On Thu, May 28, 2009 at 08:23:28AM -0700, Eric W. Biederman wrote: > I thought the network stack was the only piece of code silly enough > to hold locks while deleting sysfs files. > > Holding any lock while deleting a objects from sysfs, sysctl or proc, > is asking for serious mischief, and unfixable from the fs side. > > The usual problem is that lockdep doesn't yet understand > sysfs_deactivate which waits for any running sysfs operations to > complete before it deletes the sysfs files. > > Which means any lock you hold in a show or store method is can deadlock > with any lock you hold while deleting from sysfs. > > ext4 appears lock loose and fancy free in it's show and store methods > so it might be ok except for this issue of mmap_sem vs sysfs_mutex. > But apparently even that isn't enough to git rid of the requirement > to not hold locks when deleting objects. I ran into this problem, and I attempted to work around the fact that the core VFS layer calls the fs's put_super() while holding the BKL and the super block lock as follows at the very end of ext4's put_super() function: /* * Now that we are completely done shutting down the * superblock, we need to actually destroy the kobject. */ unlock_kernel(); unlock_super(sb); kobject_put(&sbi->s_kobj); wait_for_completion(&sbi->s_kobj_unregister); lock_super(sb); lock_kernel(); I'm pretty sure that after the first call to invalidate_inodes() in fs/super.c's generic_shutdown_super(), we really don't need to hold the BKL or the superblock lock (and let the filesystems' low-level write_super(0 and put_super() take the lock if they really need it), but we probably need to take a closer look at this to make sure it's true for all filesystems. (IIRC, I think Christoph was looking to clean up lock_super(); at least with respect to the write_super call. I don't know what his plans regarding the BKL and put_super(), though.) Fundamentally, right now it's rather painful to use sysfs from within a filesystem driver, since the generic VFS layer doesn't have any explicit kobject support functionality. - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
