On Sun, May 19, 2013 at 12:35:26PM +0300, Michael S. Tsirkin wrote: > On Thu, May 16, 2013 at 08:40:41PM +0200, Peter Zijlstra wrote: > > On Thu, May 16, 2013 at 02:16:10PM +0300, Michael S. Tsirkin wrote: > > > There are several ways to make sure might_fault > > > calling function does not sleep. > > > One is to use it on kernel or otherwise locked memory - apparently > > > nfs/sunrpc does this. As noted by Ingo, this is handled by the > > > migh_fault() implementation in mm/memory.c but not the one in > > > linux/kernel.h so in the current code might_fault() schedules > > > differently depending on CONFIG_PROVE_LOCKING, which is an undesired > > > semantical side effect. > > > > > > Another is to call pagefault_disable: in this case the page fault > > > handler will go to fixups processing and we get an error instead of > > > sleeping, so the might_sleep annotation is a false positive. > > > vhost driver wants to do this now in order to reuse socket ops > > > under a spinlock (and fall back on slower thread handler > > > on error). > > > > Are you using the assumption that spin_lock() implies preempt_disable() implies > > pagefault_disable()? Note that this assumption isn't valid for -rt where the > > spinlock becomes preemptible but we'll not disable pagefaults. > > No, I was not assuming that. What I'm trying to say is that a caller > that does something like this under a spinlock: > preempt_disable > pagefault_disable > error = copy_to_user > pagefault_enable > preempt_enable_no_resched > > is not doing anything wrong and should not get a warning, > as long as error is handled correctly later. > Right? Aside from the no_resched() thing which Steven already explained and my previous email asking why you need the preempt_disable() at all, that should indeed work. The reason I was asking was that I wasn't sure you weren't doing: spin_lock(&my_lock); error = copy_to_user(); spin_unlock(&my_lock); and expecting the copy_to_user() to always take the exception table route. This works on mainline (since spin_lock implies a preempt disable and preempt_disable is the same as pagefault_disable). However as should be clear by now, it doesn't quite work that way for -rt. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
