/proc/inode.c | 176 +++++++++++++++++++++++++++---------------------
include/linux/proc_fs.h | 6 +-
3 files changed, 133 insertions(+), 111 deletions(-)
diff --git a/fs/proc/generic.c b/fs/proc/generic.c
index 76ddae8..d6118cb 100644
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -190,14 +190,16 @@ proc_file_read(struct file *file, char __user *buf, size_t nbytes,
{
struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
ssize_t rv = -EIO;
+ const struct file_operations *fops;
- spin_lock(&pde->pde_unload_lock);
- if (!pde->proc_fops) {
- spin_unlock(&pde->pde_unload_lock);
+ rcu_read_lock();
+ fops = rcu_dereference(pde->proc_fops);
+ if (!fops) {
+ rcu_read_unlock();
return rv;
}
- pde->pde_users++;
- spin_unlock(&pde->pde_unload_lock);
+ atomic_inc(&pde->pde_users);
+ rcu_read_unlock();
rv = __proc_file_read(file, buf, nbytes, ppos);
@@ -213,13 +215,16 @@ proc_file_write(struct file *file, const char __user *buffer,
ssize_t rv = -EIO;
if (pde->write_proc) {
- spin_lock(&pde->pde_unload_lock);
- if (!pde->proc_fops) {
- spin_unlock(&pde->pde_unload_lock);
+ const struct file_operations *fops;
+
+ rcu_read_lock();
+ fops = rcu_dereference(pde->proc_fops);
+ if (!fops) {
+ rcu_read_unlock();
return rv;
}
- pde->pde_users++;
- spin_unlock(&pde->pde_unload_lock);
+ atomic_inc(&pde->pde_users);
+ rcu_read_unlock();
/* FIXME: does this routine need ppos? probably... */
rv = pde->write_proc(file, buffer, count, pde->data);
@@ -558,7 +563,7 @@ static int proc_register(struct proc_dir_entry * dir, struct proc_dir_entry * dp
if (S_ISDIR(dp->mode)) {
if (dp->proc_iops == NULL) {
- dp->proc_fops = &proc_dir_operations;
+ RCU_INIT_POINTER(dp->proc_fops, &proc_dir_operations);
dp->proc_iops = &proc_dir_inode_operations;
}
dir->nlink++;
@@ -567,7 +572,7 @@ static int proc_register(struct proc_dir_entry * dir, struct proc_dir_entry * dp
dp->proc_iops = &proc_link_inode_operations;
} else if (S_ISREG(dp->mode)) {
if (dp->proc_fops == NULL)
- dp->proc_fops = &proc_file_operations;
+ RCU_INIT_POINTER(dp->proc_fops, &proc_file_operations);
if (dp->proc_iops == NULL)
dp->proc_iops = &proc_file_inode_operations;
}
@@ -620,7 +625,8 @@ static struct proc_dir_entry *__proc_create(struct proc_dir_entry **parent,
ent->mode = mode;
ent->nlink = nlink;
atomic_set(&ent->count, 1);
- spin_lock_init(&ent->pde_unload_lock);
+ atomic_set(&ent->pde_users, 1);
+ spin_lock_init(&ent->pde_openers_lock);
INIT_LIST_HEAD(&ent->pde_openers);
out:
return ent;
@@ -744,7 +750,7 @@ struct proc_dir_entry *proc_create_data(const char *name, umode_t mode,
pde = __proc_create(&parent, name, mode, nlink);
if (!pde)
goto out;
- pde->proc_fops = proc_fops;
+ rcu_assign_pointer(pde->proc_fops, proc_fops);
pde->data = data;
if (proc_register(parent, pde) < 0)
goto out_free;
@@ -780,6 +786,7 @@ void remove_proc_entry(const char *name, struct proc_dir_entry *parent)
struct proc_dir_entry *de = NULL;
const char *fn = name;
unsigned int len;
+ DECLARE_COMPLETION_ONSTACK(c);
spin_lock(&proc_subdir_lock);
if (__xlate_proc_name(name, &parent, &fn) != 0) {
@@ -802,37 +809,30 @@ void remove_proc_entry(const char *name, struct proc_dir_entry *parent)
return;
}
- spin_lock(&de->pde_unload_lock);
/*
* Stop accepting new callers into module. If you're
* dynamically allocating ->proc_fops, save a pointer somewhere.
*/
- de->proc_fops = NULL;
- /* Wait until all existing callers into module are done. */
- if (de->pde_users > 0) {
- DECLARE_COMPLETION_ONSTACK(c);
-
- if (!de->pde_unload_completion)
- de->pde_unload_completion = &c;
-
- spin_unlock(&de->pde_unload_lock);
-
+ rcu_assign_pointer(de->proc_fops, NULL);
+ synchronize_rcu();
+ /*
+ * Wait until all existing callers into module are done.
+ * Once pde_users hits zero we are free to clean out pde_openers.
+ */
+ de->pde_unload_completion = &c;
+ if (!atomic_dec_and_test(&de->pde_users))
wait_for_completion(de->pde_unload_completion);
- spin_lock(&de->pde_unload_lock);
- }
-
+ spin_lock(&de->pde_openers_lock);
while (!list_empty(&de->pde_openers)) {
struct pde_opener *pdeo;
pdeo = list_first_entry(&de->pde_openers, struct pde_opener, lh);
list_del(&pdeo->lh);
- spin_unlock(&de->pde_unload_lock);
pdeo->release(pdeo->inode, pdeo->file);
kfree(pdeo);
- spin_lock(&de->pde_unload_lock);
}
- spin_unlock(&de->pde_unload_lock);
+ spin_unlock(&de->pde_openers_lock);
if (S_ISDIR(de->mode))
parent->nlink--;
diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 439ae688..345acd6 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -128,46 +128,41 @@ static const struct super_operations proc_sops = {
.show_options = proc_show_options,
};
-static void __pde_users_dec(struct proc_dir_entry *pde)
-{
- pde->pde_users--;
- if (pde->pde_unload_completion && pde->pde_users == 0)
- complete(pde->pde_unload_completion);
-}
-
void pde_users_dec(struct proc_dir_entry *pde)
{
- spin_lock(&pde->pde_unload_lock);
- __pde_users_dec(pde);
- spin_unlock(&pde->pde_unload_lock);
+ if (atomic_dec_and_test(&pde->pde_users) && pde->pde_unload_completion)
+ complete(pde->pde_unload_completion);
}
static loff_t proc_reg_llseek(struct file *file, loff_t offset, int whence)
{
+ const struct file_operations *fops;
struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
loff_t rv = -EINVAL;
loff_t (*llseek)(struct file *, loff_t, int);
- spin_lock(&pde->pde_unload_lock);
+ rcu_read_lock();
+ fops = rcu_dereference(pde->proc_fops);
/*
* remove_proc_entry() is going to delete PDE (as part of module
* cleanup sequence). No new callers into module allowed.
*/
- if (!pde->proc_fops) {
- spin_unlock(&pde->pde_unload_lock);
+ if (!fops) {
+ rcu_read_unlock();
return rv;
}
/*
* Bump refcount so that remove_proc_entry will wail for ->llseek to
* complete.
*/
- pde->pde_users++;
+ atomic_inc(&pde->pde_users);
+
/*
- * Save function pointer under lock, to protect against ->proc_fops
- * NULL'ifying right after ->pde_unload_lock is dropped.
+ * Save function pointer under rcu lock, to protect against
+ * ->proc_fops NULL'ifying by remove_proc_entry.
*/
- llseek = pde->proc_fops->llseek;
- spin_unlock(&pde->pde_unload_lock);
+ llseek = fops->llseek;
+ rcu_read_unlock();
if (!llseek)
llseek = default_llseek;
@@ -182,15 +177,17 @@ static ssize_t proc_reg_read(struct file *file, char __user *buf, size_t count,
struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
ssize_t rv = -EIO;
ssize_t (*read)(struct file *, char __user *, size_t, loff_t *);
+ const struct file_operations *fops;
- spin_lock(&pde->pde_unload_lock);
- if (!pde->proc_fops) {
- spin_unlock(&pde->pde_unload_lock);
+ rcu_read_lock();
+ fops = rcu_dereference(pde->proc_fops);
+ if (!fops) {
+ rcu_read_unlock();
return rv;
}
- pde->pde_users++;
- read = pde->proc_fops->read;
- spin_unlock(&pde->pde_unload_lock);
+ atomic_inc(&pde->pde_users);
+ read = fops->read;
+ rcu_read_unlock();
if (read)
rv = read(file, buf, count, ppos);
@@ -204,15 +201,17 @@ static ssize_t proc_reg_write(struct file *file, const char __user *buf, size_t
struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
ssize_t rv = -EIO;
ssize_t (*write)(struct file *, const char __user *, size_t, loff_t *);
+ const struct file_operations *fops;
- spin_lock(&pde->pde_unload_lock);
- if (!pde->proc_fops) {
- spin_unlock(&pde->pde_unload_lock);
+ rcu_read_lock();
+ fops = rcu_dereference(pde->proc_fops);
+ if (!fops) {
+ rcu_read_unlock();
return rv;
}
- pde->pde_users++;
- write = pde->proc_fops->write;
- spin_unlock(&pde->pde_unload_lock);
+ atomic_inc(&pde->pde_users);
+ write = fops->write;
+ rcu_read_unlock();
if (write)
rv = write(file, buf, count, ppos);
@@ -226,15 +225,17 @@ static unsigned int proc_reg_poll(struct file *file, struct poll_table_struct *p
struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
unsigned int rv = DEFAULT_POLLMASK;
unsigned int (*poll)(struct file *, struct poll_table_struct *);
+ const struct file_operations *fops;
- spin_lock(&pde->pde_unload_lock);
- if (!pde->proc_fops) {
- spin_unlock(&pde->pde_unload_lock);
+ rcu_read_lock();
+ fops = rcu_dereference(pde->proc_fops);
+ if (!fops) {
+ rcu_read_unlock();
return rv;
}
- pde->pde_users++;
- poll = pde->proc_fops->poll;
- spin_unlock(&pde->pde_unload_lock);
+ atomic_inc(&pde->pde_users);
+ poll = fops->poll;
+ rcu_read_unlock();
if (poll)
rv = poll(file, pts);
@@ -248,15 +249,17 @@ static long proc_reg_unlocked_ioctl(struct file *file, unsigned int cmd, unsigne
struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
long rv = -ENOTTY;
long (*ioctl)(struct file *, unsigned int, unsigned long);
+ const struct file_operations *fops;
- spin_lock(&pde->pde_unload_lock);
- if (!pde->proc_fops) {
- spin_unlock(&pde->pde_unload_lock);
+ rcu_read_lock();
+ fops = rcu_dereference(pde->proc_fops);
+ if (!fops) {
+ rcu_read_unlock();
return rv;
}
- pde->pde_users++;
- ioctl = pde->proc_fops->unlocked_ioctl;
- spin_unlock(&pde->pde_unload_lock);
+ atomic_inc(&pde->pde_users);
+ ioctl = fops->unlocked_ioctl;
+ rcu_read_unlock();
if (ioctl)
rv = ioctl(file, cmd, arg);
@@ -271,15 +274,17 @@ static long proc_reg_compat_ioctl(struct file *file, unsigned int cmd, unsigned
struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
long rv = -ENOTTY;
long (*compat_ioctl)(struct file *, unsigned int, unsigned long);
+ const struct file_operations *fops;
- spin_lock(&pde->pde_unload_lock);
- if (!pde->proc_fops) {
- spin_unlock(&pde->pde_unload_lock);
+ rcu_read_lock();
+ fops = rcu_dereference(pde->proc_fops);
+ if (!fops) {
+ rcu_read_unlock();
return rv;
}
- pde->pde_users++;
- compat_ioctl = pde->proc_fops->compat_ioctl;
- spin_unlock(&pde->pde_unload_lock);
+ atomic_inc(&pde->pde_users);
+ compat_ioctl = fops->compat_ioctl;
+ rcu_read_unlock();
if (compat_ioctl)
rv = compat_ioctl(file, cmd, arg);
@@ -294,15 +299,17 @@ static int proc_reg_mmap(struct file *file, struct vm_area_struct *vma)
struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
int rv = -EIO;
int (*mmap)(struct file *, struct vm_area_struct *);
+ const struct file_operations *fops;
- spin_lock(&pde->pde_unload_lock);
- if (!pde->proc_fops) {
- spin_unlock(&pde->pde_unload_lock);
+ rcu_read_lock();
+ fops = rcu_dereference(pde->proc_fops);
+ if (!fops) {
+ rcu_read_unlock();
return rv;
}
- pde->pde_users++;
- mmap = pde->proc_fops->mmap;
- spin_unlock(&pde->pde_unload_lock);
+ atomic_inc(&pde->pde_users);
+ mmap = fops->mmap;
+ rcu_read_unlock();
if (mmap)
rv = mmap(file, vma);
@@ -318,6 +325,7 @@ static int proc_reg_open(struct inode *inode, struct file *file)
int (*open)(struct inode *, struct file *);
int (*release)(struct inode *, struct file *);
struct pde_opener *pdeo;
+ const struct file_operations *fops;
/*
* What for, you ask? Well, we can have open, rmmod, remove_proc_entry
@@ -333,32 +341,33 @@ static int proc_reg_open(struct inode *inode, struct file *file)
if (!pdeo)
return -ENOMEM;
- spin_lock(&pde->pde_unload_lock);
- if (!pde->proc_fops) {
- spin_unlock(&pde->pde_unload_lock);
+ rcu_read_lock();
+ fops = rcu_dereference(pde->proc_fops);
+ if (!fops) {
+ rcu_read_unlock();
kfree(pdeo);
return -ENOENT;
}
- pde->pde_users++;
- open = pde->proc_fops->open;
- release = pde->proc_fops->release;
- spin_unlock(&pde->pde_unload_lock);
+ atomic_inc(&pde->pde_users);
+ open = fops->open;
+ release = fops->release;
+ rcu_read_unlock();
if (open)
rv = open(inode, file);
- spin_lock(&pde->pde_unload_lock);
if (rv == 0 && release) {
/* To know what to release. */
pdeo->inode = inode;
pdeo->file = file;
/* Strictly for "too late" ->release in proc_reg_release(). */
pdeo->release = release;
+ spin_lock(&pde->pde_openers_lock);
list_add(&pdeo->lh, &pde->pde_openers);
+ spin_unlock(&pde->pde_openers_lock);
} else
kfree(pdeo);
- __pde_users_dec(pde);
- spin_unlock(&pde->pde_unload_lock);
+ pde_users_dec(pde);
return rv;
}
@@ -367,10 +376,14 @@ static struct pde_opener *find_pde_opener(struct proc_dir_entry *pde,
{
struct pde_opener *pdeo;
+ spin_lock(&pde->pde_openers_lock);
list_for_each_entry(pdeo, &pde->pde_openers, lh) {
- if (pdeo->inode == inode && pdeo->file == file)
+ if (pdeo->inode == inode && pdeo->file == file) {
+ spin_unlock(&pde->pde_openers_lock);
return pdeo;
+ }
}
+ spin_unlock(&pde->pde_openers_lock);
return NULL;
}
@@ -380,10 +393,13 @@ static int proc_reg_release(struct inode *inode, struct file *file)
int rv = 0;
int (*release)(struct inode *, struct file *);
struct pde_opener *pdeo;
+ const struct file_operations *fops;
- spin_lock(&pde->pde_unload_lock);
pdeo = find_pde_opener(pde, inode, file);
- if (!pde->proc_fops) {
+ rcu_read_lock();
+ fops = rcu_dereference(pde->proc_fops);
+ if (!fops) {
+ rcu_read_unlock();
/*
* Can't simply exit, __fput() will think that everything is OK,
* and move on to freeing struct file. remove_proc_entry() will
@@ -393,21 +409,23 @@ static int proc_reg_release(struct inode *inode, struct file *file)
* But if opener is removed from list, who will ->release it?
*/
if (pdeo) {
+ spin_lock(&pde->pde_openers_lock);
list_del(&pdeo->lh);
- spin_unlock(&pde->pde_unload_lock);
+ spin_unlock(&pde->pde_openers_lock);
rv = pdeo->release(inode, file);
kfree(pdeo);
- } else
- spin_unlock(&pde->pde_unload_lock);
+ }
return rv;
}
- pde->pde_users++;
- release = pde->proc_fops->release;
+ atomic_inc(&pde->pde_users);
+ release = fops->release;
+ rcu_read_unlock();
if (pdeo) {
+ spin_lock(&pde->pde_openers_lock);
list_del(&pdeo->lh);
- kfree(pdeo);
+ spin_unlock(&pde->pde_openers_lock);
}
- spin_unlock(&pde->pde_unload_lock);
+ kfree(pdeo);
if (release)
rv = release(inode, file);
@@ -446,6 +464,7 @@ static const struct file_operations proc_reg_file_ops_no_compat = {
struct inode *proc_get_inode(struct super_block *sb, struct proc_dir_entry *de)
{
struct inode * inode;
+ const struct file_operations *fops;
inode = iget_locked(sb, de->low_ino);
if (!inode)
@@ -465,19 +484,22 @@ struct inode *proc_get_inode(struct super_block *sb, struct proc_dir_entry *de)
set_nlink(inode, de->nlink);
if (de->proc_iops)
inode->i_op = de->proc_iops;
- if (de->proc_fops) {
+ rcu_read_lock();
+ fops = rcu_dereference(de->proc_fops);
+ if (fops) {
if (S_ISREG(inode->i_mode)) {
#ifdef CONFIG_COMPAT
- if (!de->proc_fops->compat_ioctl)
+ if (!fops->compat_ioctl)
inode->i_fop =
&proc_reg_file_ops_no_compat;
else
#endif
inode->i_fop = &proc_reg_file_ops;
} else {
- inode->i_fop = de->proc_fops;
+ inode->i_fop = fops;
}
}
+ rcu_read_unlock();
unlock_new_inode(inode);
} else
pde_put(de);
diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
index 32676b3..82628d9 100644
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h
@@ -68,16 +68,16 @@ struct proc_dir_entry {
* If you're allocating ->proc_fops dynamically, save a pointer
* somewhere.
*/
- const struct file_operations *proc_fops;
+ const struct file_operations __rcu *proc_fops;
struct proc_dir_entry *next, *parent, *subdir;
void *data;
read_proc_t *read_proc;
write_proc_t *write_proc;
atomic_t count; /* use count */
- int pde_users; /* number of callers into module in progress */
+ atomic_t pde_users; /* number of callers into module in progress */
struct completion *pde_unload_completion;
+ spinlock_t pde_openers_lock;
struct list_head pde_openers; /* who did ->open, but not ->release */
- spinlock_t pde_unload_lock; /* proc_fops checks and pde_users bumps */
u8 namelen;
char name[];
};
--
1.8.1.2
--
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