On Wed, Jan 23, 2013 at 03:51:47PM -0800, Andrew Morton wrote: > > note that > > * file->f_path is already pinned down by open(), path_get() does not > > provide anything extra. > > * file->f_path.dentry is already pinned by open() *and* path_get() > > just above that dget(). > > * ->d_name.name *IS* *NOT* *PROTECTED* by pinning dentry down, > > whether it's done once or thrice. > > I guess the first two are obvious (or at least, expected). But the > third isn't.
->d_name.name is changed by rename() (as one could expect). Grabbing a reference to dentry will not prevent rename() from happening. ->i_mutex on parent will, but you either need to play with retries (grab reference to parent, grab ->i_mutex, check that it's still our parent, if we'd lost the race and someone had renamed the sucker - unlock ->i_mutex, dput, repeat) *or* to have our dentry looked up with parent locked, with ->i_mutex on said parent still held (which happens to cover the majority of valid uses in fs code - ->lookup(), ->create(), ->unlink(), rename(), etc. are all called that way, so the name of dentry passed to such methods is stable for the duration of the method). ->d_lock on dentry is also sufficient, but that obviously means that you can't block while holding it. > Where should a kernel developer go to learn these things? > include/linux/dcache.h doesn't mention d_name locking rules, nor does > Documentation/filesystems/vfs.txt. See directory locking rules in there; the crucial point is that dentry name is changed by rename() *and* that results of a race can be worse than just running into a partially rewritten name - long names are allocated separately and walking through a stale pointer you might end up in freed memory. It's a mess, unfortunately, and $BIGNUM other uses of ->i_mutex make it only nastier. Once in a while I go hunting for races in that area, usally with a bunch of fixes coming out of such run ;-/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
