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Date: Wed, 28 May 2014 04:19:55 +0100
From: Al Viro <viro@...IV.linux.org.uk>
To: Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Miklos Szeredi <mszeredi@...e.cz>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: fs/dcache.c - BUG: soft lockup - CPU#5 stuck for 22s!
[systemd-udevd:1667]
On Tue, May 27, 2014 at 10:04:09AM +0300, Mika Westerberg wrote:
> On Tue, May 27, 2014 at 05:00:26AM +0100, Al Viro wrote:
> > On Tue, May 27, 2014 at 04:14:15AM +0100, Al Viro wrote:
> >
> > > As the matter of fact, let's try this instead - retry the same sucker
> > > immediately in case if trylocks fail. Comments?
> >
> > Better yet, let's take "move back to shrink list" into dentry_kill()
> > itself. Then we get consistent locking rules for dentry_kill() and
> > instead of unlock_on_failure we simply pass it NULL or the shrink
> > list to put the sucker back. Mika, could you test this one and see
> > if it fixes that livelock? The difference in behaviour is that in
> > case of trylock failure we hit that sucker again without letting
> > it ride all the way around the list, same as we do for other dentry_kill()
> > callers.
>
> I tried this patch and unfortunately it still results the same sort of
> livelock. I've attached the dmesg.
>
> I also tried the serialization patch from Linus and it seemed to fix the
> problem. After several rounds of USB memory stick plug/unplug I haven't
> seen a single "soft lockup" warning in dmesg.
>
> I'm able to reproduce the problem pretty easily, so if you have
> something else to try I'm more than happy to give it a try.
Could you try this and post the resulting log? I'd really like to understand
what's going on there - are we really hitting trylock failures there and what
dentries are involved.
diff --git a/fs/dcache.c b/fs/dcache.c
index 42ae01e..75f56a6 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -38,6 +38,7 @@
#include <linux/prefetch.h>
#include <linux/ratelimit.h>
#include <linux/list_lru.h>
+#include <linux/magic.h>
#include "internal.h"
#include "mount.h"
@@ -448,7 +449,7 @@ EXPORT_SYMBOL(d_drop);
* Returns dentry requiring refcount drop, or NULL if we're done.
*/
static struct dentry *
-dentry_kill(struct dentry *dentry, int unlock_on_failure)
+dentry_kill(struct dentry *dentry, struct list_head *shrink_list)
__releases(dentry->d_lock)
{
struct inode *inode;
@@ -464,10 +465,10 @@ dentry_kill(struct dentry *dentry, int unlock_on_failure)
inode = dentry->d_inode;
if (inode && !spin_trylock(&inode->i_lock)) {
relock:
- if (unlock_on_failure) {
- spin_unlock(&dentry->d_lock);
- cpu_relax();
- }
+ if (shrink_list)
+ d_shrink_add(dentry, shrink_list);
+ spin_unlock(&dentry->d_lock);
+ cpu_relax();
return dentry; /* try again with same dentry */
}
if (!IS_ROOT(dentry))
@@ -542,6 +543,14 @@ out:
* on the compiler to always get this right (gcc generally doesn't).
* Real recursion would eat up our stack space.
*/
+static inline void dump(const char *s, struct dentry *dentry)
+{
+ if (unlikely(dentry->d_sb->s_magic == SYSFS_MAGIC)) {
+ printk(KERN_ERR "%s[%pd4]; CPU %d PID %d [%s]\n",
+ s, dentry, smp_processor_id(),
+ task_pid_nr(current), current->comm);
+ }
+}
/*
* dput - release a dentry
@@ -579,7 +588,9 @@ repeat:
return;
kill_it:
- dentry = dentry_kill(dentry, 1);
+ if (dentry->d_inode)
+ dump("dput", dentry);
+ dentry = dentry_kill(dentry, NULL);
if (dentry)
goto repeat;
}
@@ -798,6 +809,7 @@ static void shrink_dentry_list(struct list_head *list)
while (!list_empty(list)) {
dentry = list_entry(list->prev, struct dentry, d_lru);
+again:
spin_lock(&dentry->d_lock);
/*
* The dispose list is isolated and dentries are not accounted
@@ -815,22 +827,19 @@ static void shrink_dentry_list(struct list_head *list)
continue;
}
- parent = dentry_kill(dentry, 0);
+ dump("shrink", dentry);
+ parent = dentry_kill(dentry, list);
/*
* If dentry_kill returns NULL, we have nothing more to do.
*/
if (!parent)
continue;
+ /* if trylocks have failed; just do it again */
if (unlikely(parent == dentry)) {
- /*
- * trylocks have failed and d_lock has been held the
- * whole time, so it could not have been added to any
- * other lists. Just add it back to the shrink list.
- */
- d_shrink_add(dentry, list);
- spin_unlock(&dentry->d_lock);
- continue;
+ if (dentry->d_sb->s_magic == SYSFS_MAGIC)
+ printk(KERN_ERR "A");
+ goto again;
}
/*
* We need to prune ancestors too. This is necessary to prevent
@@ -839,8 +848,10 @@ static void shrink_dentry_list(struct list_head *list)
* fragmentation.
*/
dentry = parent;
- while (dentry && !lockref_put_or_lock(&dentry->d_lockref))
- dentry = dentry_kill(dentry, 1);
+ while (dentry && !lockref_put_or_lock(&dentry->d_lockref)) {
+ dump("shrink-dput", dentry);
+ dentry = dentry_kill(dentry, NULL);
+ }
}
}
@@ -1223,6 +1234,7 @@ out:
*/
void shrink_dcache_parent(struct dentry *parent)
{
+ dump("shrink_dcache_parent", parent);
for (;;) {
struct select_data data;
@@ -1331,6 +1343,8 @@ int check_submounts_and_drop(struct dentry *dentry)
goto out;
}
+ dump("check_submounts_and_drop", dentry);
+
for (;;) {
struct select_data data;
--
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