Re: bit fields && data tearing

Mikael Pettersson
2014-09-04T08:57:40Z

Benjamin Herrenschmidt writes:
> On Wed, 2014-09-03 at 18:51 -0400, Peter Hurley wrote:
>
> > Apologies for hijacking this thread but I need to extend this discussion
> > somewhat regarding what a compiler might do with adjacent fields in a structure.
> >
> > The tty subsystem defines a large aggregate structure, struct tty_struct.
> > Importantly, several different locks apply to different fields within that
> > structure; ie., a specific spinlock will be claimed before updating or accessing
> > certain fields while a different spinlock will be claimed before updating or
> > accessing certain _adjacent_ fields.
> >
> > What is necessary and sufficient to prevent accidental false-sharing?
> > The patch below was flagged as insufficient on ia64, and possibly ARM.
>
> We expect native aligned scalar types to be accessed atomically (the
> read/modify/write of a larger quantity that gcc does on some bitfield
> cases has been flagged as a gcc bug, but shouldn't happen on normal
> scalar types).
>
> I am not 100% certain of "bool" here, I assume it's treated as a normal
> scalar and thus atomic but if unsure, you can always use int.

Please use an aligned int or long. Some machines cannot do atomic
accesses on sub-int/long quantities, so 'bool' may cause unexpected
rmw cycles on adjacent fields.

/Mikael

>
> Another option is to use the atomic bitops and make these bits in a
> bitmask but that is probably unnecessary if you have locks already.
>
> Cheers,
> Ben.
>
>
> > Regards,
> > Peter Hurley
> >
> > --- >% ---
> > Subject: [PATCH 21/26] tty: Convert tty_struct bitfield to bools
> >
> > The stopped, hw_stopped, flow_stopped and packet bits are smp-unsafe
> > and interrupt-unsafe. For example,
> >
> > CPU 0 | CPU 1
> > |
> > tty->flow_stopped = 1 | tty->hw_stopped = 0
> >
> > One of these updates will be corrupted, as the bitwise operation
> > on the bitfield is non-atomic.
> >
> > Ensure each flag has a separate memory location, so concurrent
> > updates do not corrupt orthogonal states.
> >
> > Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
> > ---
> > include/linux/tty.h | 5 ++++-
> > 1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/tty.h b/include/linux/tty.h
> > index 1c3316a..7cf61cb 100644
> > --- a/include/linux/tty.h
> > +++ b/include/linux/tty.h
> > @@ -261,7 +261,10 @@ struct tty_struct {
> > unsigned long flags;
> > int count;
> > struct winsize winsize; /* winsize_mutex */
> > - unsigned char stopped:1, hw_stopped:1, flow_stopped:1, packet:1;
> > + bool stopped;
> > + bool hw_stopped;
> > + bool flow_stopped;
> > + bool packet;
> > unsigned char ctrl_status; /* ctrl_lock */
> > unsigned int receive_room; /* Bytes free for queue */
> > int flow_change;
>
>
> --
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