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. --- 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>