Re: [PATCH v2 00/10] uaccess: better might_sleep/might_fault behavior

From: Michael S. Tsirkin
Date: Wed May 22 2013 - 07:09:16 EST

On Wed, May 22, 2013 at 12:19:16PM +0200, Peter Zijlstra wrote:
> On Wed, May 22, 2013 at 11:25:36AM +0200, Arnd Bergmann wrote:
> > Calling might_fault() for every __get_user/__put_user is rather expensive
> > because it turns what should be a single instruction (plus fixup) into an
> > external function call.
> We could hide it all behind CONFIG_DEBUG_ATOMIC_SLEEP just like
> might_sleep() is. I'm not sure there's a point to might_fault() when
> might_sleep() is a NOP.

The patch that you posted gets pretty close.
E.g. I'm testing this now:
+#define might_fault() do { \
+ if (_might_fault()) \
+ __might_sleep(__FILE__, __LINE__, 0); \
+ might_resched(); \
+} while(0)

So if might_sleep is a NOP, __might_sleep and might_resched are NOPs
so compiler will optimize this all out.

However, in a related thread, you pointed out that might_sleep is not a NOP if

Do you think we should drop the preemption point in might_fault?
Only copy_XX_user?
Only __copy_XXX_user ?

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at