Messages in this thread | | | Date | Tue, 8 Feb 2022 08:26:40 -1000 | From | Tejun Heo <> | Subject | Re: [PATCH v5 2/2] kernfs: Replace per-fs global rwsem with per-fs hashed rwsem. |
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Hello,
On Sun, Feb 06, 2022 at 12:09:25PM +1100, Imran Khan wrote: > Having a single rwsem to synchronize all operations across a kernfs > based file system (cgroup, sysfs etc.) does not scale well. Replace > it with a hashed rwsem to reduce contention around single per-fs > rwsem. > Also introduce a perfs rwsem to protect per-fs list of kernfs_super_info.
Can you split the two conversions into separate patches? Also, I'm not sure about the per-fs hashtable as before.
> static bool kernfs_active(struct kernfs_node *kn) > { > - lockdep_assert_held(&kernfs_root(kn)->kernfs_rwsem); > + int idx = hash_ptr(kn, NR_KERNFS_LOCK_BITS); > + > + lockdep_assert_held(&kernfs_root(kn)->kernfs_rwsem[idx]);
Please encapsulate this into a function. If possible, it'd be ideal if the conversion can be done in steps - e.g. first introduce lock encapsulation interface while leaving locking itself alone and then switch the actual locking.
> -static void kernfs_drain(struct kernfs_node *kn) > - __releases(&kernfs_root(kn)->kernfs_rwsem) > - __acquires(&kernfs_root(kn)->kernfs_rwsem) > +static void kernfs_drain(struct kernfs_node *kn, struct kernfs_node *anc) > + __releases(&kernfs_root(anc)->kernfs_rwsem[a_idx]) > + __acquires(&kernfs_root(anc)->kernfs_rwsem[a_idx]) > { > struct kernfs_root *root = kernfs_root(kn); > + int a_idx = hash_ptr(anc, NR_KERNFS_LOCK_BITS); > > - lockdep_assert_held_write(&root->kernfs_rwsem); > - WARN_ON_ONCE(kernfs_active(kn)); > + lockdep_assert_held_write(&root->kernfs_rwsem[a_idx]); > + WARN_ON_ONCE(atomic_read(&kn->active) >= 0);
Ditto, I'd much prefer to see the lock lookup and accompanying operations to be encapsulated somehow.
> @@ -1588,37 +1597,65 @@ int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent, > const char *new_name, const void *new_ns) > { > struct kernfs_node *old_parent; > - struct kernfs_root *root; > const char *old_name = NULL; > - int error; > + int error, idx, np_idx, p_idx; > > /* can't move or rename root */ > if (!kn->parent) > return -EINVAL; > > - root = kernfs_root(kn); > - down_write(&root->kernfs_rwsem); > + /* > + * Take lock of node's old (current) parent. > + * If new parent has a different lock, then take that > + * lock as well. > + */ > + idx = hash_ptr(kn, NR_KERNFS_LOCK_BITS); > + p_idx = hash_ptr(kn->parent, NR_KERNFS_LOCK_BITS); > + np_idx = hash_ptr(new_parent, NR_KERNFS_LOCK_BITS); > + > + /* > + * Take only kn's lock. The subsequent kernfs_put > + * may free up old_parent so if old_parent has a > + * different lock, we will explicitly release that. > + */ > + down_write_kernfs_rwsem(kn, LOCK_SELF, 0); > + > + if (idx != np_idx) /* new parent hashes to different lock */ > + down_write_kernfs_rwsem(new_parent, LOCK_SELF, 1); > + > + /* old_parent hashes to a different lock */ > + if (idx != p_idx && p_idx != np_idx) > + down_write_kernfs_rwsem(kn->parent, LOCK_SELF, 2);
Can't this lead to ABBA deadlock? When double locking, the locking order should always be consistent. If we were doing per-kernfs_node lock, child -> parent ordering works but we're hashing locks, so that doesn't work anymore - one child-parent combo can lock A then B while the other child-parent combo hash the other way around and lock B then A. The only order we can define is in terms of the locks themselves - e.g. if the address (or index) of lock A < lock B, then we lock A first whether that maps to the child or parent.
Also, please encapsulate double locking in a set of functions. We really don't wanna see all the details in the users.
> --- a/fs/kernfs/kernfs-internal.h > +++ b/fs/kernfs/kernfs-internal.h > @@ -19,6 +19,9 @@ > #include <linux/kernfs.h> > #include <linux/fs_context.h> > > +#define LOCK_SELF 0 > +#define LOCK_SELF_AND_PARENT 1
I get that this is private header but can you please add some identifying prefix and make them enums? That makes it way easier for debuggers, bpf and tracing.
> +/* > + * If both node and it's parent need locking, > + * lock child first so that kernfs_rename_ns > + * does not change the parent, leaving us > + * with old parent here. > + */
Please reflow it close to 80 chars and ditto with above. We can't follow child -> parent order with hashed locks.
Thanks.
-- tejun
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