I wanted to wrap a bunch of ida_simple_get calls into their own
locking, until I dug around and read the original commit message.
Stuff like this should imo be added to the kernel doc, let's do that.
Cc: Mel Gorman <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Andrew Morton <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
---
lib/idr.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)
diff --git a/lib/idr.c b/lib/idr.c
index 6098336df267..5508d7f6d7be 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -927,6 +927,9 @@ EXPORT_SYMBOL(ida_pre_get);
* and go back to the ida_pre_get() call. If the ida is full, it will
* return %-ENOSPC.
*
+ * Note that callers must ensure that concurrent access to @ida is not possible.
+ * When simplicity trumps concurrency needs look at ida_simple_get() instead.
+ *
* @p_id returns a value in the range @starting_id ... %0x7fffffff.
*/
int ida_get_new_above(struct ida *ida, int starting_id, int *p_id)
@@ -1073,6 +1076,10 @@ EXPORT_SYMBOL(ida_destroy);
* Allocates an id in the range start <= id < end, or returns -ENOSPC.
* On memory allocation failure, returns -ENOMEM.
*
+ * Compared to ida_get_new_above() this function does its own locking and hence
+ * is recommended everywhere where simplicity is preferred over the last bit of
+ * speed.
+ *
* Use ida_simple_remove() to get rid of an id.
*/
int ida_simple_get(struct ida *ida, unsigned int start, unsigned int end,
@@ -1119,6 +1126,13 @@ EXPORT_SYMBOL(ida_simple_get);
* ida_simple_remove - remove an allocated id.
* @ida: the (initialized) ida.
* @id: the id returned by ida_simple_get.
+ *
+ * Use to release an id allocated with ida_simple_get().
+ *
+ * Compared to ida_get_new_above() this function does its own locking and hence
+ * is recommended everywhere where simplicity is preferred over the last bit of
+ * speed.
+ *
*/
void ida_simple_remove(struct ida *ida, unsigned int id)
{
--
2.10.1
Hello, Daniel.
On Wed, Oct 26, 2016 at 04:27:39PM +0200, Daniel Vetter wrote:
> I wanted to wrap a bunch of ida_simple_get calls into their own
> locking, until I dug around and read the original commit message.
> Stuff like this should imo be added to the kernel doc, let's do that.
Generally agreed but some nits below.
> @@ -927,6 +927,9 @@ EXPORT_SYMBOL(ida_pre_get);
> * and go back to the ida_pre_get() call. If the ida is full, it will
> * return %-ENOSPC.
> *
> + * Note that callers must ensure that concurrent access to @ida is not possible.
> + * When simplicity trumps concurrency needs look at ida_simple_get() instead.
Maybe we can make it a bit less dramatic?
> @@ -1073,6 +1076,10 @@ EXPORT_SYMBOL(ida_destroy);
> * Allocates an id in the range start <= id < end, or returns -ENOSPC.
> * On memory allocation failure, returns -ENOMEM.
> *
> + * Compared to ida_get_new_above() this function does its own locking and hence
> + * is recommended everywhere where simplicity is preferred over the last bit of
> + * speed.
Hmm... so, this isn't necessarily about speed. For example, id
allocation might have to happen inside a spinlock which protects a
larger scope. To guarantee GFP_KERNEL allocation behavior in such
cases, the caller would have to call ida_pre_get() outside the said
spinlock and then call ida_get_new_above() inside the lock.
I think it'd be better to explain what the simple version does and
expects and then say that unless there are specific requirements using
the simple version is recommended.
Thanks.
--
tejun
On Wed, Oct 26, 2016 at 10:39:29AM -0400, Tejun Heo wrote:
> Hello, Daniel.
>
> On Wed, Oct 26, 2016 at 04:27:39PM +0200, Daniel Vetter wrote:
> > I wanted to wrap a bunch of ida_simple_get calls into their own
> > locking, until I dug around and read the original commit message.
> > Stuff like this should imo be added to the kernel doc, let's do that.
>
> Generally agreed but some nits below.
I value good docs but I suck at typing them ;-)
> > @@ -927,6 +927,9 @@ EXPORT_SYMBOL(ida_pre_get);
> > * and go back to the ida_pre_get() call. If the ida is full, it will
> > * return %-ENOSPC.
> > *
> > + * Note that callers must ensure that concurrent access to @ida is not possible.
> > + * When simplicity trumps concurrency needs look at ida_simple_get() instead.
>
> Maybe we can make it a bit less dramatic?
What about?
"Note that callers must ensure that concurrent access to @ida is not possible.
See ida_simple_get() for a varaint which takes care of locking.
>
>
> > @@ -1073,6 +1076,10 @@ EXPORT_SYMBOL(ida_destroy);
> > * Allocates an id in the range start <= id < end, or returns -ENOSPC.
> > * On memory allocation failure, returns -ENOMEM.
> > *
> > + * Compared to ida_get_new_above() this function does its own locking and hence
> > + * is recommended everywhere where simplicity is preferred over the last bit of
> > + * speed.
>
> Hmm... so, this isn't necessarily about speed. For example, id
> allocation might have to happen inside a spinlock which protects a
> larger scope. To guarantee GFP_KERNEL allocation behavior in such
> cases, the caller would have to call ida_pre_get() outside the said
> spinlock and then call ida_get_new_above() inside the lock.
Hm, ida_simple_get does that for you already ...
> I think it'd be better to explain what the simple version does and
> expects and then say that unless there are specific requirements using
> the simple version is recommended.
What about:
"Compared to ida_get_new_above() this function does its own locking, and
should be used unless there are special requirements."
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
Hello, Daniel.
On Wed, Oct 26, 2016 at 09:25:25PM +0200, Daniel Vetter wrote:
> > > + * Note that callers must ensure that concurrent access to @ida is not possible.
> > > + * When simplicity trumps concurrency needs look at ida_simple_get() instead.
> >
> > Maybe we can make it a bit less dramatic?
>
> What about?
>
> "Note that callers must ensure that concurrent access to @ida is not possible.
> See ida_simple_get() for a varaint which takes care of locking.
Yeah, that reads easier to me.
> > Hmm... so, this isn't necessarily about speed. For example, id
> > allocation might have to happen inside a spinlock which protects a
> > larger scope. To guarantee GFP_KERNEL allocation behavior in such
> > cases, the caller would have to call ida_pre_get() outside the said
> > spinlock and then call ida_get_new_above() inside the lock.
>
> Hm, ida_simple_get does that for you already ...
Here's an example.
spin_lock();
do some stuff;
something->id = ida_simple_get(some gfp flag);
do some stuff;
spin_unlock();
In this scenario, you can't use sleeping GFPs for ida_simple_get()
because it does preloading inside it. What one has to do is...
ida_pre_get(GFP_KERNEL);
spin_lock();
do some stuff;
something->id = ida_get_new_above(GFP_NOWAIT);
do some stuff;
spin_unlock();
So, I guess it can be sometimes about avoiding the extra locking
overhead but it's more often about separating out allocation context
into an earlier call.
> > I think it'd be better to explain what the simple version does and
> > expects and then say that unless there are specific requirements using
> > the simple version is recommended.
>
> What about:
>
> "Compared to ida_get_new_above() this function does its own locking, and
> should be used unless there are special requirements."
Yeah, looks good to me.
Thanks.
--
tejun
On Wed, Oct 26, 2016 at 04:07:25PM -0400, Tejun Heo wrote:
> Hello, Daniel.
>
> On Wed, Oct 26, 2016 at 09:25:25PM +0200, Daniel Vetter wrote:
> > > > + * Note that callers must ensure that concurrent access to @ida is not possible.
> > > > + * When simplicity trumps concurrency needs look at ida_simple_get() instead.
> > >
> > > Maybe we can make it a bit less dramatic?
> >
> > What about?
> >
> > "Note that callers must ensure that concurrent access to @ida is not possible.
> > See ida_simple_get() for a varaint which takes care of locking.
>
> Yeah, that reads easier to me.
>
> > > Hmm... so, this isn't necessarily about speed. For example, id
> > > allocation might have to happen inside a spinlock which protects a
> > > larger scope. To guarantee GFP_KERNEL allocation behavior in such
> > > cases, the caller would have to call ida_pre_get() outside the said
> > > spinlock and then call ida_get_new_above() inside the lock.
> >
> > Hm, ida_simple_get does that for you already ...
>
> Here's an example.
>
> spin_lock();
> do some stuff;
> something->id = ida_simple_get(some gfp flag);
> do some stuff;
> spin_unlock();
>
> In this scenario, you can't use sleeping GFPs for ida_simple_get()
> because it does preloading inside it. What one has to do is...
>
> ida_pre_get(GFP_KERNEL);
> spin_lock();
> do some stuff;
> something->id = ida_get_new_above(GFP_NOWAIT);
> do some stuff;
> spin_unlock();
>
> So, I guess it can be sometimes about avoiding the extra locking
> overhead but it's more often about separating out allocation context
> into an earlier call.
Hm yeah, calling ida_simple_get in that case isn't recommend really.
> > > I think it'd be better to explain what the simple version does and
> > > expects and then say that unless there are specific requirements using
> > > the simple version is recommended.
> >
> > What about:
> >
> > "Compared to ida_get_new_above() this function does its own locking, and
> > should be used unless there are special requirements."
>
> Yeah, looks good to me.
Ok, will respin, thanks for the review comments.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch