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Date: Tue, 10 Aug 2021 21:39:52 +0200
From: Mete Polat <metepolat2000@...il.com>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: Michel Lespinasse <michel@...pinasse.org>,
Davidlohr Bueso <dbueso@...e.de>,
Andrew Morton <akpm@...ux-foundation.org>,
Lukas Bulwahn <lukas.bulwahn@...il.com>,
Peter Zijlstra <peterz@...radead.org>,
Jesper Nilsson <jesper@....nu>, Arnd Bergmann <arnd@...db.de>,
David Woodhouse <dwmw2@...radead.org>,
Ingo Molnar <mingo@...nel.org>,
Randy Dunlap <rdunlap@...radead.org>,
kernel-janitors@...r.kernel.org,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-m68k <linux-m68k@...ts.linux-m68k.org>
Subject: Re: [PATCH v2] rbtree: remove unneeded explicit alignment in struct
rb_node
On Mon, Aug 09, 2021 at 02:25:35PM +0200, Geert Uytterhoeven wrote:
> Hi Mete,
>
> On Fri, Aug 6, 2021 at 5:22 PM Mete Polat <metepolat2000@...il.com> wrote:
> > Commit e977145aeaad ("[RBTREE] Add explicit alignment to sizeof(long) for
> > struct rb_node.") adds an explicit alignment to the struct rb_node due to
> > some speciality of the CRIS architecture.
> >
> > The support for the CRIS architecture was removed with commit c690eddc2f3b
> > ("CRIS: Drop support for the CRIS port")
> >
> > So, remove this now unneeded explicit alignment in struct rb_node as well.
> >
> > This basically reverts commit e977145aeaad ("[RBTREE] Add explicit
> > alignment to sizeof(long) for struct rb_node.").
> >
> > The rbtree node color is stored in the LSB of '__rb_parent_color'.
> > Only mask the first bit in '__rb_parent()', otherwise it modifies the
> > node's parent address on m68k.
> >
> > Reported-by: Randy Dunlap <rdunlap@...radead.org>
> > Reported-by: Mete Polat <metepolat2000@...il.com>
> > Signed-off-by: Lukas Bulwahn <lukas.bulwahn@...il.com>
> > Signed-off-by: Mete Polat <metepolat2000@...il.com>
> > ---
> > I have tested it on x86, but not on m68k. Can you ack that Geert?
>
> Unfortunately not. Just booting on ARAnyM gives:
>
> Unable to handle kernel access at virtual address 84abc227
> Oops: 00000000
> Modules linked in:
> PC: [<b45b0005>] 0xb45b0005
> SR: 2700 SP: fc24ede1 a2: 01322b40
> d0: b45aff7c d1: 00002708 d2: 00000000 d3: 0033a6a4
> d4: 00000005 d5: 95847357 a0: b45b0005 a1: 000f4240
> Process sh (pid: 402, task=672891a8)
> Frame format=7 eff addr=b45b0005 ssw=0546 faddr=b45b0005
> wb 1 stat/addr/data: 0000 00000000 00000000
> wb 2 stat/addr/data: 0000 00000000 00000000
> wb 3 stat/addr/data: 0000 b45b0005 00000000
> push data: 00000000 00000000 00000000 00000000
> Stack from 0131debc:
> 0004ddf4 0122e0a4 0122e0a4 00477c26 00000000 00000000 00000005 95847357
> 00002708 0004558e 0033ab30 00000002 01003180 0047273c 0131df88 0033a6a4
> 00477c3a 00477c24 0004dfee 00000005 95847357 00002708 0000000f 00000001
> 9915a000 0000000d 0004d430 01322b40 00000001 00002604 00000000 00006e50
> 00000001 00000000 0004560c 0000000d 00000000 00000000 efc3aa78 efc3aa78
> c0158c0c 8009ff68 0047273c efc3aa64 c01537a8 0131df8c 000456b6 0047273c
> Call Trace: [<0004ddf4>] __hrtimer_run_queues.constprop.0+0xba/0x144
> [<00002708>] calibrate_delay+0xd6/0x18a
> [<0004558e>] __irq_wake_thread+0x0/0x40
> [<0033ab30>] printk+0x0/0x18
> [<0033a6a4>] warn_slowpath_fmt+0x0/0x62
> [<0004dfee>] hrtimer_run_queues+0xb4/0xc0
> [<00002708>] calibrate_delay+0xd6/0x18a
> [<0004d430>] update_process_times+0x38/0x8c
> [<00002604>] wait_for_initramfs+0x30/0x58
> [<00006e50>] mfp_timer_c_handler+0x2c/0x3c
> [<0004560c>] __handle_irq_event_percpu+0x3e/0xcc
> [<000456b6>] handle_irq_event_percpu+0x1c/0x52
> [<0004570a>] handle_irq_event+0x1e/0x30
> [<00047aa2>] handle_simple_irq+0x4a/0x4e
> [<00045062>] handle_irq_desc+0x1e/0x28
> [<00002cac>] do_IRQ+0x20/0x32
> [<00002bcc>] user_irqvec_fixup+0xc/0x14
> [<0010c00b>] mb_cache_create+0x47/0xdc
>
> Code: dde0 dde0 dde0 dde0 dde0 dde0 dde0 dde0 Bad PC value.
> Disabling lock debugging due to kernel taint
> Kernel panic - not syncing: Aiee, killing interrupt handler!
> ---[ end Kernel panic - not syncing: Aiee, killing interrupt handler! ]---
>
> > --- a/include/linux/rbtree.h
> > +++ b/include/linux/rbtree.h
> > @@ -25,8 +25,7 @@ struct rb_node {
> > unsigned long __rb_parent_color;
> > struct rb_node *rb_right;
> > struct rb_node *rb_left;
> > -} __attribute__((aligned(sizeof(long))));
> > - /* The alignment might seem pointless, but allegedly CRIS needs it */
> > +};
> >
> > struct rb_root {
> > struct rb_node *rb_node;
> > diff --git a/include/linux/rbtree_augmented.h b/include/linux/rbtree_augmented.h
> > index d1c53e9d8c75..94b6a0f4499e 100644
> > --- a/include/linux/rbtree_augmented.h
> > +++ b/include/linux/rbtree_augmented.h
> > @@ -145,7 +145,7 @@ RB_DECLARE_CALLBACKS(RBSTATIC, RBNAME, \
> > #define RB_RED 0
> > #define RB_BLACK 1
> >
> > -#define __rb_parent(pc) ((struct rb_node *)(pc & ~3))
> > +#define __rb_parent(pc) ((struct rb_node *)(pc & ~1))
>
> Are you sure you can do this, i.e. that nothing else relies on the
> alignment to (at least) 4 bytes?
I am quite sure that the RBT algorithm itself only relies on the LSB,
but I am unsure yet if RCU requires this natural alignment as Peter
mentioned.
>
> There is another definition of rb_parent(). If I make a similar change there:
>
> --- a/include/linux/rbtree.h
> +++ b/include/linux/rbtree.h
> @@ -31,7 +31,7 @@ struct rb_root {
> struct rb_node *rb_node;
> };
>
> -#define rb_parent(r) ((struct rb_node *)((r)->__rb_parent_color & ~3))
> +#define rb_parent(r) ((struct rb_node *)((r)->__rb_parent_color & ~1))
>
> #define RB_ROOT (struct rb_root) { NULL, }
> #define rb_entry(ptr, type, member) container_of(ptr, type, member)
>
> the system boots on ARAnyM, but I prefer to be a bit cautious...
Ah yes I forgot that the same makro is also defined in the other header
file as well; and similary in some other locations:
./scripts/gdb/linux/rbtree.py:
parent = gdb.Value(node['__rb_parent_color'] & ~3)
./tools/include/linux/rbtree.h:
#define rb_parent(r) ((struct rb_node *)((r)->__rb_parent_color & ~3))
>
> Gr{oetje,eeting}s,
>
> Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
> -- Linus Torvalds
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