On Wed 20-06-18 20:35:15, Matthew Wilcox wrote: > On Thu, Jun 21, 2018 at 11:02:58AM +0800, Jia-Ju Bai wrote: > > The kernel may sleep with holding a spinlock. > > The function call path (from bottom to top) in Linux-4.16.7 is: > > > > [FUNC] schedule > > lib/percpu-refcount.c, 222: > > schedule in __percpu_ref_switch_mode > > lib/percpu-refcount.c, 339: > > __percpu_ref_switch_mode in percpu_ref_kill_and_confirm > > ./include/linux/percpu-refcount.h, 127: > > percpu_ref_kill_and_confirm in percpu_ref_kill > > mm/backing-dev.c, 545: > > percpu_ref_kill in cgwb_kill > > mm/backing-dev.c, 576: > > cgwb_kill in cgwb_create > > mm/backing-dev.c, 573: > > _raw_spin_lock_irqsave in cgwb_create > > > > This bug is found by my static analysis tool (DSAC-2) and checked by my > > code review. > > I disagree with your code review. > > * If the previous ATOMIC switching hasn't finished yet, wait for > * its completion. If the caller ensures that ATOMIC switching > * isn't in progress, this function can be called from any context. > > I believe cgwb_kill is always called under the spinlock, so we will never > sleep because the percpu ref will never be switching to atomic mode. You are right that the sleep under spinlock never happens. And the reason is that percpu_ref_kill() never results in blocking - it does call percpu_ref_kill_and_confirm() but the 'confirm' argument is NULL and thus even percpu_ref_kill_and_confirm() never blocks. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
