On Fri, Oct 30, 2020 at 08:51:21PM -0300, Jason Gunthorpe wrote: > On Fri, Oct 30, 2020 at 06:52:50PM -0400, Peter Xu wrote: ... > > > > diff --git a/mm/memory.c b/mm/memory.c > > > index c48f8df6e50268..294c2c3c4fe00d 100644 > > > +++ b/mm/memory.c > > > @@ -1171,6 +1171,12 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma) > > > mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_PAGE, > > > 0, src_vma, src_mm, addr, end); > > > mmu_notifier_invalidate_range_start(&range); > > > + /* > > > + * The read side doesn't spin, it goes to the mmap_lock, so the > > > + * raw version is used to avoid disabling preemption here > > > + */ > > > + mmap_assert_write_locked(src_mm); > > > + raw_write_seqcount_t_begin(&src_mm->write_protect_seq); > > > > Would raw_write_seqcount_begin() be better here? > > Hum.. > > I felt no because it had the preempt stuff added into it, however it > would work - __seqcount_lock_preemptible() == false for the seqcount_t > case (see below) > > Looking more closely, maybe the right API to pick is > write_seqcount_t_begin() and write_seqcount_t_end() ?? > No, that's not the right API: it is also internal to seqlock.h. Please stick with the official exported API: raw_write_seqcount_begin(). It should satisfy your needs, and the raw_*() variant is created exactly for contexts wishing to avoid the lockdep checks (e.g. NMI handlers cannot invoke lockdep, etc.) > However, no idea what the intention of the '*_seqcount_t_*' family is > - it only seems to be used to implement the seqlock.. > Exactly. '*_seqcount_t_*' is a seqlock.h implementation detail, and it has _zero_ relevance to what is discussed in this thread actually. ... > Ahmed explained in commit 8117ab508f the reason the seqcount_t write > side has preemption disabled is because it can livelock RT kernels if > the read side is spinning after preempting the write side. eg look at > how __read_seqcount_begin() is implemented: > > while ((seq = __seqcount_sequence(s)) & 1) \ > cpu_relax(); \ > > However, in this patch, we don't spin on the read side. > > If the read side collides with a writer it immediately goes to the > mmap_lock, which is sleeping, and so it will sort itself out properly, > even if it was preempted. > Correct. Thanks, -- Ahmed Darwish Linutronix GmbH
