On Tue, Feb 24, 2015 at 01:34:06PM +0300, Vladimir Davydov wrote: > On Mon, Feb 23, 2015 at 11:12:22AM -0500, Konrad Rzeszutek Wilk wrote: > > Thank you for posting these patches. I was wondering if you had > > run through some of the different combinations that you can > > load the filesystems/tmem drivers in random order? The #4 patch > > deleted a nice chunk of documentation that outlines the different > > combinations. > > Yeah, I admit the synchronization between cleancache_register_ops and > cleancache_init_fs is far not obvious. I should have updated the comment > instead of merely dropping it, sorry. What about the following patch > proving correctness of register_ops-vs-init_fs synchronization? It is > meant to be applied incrementally on top of patch #4. Just fold it in please. But more importantly - I was wondering if you had run throught the different combinations it outlines? > --- > diff --git a/mm/cleancache.c b/mm/cleancache.c > index fbdaf9c77d7a..8fc50811119b 100644 > --- a/mm/cleancache.c > +++ b/mm/cleancache.c > @@ -54,6 +54,57 @@ int cleancache_register_ops(struct cleancache_ops *ops) > if (cmpxchg(&cleancache_ops, NULL, ops)) > return -EBUSY; > > + /* > + * A cleancache backend can be built as a module and hence loaded after > + * a cleancache enabled filesystem has called cleancache_init_fs. To > + * handle such a scenario, here we call ->init_fs or ->init_shared_fs > + * for each active super block. To differentiate between local and > + * shared filesystems, we temporarily initialize sb->cleancache_poolid > + * to CLEANCACHE_NO_BACKEND or CLEANCACHE_NO_BACKEND_SHARED > + * respectively in case there is no backend registered at the time > + * cleancache_init_fs or cleancache_init_shared_fs is called. > + * > + * Since filesystems can be mounted concurrently with cleancache > + * backend registration, we have to be careful to guarantee that all > + * cleancache enabled filesystems that has been mounted by the time > + * cleancache_register_ops is called has got and all mounted later will > + * get cleancache_poolid. This is assured by the following statements > + * tied together: > + * > + * a) iterate_supers skips only those super blocks that has started > + * ->kill_sb > + * > + * b) if iterate_supers encounters a super block that has not finished > + * ->mount yet, it waits until it is finished > + * > + * c) cleancache_init_fs is called from ->mount and > + * cleancache_invalidate_fs is called from ->kill_sb > + * > + * d) we call iterate_supers after cleancache_ops has been set > + * > + * From a) it follows that if iterate_supers skips a super block, then > + * either the super block is already dead, in which case we do not need > + * to bother initializing cleancache for it, or it was mounted after we > + * initiated iterate_supers. In the latter case, it must have seen > + * cleancache_ops set according to d) and initialized cleancache from > + * ->mount by itself according to c). This proves that we call > + * ->init_fs at least once for each active super block. > + * > + * From b) and c) it follows that if iterate_supers encounters a super > + * block that has already started ->init_fs, it will wait until ->mount > + * and hence ->init_fs has finished, then check cleancache_poolid, see > + * that it has already been set and therefore do nothing. This proves > + * that we call ->init_fs no more than once for each super block. > + * > + * Combined together, the last two paragraphs prove the function > + * correctness. > + * > + * Note that various cleancache callbacks may proceed before this > + * function is called or even concurrently with it, but since > + * CLEANCACHE_NO_BACKEND is negative, they will all result in a noop > + * until the corresponding ->init_fs has been actually called and > + * cleancache_ops has been set. > + */ > iterate_supers(cleancache_register_ops_sb, NULL); > return 0; > } -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>