On Tue, Feb 12, 2019 at 08:45:35AM +0100, Jan Kara wrote: > On Mon 11-02-19 09:56:53, Matthew Wilcox wrote: > > On Mon, Feb 11, 2019 at 06:48:46PM +0100, Jan Kara wrote: > > > On Mon 11-02-19 13:59:24, Linux Upstream wrote: > > > > > > > > > >> Signed-off-by: Chintan Pandya <chintan.pan...@oneplus.com> > > > > > > > > > > NAK. > > > > > > > > > > This is bound to regress some stuff. Now agreed that using non-atomic > > > > > ops is tricky, but many are in places where we 'know' there can't be > > > > > concurrency. > > > > > > > > > > If you can show any single one is wrong, we can fix that one, but > > > > > we're > > > > > not going to blanket remove all this just because. > > > > > > > > Not quite familiar with below stack but from crash dump, found that this > > > > was another stack running on some other CPU at the same time which also > > > > updates page cache lru and manipulate locks. > > > > > > > > [84415.344577] [20190123_21:27:50.786264]@1 preempt_count_add+0xdc/0x184 > > > > [84415.344588] [20190123_21:27:50.786276]@1 > > > > workingset_refault+0xdc/0x268 > > > > [84415.344600] [20190123_21:27:50.786288]@1 > > > > add_to_page_cache_lru+0x84/0x11c > > > > [84415.344612] [20190123_21:27:50.786301]@1 > > > > ext4_mpage_readpages+0x178/0x714 > > > > [84415.344625] [20190123_21:27:50.786313]@1 ext4_readpages+0x50/0x60 > > > > [84415.344636] [20190123_21:27:50.786324]@1 > > > > __do_page_cache_readahead+0x16c/0x280 > > > > [84415.344646] [20190123_21:27:50.786334]@1 filemap_fault+0x41c/0x588 > > > > [84415.344655] [20190123_21:27:50.786343]@1 ext4_filemap_fault+0x34/0x50 > > > > [84415.344664] [20190123_21:27:50.786353]@1 __do_fault+0x28/0x88 > > > > > > > > Not entirely sure if it's racing with the crashing stack or it's simply > > > > overrides the the bit set by case 2 (mentioned in 0/2). > > > > > > So this is interesting. Looking at __add_to_page_cache_locked() nothing > > > seems to prevent __SetPageLocked(page) in add_to_page_cache_lru() to get > > > reordered into __add_to_page_cache_locked() after page is actually added > > > to > > > the xarray. So that one particular instance might benefit from atomic > > > SetPageLocked or a barrier somewhere between __SetPageLocked() and the > > > actual addition of entry into the xarray. > > > > There's a write barrier when you add something to the XArray, by virtue > > of the call to rcu_assign_pointer(). > > OK, I've missed rcu_assign_pointer(). Thanks for correction... but... > rcu_assign_pointer() is __smp_store_release(&p, v) and that on x86 seems to > be: > > barrier(); \ > WRITE_ONCE(*p, v); \ > > which seems to provide a compiler barrier but not an SMP barrier? So is x86 > store ordering strong enough to make writes appear in the right order? So far > I didn't think so... What am I missing?
X86 is TSO, and that is strong enough for this. The only visible reordering on TSO is due to the store-buffer, that is: writes may happen after later reads.
