Re: [PATCH 1/2] fs: sysfs: Do dcache-related updates to sysfs dentries under sysfs_mutex

Mel Gorman [off-list ref] writes:

On Wed, Jan 11, 2012 at 09:11:27AM -0800, Eric W. Biederman wrote:

quoted

quoted

In Miklos's case, the problem is with the bonding driver but during
CPU online or offline, a number of dentries are being created and
deleted and this deadlock is also being hit. Looking at sysfs, there
is a global sysfs_mutex that protects the sysfs directory tree from
concurrent reclaims. Almost all operations involving directory inodes
and dentries take place under the sysfs_mutex - linking, unlinking,
patch searching lookup, renames and readdir. d_invalidate is slightly
different. It is mostly under the mutex but if the dentry has to be
removed from the dcache, the mutex is dropped.

The sysfs_mutex protects the sysfs data structures not the vfs.

Ok.

quoted

quoted

Where as Miklos' patch changes dcache, this patch changes sysfs to
consistently hold the mutex for dentry-related operations. Once
applied, this particular bug with CPU hotadd/hotremove no longer
occurs.

After taking a quick skim over the code to reacquaint myself with
it appears that the usage in sysfs is idiomatic.  That is sysfs
uses shrink_dcache_parent without a lock and in a context where
the right race could trigger this deadlock.

Yes.

quoted

And in particular I expect you could trigger the same deadlock in
proc, nfs, and gfs2 with if you can get the timing right.

Agreed. When the dcache-specific fix was being discussed on an external
bugzilla, this came up. It's probably easiest to race in sysfs because
it's possible to create/delete directories faster than is possible
for proc, nfs or gfs2.

I expect we see the race in sysfs because of uevents that get triggered
on hotplug.  So a lot is occurring around the time of the race.  You can
get to shrink_dcache_parent with fork/exit in proc which is a lot easier
to trigger.  But usually in fork/exec you don't have the dentries cached...

Since I wrote this patch, the dcache specific fix was finished, merged
and I expect it'll make it to stable. Assuming that happens, this patch
will no longer be required.

Sounds good.

Eric

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>