Messages in this thread Patch in this message | | | From | Neil Brown <> | Date | Mon, 10 May 2004 13:03:18 +1000 | Subject | Re: d_splice_alias() problem. |
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On Tuesday May 4, gnb@melbourne.sgi.com wrote: > > > > > > --- linux.orig/fs/dcache.c Mon May 3 21:46:30 2004 > > > +++ linux/fs/dcache.c Mon May 3 21:49:07 2004 > > > @@ -255,8 +255,8 @@ > > > > > > static inline struct dentry * __dget_locked(struct dentry *dentry) > > > { > > > - atomic_inc(&dentry->d_count); > > > - if (atomic_read(&dentry->d_count) == 1) { > > > + if (atomic_inc(&dentry->d_count) == 1) { > > > > One problem with this is that (in include/asm-i386/atomic.h at least): > > static __inline__ void atomic_inc(atomic_t *v) > > Ok, how about this...it's portable, and not racy, but may perturb the > logic slightly by also taking dentries off the unused list in the case > where they already had d_count>=1. I'm not sure how significant that is. > In any case this also passes my tests.
I think this patch is good and needed.
I think the race can happen if: dentry->d_count == 1, not on list
thread 1 thread 2 enter __dget_locked enter dput atomic_inc(d_count) (now 2) atomic_dec_and_lock(d_count...) (now 1) if(atomic_read(d_count)==1 .... remove from list
This will remove it from the unused list when it isn't on, and will decrement nr_unused which, as you say, is bad.
I don't think there can be any problem with taking dentries with a non-zero d_count off the unused_list randomly (providing dcache_lock is held of course).
The comment at the top of d_lookup says that it doesn't take dentries off the unused_list, but instead leaves that task to assorted other code.
prune_dcache and shrink_dcache_sb and select_parent will all quietly remove such dentries from the unused list and there is no reason that __dget_locked shouldn't aswell. Possibly that patch should update the d_lookup comment to add __dget_locked to the set of functions that clean up after it.
NeilBrown
> > > --- linux.orig/fs/dcache.c Mon May 3 21:46:30 2004 > +++ linux/fs/dcache.c Tue May 4 14:34:44 2004 > @@ -256,7 +256,7 @@ > static inline struct dentry * __dget_locked(struct dentry *dentry) > { > atomic_inc(&dentry->d_count); > - if (atomic_read(&dentry->d_count) == 1) { > + if (!list_empty(&dentry->d_lru)) { > dentry_stat.nr_unused--; > list_del_init(&dentry->d_lru); > } > @@ -663,6 +663,7 @@ > if (gfp_mask & __GFP_FS) > prune_dcache(nr); > } > + BUG_ON(dentry_stat.nr_unused < 0); > return dentry_stat.nr_unused; > } > > > > Greg. > -- > Greg Banks, R&D Software Engineer, SGI Australian Software Group. > I don't speak for SGI. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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