On 02/15/2013 04:12 PM, Andrew Morton wrote:
On Fri, 15 Feb 2013 14:47:54 -0600
Nathan Zimmer <nzim...@sgi.com> wrote:
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 replaces the lock with 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/resend of what was orignally suggested by Eric Dumazet some
time ago.
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
...
diff --git a/fs/proc/generic.c b/fs/proc/generic.c
index 76ddae8..6896a70 100644
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -191,13 +191,16 @@ 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;
- spin_lock(&pde->pde_unload_lock);
- if (!pde->proc_fops) {
- spin_unlock(&pde->pde_unload_lock);
+ const struct file_operations *fops;
There's now a stray newline in the definitions section.
Noted and corrected, in a few places.
+ 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();
So what's up with pde_users? Seems that it's atomic_t *and* uses a
form of RCU protection. We can't make it a plain old integer because
it's modified under rcu_read_lock() and we can't move the atomic_inc()
outside rcu_read_lock() because of the synchronization games in
remove_proc_entry()?
The intent of pde_users is to let us know when it is safe to clean out
the pde_openers.
I probably should comment this.
rv = __proc_file_read(file, buf, nbytes, ppos);
...
@@ -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. */
+ DECLARE_COMPLETION_ONSTACK(c);
This should have generated a c99-style definition warning. Did your
compiler version not do this?
A clear over site on my part.
+ 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--;
...
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.
*/
This comment needs updating.
However, it doesn't appear to be true any more. With this patch we no
longer set ->fops to NULL in remove_proc_entry(). (What replaced that
logic?)
So are all these games with local variable `llseek' still needed?
afaict the increment of pde_users will stabilize ->fops?
We still are setting de->proc_fops to NULL to prevent new callers.
Also we still have to save fops-> since we cannot use fops outside the
rcu_read_un/lock.
Unless I misunderstood your question.
But yes the comment needs to be updated.
- llseek = pde->proc_fops->llseek;
- spin_unlock(&pde->pde_unload_lock);
+ llseek = fops->llseek;
+ rcu_read_unlock();
if (!llseek)
llseek = default_llseek;
@@ -182,15 +176,17 @@ 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();
Many dittoes.
if (read)
rv = read(file, buf, count, ppos);
@@ -204,15 +200,17 @@ 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();
...
Thanks,
Nate
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