Messages in this thread | | | Date | Fri, 25 Jan 2013 09:49:53 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [tracepoint] cargo-culting considered harmful... |
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* Al Viro (viro@ZenIV.linux.org.uk) wrote: > 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.
Hi Al,
I agree that the tracepoint example should be removed. There is one extra piece of module code I think would require fixing (see below).
> > ->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 ;-/
In the light of what you are saying here, am I right to think that the following code is broken wrt locking wrt use of filp->f_dentry->d_name.name ?
static void lttng_enumerate_task_fd(struct lttng_session *session, struct task_struct *p, char *tmp) { struct fdtable *fdt; struct file *filp; unsigned int i; const unsigned char *path;
task_lock(p); if (!p->files) goto unlock_task; spin_lock(&p->files->file_lock); fdt = files_fdtable(p->files); for (i = 0; i < fdt->max_fds; i++) { filp = fcheck_files(p->files, i); if (!filp) continue; path = d_path(&filp->f_path, tmp, PAGE_SIZE); /* Make sure we give at least some info */ trace_lttng_statedump_file_descriptor(session, p, i, IS_ERR(path) ? filp->f_dentry->d_name.name : path); } spin_unlock(&p->files->file_lock); unlock_task: task_unlock(p); }
Since tracepoints never block, holding the ->d_lock around the trace_lttng_statedump_file_descriptor() tracepoint should probably be enough to make it correct. Am I missing anything ?
Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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