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SubjectRe: [ANNOUNCE] 3.2.9-rt16
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On Tue, 2012-03-06 at 11:20 +0100, Thomas Gleixner wrote:
> Dear RT Folks,
>
> I'm pleased to announce the 3.2.9-rt16 release.
>
> Changes vs. 3.2.9-rt15:
>
> * cpu hotplug lock init fix [ Steven ]
>
> * seqlock fix CONFIG typo
>

Note, yesterday while running some stress tests I hit a live lock here:

static inline struct dentry *dentry_kill(struct dentry *dentry, int ref)
__releases(dentry->d_lock)
{
struct inode *inode;
struct dentry *parent;

inode = dentry->d_inode;
if (inode && !spin_trylock(&inode->i_lock)) {
relock:
seq_spin_unlock(&dentry->d_lock);
cpu_relax();
return dentry; /* try again with same dentry */
}
if (IS_ROOT(dentry))
parent = NULL;
else
parent = dentry->d_parent;
if (parent && !seq_spin_trylock(&parent->d_lock)) {
if (inode)
spin_unlock(&inode->i_lock);
goto relock;
}


When it fails to grab either the inode->i_lock or the parent->d_lock it
returns back to dput() and dput() will retry. We get into another one of
these cases where we can spin blocking the holder of the locks.

I experimented with adding a grab lock of the inode->i_lock or
parent->d_lock if they existed (required initializing parent to NULL),
which seemed to help a lot, but then eventually it locked up. As I'm not
sure its safe to grab them straight here even after we release the
dentry->d_lock. I'll have to enable full lockdep to see if this breaks
the ordering.

I haven't looked too deeply into this code yet, but I'm assuming that
dput() can be called where we can't just take the inode or parent lock?

-- Steve




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