On Wed, Apr 29, 2020 at 07:22:13AM +0200, Manfred Spraul wrote:
> Hello together,
>
> On 4/28/20 1:14 PM, Matthew Wilcox wrote:
> > On Tue, Apr 28, 2020 at 03:47:36AM +0000, Wei Yongjun wrote:
> > > The function ipc_id_alloc() is called from ipc_addid(), in which
> > > a spin lock is held, so we should use GFP_ATOMIC instead.
> > >
> > > Fixes: de5738d1c364 ("ipc: convert ipcs_idr to XArray")
> > > Signed-off-by: Wei Yongjun <[email protected]>
> > I see why you think that, but it's not true. Yes, we hold a spinlock, but
> > the spinlock is in an object which is not reachable from any other CPU.
>
> Is it really allowed that spin_lock()/spin_unlock may happen on different
> cpus?
>
> CPU1: spin_lock()
>
> CPU1: schedule() -> sleeps
>
> CPU2: -> schedule() returns
>
> CPU2: spin_unlock().
I think that is allowed, but I'm not an expert in the implementations.
> > Converting to GFP_ATOMIC is completely wrong.
>
> What is your solution proposal?
>
> xa_store() also gets a gfp_ flag. Thus even splitting _alloc() and _store()
> won't help
>
> xa_alloc(,entry=NULL,)
> new->seq = ...
> spin_lock();
> xa_store(,entry=new,GFP_KERNEL);
While it takes GFP flags, it won't do any allocation if it's overwriting
an allocated entry.
diff --git a/ipc/util.c b/ipc/util.c
index 0f6b0e0aa17e..b929ab0072a5 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -19,8 +19,8 @@
*
* General sysv ipc locking scheme:
* rcu_read_lock()
- * obtain the ipc object (kern_ipc_perm) by looking up the id in an
idr
- * tree.
+ * obtain the ipc object (kern_ipc_perm) by looking up the id in an
+ * xarray.
* - perform initial checks (capabilities, auditing and permission,
* etc).
* - perform read-only operations, such as INFO command, that
@@ -209,14 +209,14 @@ static inline int ipc_id_alloc(struct ipc_ids *ids,
struct kern_ipc_perm *new)
u32 idx;
int err;
+ xa_lock(&ids->ipcs);
+
if (get_restore_id(ids) < 0) {
int max_idx;
max_idx = max(ids->in_use*3/2, ipc_min_cycle);
max_idx = min(max_idx, ipc_mni) - 1;
- xa_lock(&ids->ipcs);
-
err = __xa_alloc_cyclic(&ids->ipcs, &idx, NULL,
XA_LIMIT(0, max_idx), &ids->next_idx,
GFP_KERNEL);
@@ -224,24 +224,31 @@ static inline int ipc_id_alloc(struct ipc_ids *ids,
struct kern_ipc_perm *new)
ids->seq++;
if (ids->seq >= ipcid_seq_max())
ids->seq = 0;
+ err = 0;
}
- if (err >= 0) {
+ if (!err) {
new->seq = ids->seq;
new->id = (new->seq << ipcmni_seq_shift()) + idx;
- /* xa_store contains a write barrier */
- __xa_store(&ids->ipcs, idx, new, GFP_KERNEL);
}
-
- xa_unlock(&ids->ipcs);
} else {
new->id = get_restore_id(ids);
new->seq = ipcid_to_seqx(new->id);
idx = ipcid_to_idx(new->id);
- err = xa_insert(&ids->ipcs, idx, new, GFP_KERNEL);
+ err = __xa_insert(&ids->ipcs, idx, NULL, GFP_KERNEL);
set_restore_id(ids, -1);
}
+ /*
+ * Hold the spinlock so that nobody else can access the object
+ * once they can find it. xa_store contains a write barrier so
+ * all previous stores to 'new' will be visible.
+ */
+ spin_lock(&new->lock);
+ if (!err)
+ __xa_store(&ids->ipcs, idx, new, GFP_NOWAIT);
+ xa_unlock(&ids->ipcs);
+
if (err == -EBUSY)
return -ENOSPC;
if (err < 0)
@@ -255,7 +262,7 @@ static inline int ipc_id_alloc(struct ipc_ids *ids, struct
kern_ipc_perm *new)
* @new: new ipc permission set
* @limit: limit for the number of used ids
*
- * Add an entry 'new' to the ipc ids idr. The permissions object is
+ * Add an entry 'new' to the ipc ids. The permissions object is
* initialised and the first free entry is set up and the index assigned
* is returned. The 'new' entry is returned in a locked state on success.
*
@@ -270,7 +277,7 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm
*new, int limit)
kgid_t egid;
int idx;
- /* 1) Initialize the refcount so that ipc_rcu_putref works */
+ /* Initialize the refcount so that ipc_rcu_putref works */
refcount_set(&new->refcount, 1);
if (limit > ipc_mni)
@@ -279,12 +286,7 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm
*new, int limit)
if (ids->in_use >= limit)
return -ENOSPC;
- /*
- * 2) Hold the spinlock so that nobody else can access the object
- * once they can find it
- */
spin_lock_init(&new->lock);
- spin_lock(&new->lock);
current_euid_egid(&euid, &egid);
new->cuid = new->uid = euid;
new->gid = new->cgid = egid;
@@ -588,7 +590,7 @@ void ipc64_perm_to_ipc_perm(struct ipc64_perm *in, struct
ipc_perm *out)
* @ids: ipc identifier set
* @id: ipc id to look for
*
- * Look for an id in the ipc ids idr and return associated ipc object.
+ * Look for an id in the ipc ids and return associated ipc object.
*
* Call inside the RCU critical section.
* The ipc object is *not* locked on exit.
