On Wed, 15 Apr 2020 13:10:18 -0400
Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
> On 4/14/20 8:08 AM, Cornelia Huck wrote:
> > On Tue, 7 Apr 2020 15:20:03 -0400
> > Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
> >> @@ -995,9 +996,11 @@ int ap_parse_mask_str(const char *str,
> >> newmap = kmalloc(size, GFP_KERNEL);
> >> if (!newmap)
> >> return -ENOMEM;
> >> - if (mutex_lock_interruptible(lock)) {
> >> - kfree(newmap);
> >> - return -ERESTARTSYS;
> >> + if (lock) {
> >> + if (mutex_lock_interruptible(lock)) {
> >> + kfree(newmap);
> >> + return -ERESTARTSYS;
> >> + }
> > This whole function is a bit odd. It seems all masks we want to
> > manipulate are always guarded by the ap_perms_mutex, and the need for
> > allowing lock == NULL comes from wanting to call this function with the
> > ap_perms_mutex already held.
> >
> > That would argue for a locked/unlocked version of this function... but
> > looking at it, why do we lock the way we do? The one thing this
> > function (prior to this patch) does outside of the holding of the mutex
> > is the allocation and freeing of newmap. But with this patch, we do the
> > allocation and freeing of newmap while holding the mutex. Something
> > seems a bit weird here.
>
> Note that the ap_parse_mask function copies the newmap
> to the bitmap passed in as a parameter to the function.
> Prior to the introduction of this patch, the calling functions - i.e.,
> apmask_store(), aqmask_store() and ap_perms_init() - passed
> in the actual bitmap (i.e., ap_perms.apm or ap_perms aqm),
> so the ap_perms were changed directly by this function.
>
> With this patch, the apmask_store() and aqmask_store()
> functions now pass in a copy of those bitmaps. This is so
> we can verify that any APQNs being removed are not
> in use by the vfio_ap device driver before committing the
> change to ap_perms. Consequently, it is now necessary
> to take the lock for the until the changes are committed.
Yes, but every caller actually takes the mutex before calling this
function already :)
> Having explained that, you make a valid argument that
> this calls for a locked/unlocked version of this function, so
> I will modify this patch to that effect.
Ok.
The other thing I found weird is that the function does
alloc newmap -> grab mutex -> do manipulation -> release mutex -> free newmap
while the new callers do
(mutex already held) -> alloc newmap
so why grab/release the mutex the way the function does now? IOW, why
not have an unlocked __ap_parse_mask_string() and do
int ap_parse_mask_string(...)
{
int rc;
if (mutex_lock_interruptible(&ap_perms_mutex))
return -ERESTARTSYS;
rc = __ap_parse_mask_string(...);
mutex_unlock(&ap_perms_mutex);
return rc;
}
