James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> writes: >> >> My take is simply that a correct user has to wait until no one else >> >> can find the kobject before calling kobject_del. At which point >> >> races are impossible, and it doesn't matter if sysfs_mutex is held >> >> across the entire operation. >> > >> > I'm afraid this one isn't a valid assumption. If you look in SCSI, >> > you'll see we do get objects after they've been removed from visibility. >> > We use it as part of the state model for how our objects work (objects >> > removed from visibility are dying, but we still need them to be findable >> > (and gettable). >> >> I was not precise enough. It appears I overlooked the fact that >> kobject_del is not always called from kobject_put by way of >> kobject_release. > > OK ... just so you understand, I'm thinking about the device model > rather than kobjects. device_del() can't be called from release methods > because they're often called from interrupt context and the mutex > requirements in device_del() mean it needs user context. Makes sense. >> Strictly the requirement is that after kobject_del we don't add, >> remove or otherwise manipulate sysfs attributes. That is we don't >> call any of: >> >> sysfs_add_file >> sysfs_create_file >> sysfs_create_bin_file >> sysfs_remove_file >> sysfs_remove_bin_file >> sysfs_create_link >> sysfs_remove_link >> sysfs_create_group >> sysfs_remove_group >> sysfs_create_subdir >> sysfs_remove_subdir >> >> >> Those all either oops or BUG today if you try it. So I can't see how >> a subsystem could depend on those working. > > It doesn't; you've altered your requirement. We can fully buy into this > new relaxed one. My apologies for misstating it earlier. Sometimes translating what is happening in sysfs up to the device model can be a bit of a challenge. At the sysfs layer the requirement is all the same. Don't mess with a directory as or after you have deleted it. To recap, my change that Tejun has a problem with is simply that I have refactored sysfs_remove_dir so that if there are directory entries present. A very fast observer in the kernel or in user space can see each directory entry being deleted individually. Before I delete the directory itself. This is because I now drop and reacquire the sysfs_mutex in between each delete. As the upper layers must already avoid messing with the attributes of a sysfs directory from the time we call kobject_del I don't see that this makes any difference to them. >> Also there is sysfs_remove_dir (on a subdirectory) aka kobject_del on >> a child object after kobject_del on the parent object. >> >> As best I can tell that only works by fluke today. > > Yes, that's an artifact of the fact that the reference counted lifecycle > is on release ... del just happens at a certain point in it. We don't > hold any counters that tell us what the visibility of our children are, > so it's possible to make a parent invisible by calling del simply > because you don't know. Strictly speaking my changes don't affect this part either except to issue a warning that something unexpected is going on. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
