On Wed, Jan 11, 2012 at 09:11:27AM -0800, Eric W. Biederman wrote: > > 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. > > 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. > 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 don't think adding a work-around for the bug in shrink_dcache_parent > is going to do anything except hide the bug in the VFS, and > unnecessarily increase the sysfs_mutex hold times. > Ok. > I may be blind but I don't see a reason at this point to rush out an > incomplete work-around for the bug in shrink_dcahce_parent instead of > actually fixing shrink_dcache_parent. > 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. Thanks Eric. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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@xxxxxxxxx";> email@xxxxxxxxx </a>