Re: [PATCH] parisc: Optimize per-pagetable spinlocks (v11)

[Guenter Roeck <linux@roeck-us.net>]

On Thu, Feb 11, 2021 at 09:38:25AM -0500, John David Anglin wrote:
> On 2021-02-10 8:20 p.m., Guenter Roeck wrote:
> > On Wed, Feb 10, 2021 at 01:57:42PM -0500, John David Anglin wrote:
> >> On 2021-02-10 12:23 p.m., Helge Deller wrote:
> >>> On 2/10/21 3:52 PM, Guenter Roeck wrote:
> >>>> On Wed, Jan 27, 2021 at 10:18:51PM +0100, Helge Deller wrote:
> >>>>> On parisc a spinlock is stored in the next page behind the pgd which
> >>>>> protects against parallel accesses to the pgd. That's why one additional
> >>>>> page (PGD_ALLOC_ORDER) is allocated for the pgd.
> >>>>>
> >>>>> Matthew Wilcox suggested that we instead should use a pointer in the
> >>>>> struct page table for this spinlock and noted, that the comments for the
> >>>>> PGD_ORDER and PMD_ORDER defines were wrong.
> >>>>>
> >>>>> Both suggestions are addressed in this patch. The pgd spinlock
> >>>>> (parisc_pgd_lock) is stored in the struct page table. In
> >>>>> switch_mm_irqs_off() the physical address of this lock is loaded into
> >>>>> cr28 (tr4) and the pgd into cr25, so that the fault handlers can
> >>>>> directly access the lock.
> >>>>>
> >>>>> The currently implemened Hybrid L2/L3 page table scheme (where the pmd
> >>>>> is adjacent to the pgd) is dropped now too.
> >>>>>
> >>>>> Suggested-by: Matthew Wilcox <willy@infradead.org>
> >>>>> Fixes: b37d1c1898b2 ("parisc: Use per-pagetable spinlock")
> >>>>> Signed-off-by: Helge Deller <deller@gmx.de>
> >>>>> Signed-off-by: John David Anglin <dave.anglin@bell.net>
> >>>> This patch results in:
> >>>>
> >>>> BUG: spinlock recursion on CPU#0, swapper/0/1
> >>>>   lock: 0x12226d14, .magic: dead4ead, .owner: swapper/0/1, .owner_cpu: 0
> >>>> CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.11.0-rc7-next-20210209-32bit #1
> >>>> Hardware name: 9000/778/B160L
> >>>> Backtrace:
> >>>>   [<1019f9bc>] show_stack+0x34/0x48
> >>>>   [<10a65278>] dump_stack+0x94/0x114
> >>>>   [<10219f4c>] spin_dump+0x8c/0xb8
> >>>>   [<1021a0b4>] do_raw_spin_lock+0xdc/0x108
> >>>>   [<10a7367c>] _raw_spin_lock_irqsave+0x30/0x48
> >>>>   [<102bf41c>] handle_mm_fault+0x5e8/0xdb0
> >>>>   [<102b813c>] __get_user_pages.part.0+0x1b0/0x3d4
> >>>>   [<102b8900>] __get_user_pages_remote+0x134/0x34c
> >>>>   [<102b8b80>] get_user_pages_remote+0x68/0x90
> >>>>   [<102fccb0>] get_arg_page+0x94/0xd8
> >>>>   [<102fdd84>] copy_string_kernel+0xc4/0x234
> >>>>   [<102fe70c>] kernel_execve+0xcc/0x1a4
> >>>>   [<10a58d94>] run_init_process+0xbc/0xe0
> >>>>   [<10a70d50>] kernel_init+0x98/0x13c
> >>>>   [<1019a01c>] ret_from_kernel_thread+0x1c/0x24
> >>>>
> >>>> when trying to boot parisc/hppa images in qemu. Reverting this patch fixes
> >>>> the problem.
> >>> True, I can reproduce the problem.
> >>> With CONFIG_DEBUG_SPINLOCK=y you get the backtrace above.
> >>> With  CONFIG_DEBUG_SPINLOCK=n it just hangs.
> >>> Happenes with 32-bit kernel with SMP kernel, even if only one virtual CPU is started.
> >> Which is quite puzzling since most spin locks are optimized in this case case.  Strikes me we
> >> have a lock that's not initialized.
> >>
> >> It's not obvious how this relates to the patch.  get_arg_page() calls get_user_pages_remote() with
> > The fact that reverting it fixes the problem is a good indication
> > that the problem does relate to this patch.
> >
> > As for how - no idea. That is not my area of expertise.
> I built Helge's for-next tree both with CONFIG_DEBUG_SPINLOCK=y and CONFIG_DEBUG_SPINLOCK=n.  Both
> builds work fine on c8000.
> 
> The only thing that might have changed in the patch is the alignment of the lock used for page table updates.
> Qemu only support PA 1.x instructions.  The ldcw instruction needs 16-byte alignment on real hardware and
> there is code to dynamically align lock pointers to 16-byte alignment.  The c8000 supports PA 2.0 instructions
> and the ldcw,co instruction only needs 4-byte alignment.  Perhaps there is an issue with the dynamic alignment
> of the lock pointer or the lock initialization in the PA 1.x build for qemu.
> 

The first lock is acquired in mm/memory.c from line 3565:

        vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
                        &vmf->ptl);

The second (recursive) lock is acquired from line 3587 in the same
function:

        set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);

Source code lines are from next-20210211. I confirmed with debug code
that the lock address passed to do_raw_spin_lock() is the same in both
calls.

Guenter