netdev - RE: [PATCH net-next v3 2/5] bitops: add non-atomic bitops for pointers

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Date:   Thu, 24 Jun 2021 13:08:36 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Toke Høiland-Jørgensen' <toke@...hat.com>,
        "Kumar Kartikeya Dwivedi" <memxor@...il.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "John Fastabend" <john.fastabend@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: RE: [PATCH net-next v3 2/5] bitops: add non-atomic bitops for
 pointers

From: Toke Høiland-Jørgensen
> Sent: 23 June 2021 12:09
> Kumar Kartikeya Dwivedi <memxor@...il.com> writes:
> 
> > On Wed, Jun 23, 2021 at 04:03:06AM IST, Toke Høiland-Jørgensen wrote:
> >> Kumar Kartikeya Dwivedi <memxor@...il.com> writes:
> >>
> >> > On Wed, Jun 23, 2021 at 03:22:51AM IST, Toke Høiland-Jørgensen wrote:
> >> >> Kumar Kartikeya Dwivedi <memxor@...il.com> writes:
> >> >>
> >> >> > cpumap needs to set, clear, and test the lowest bit in skb pointer in
> >> >> > various places. To make these checks less noisy, add pointer friendly
> >> >> > bitop macros that also do some typechecking to sanitize the argument.
> >> >> >
> >> >> > These wrap the non-atomic bitops __set_bit, __clear_bit, and test_bit
> >> >> > but for pointer arguments. Pointer's address has to be passed in and it
> >> >> > is treated as an unsigned long *, since width and representation of
> >> >> > pointer and unsigned long match on targets Linux supports. They are
> >> >> > prefixed with double underscore to indicate lack of atomicity.
> >> >> >
> >> >> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> >> >> > ---
> >> >> >  include/linux/bitops.h    | 19 +++++++++++++++++++
> >> >> >  include/linux/typecheck.h | 10 ++++++++++
> >> >> >  2 files changed, 29 insertions(+)
> >> >> >
> >> >> > diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> >> >> > index 26bf15e6cd35..a9e336b9fa4d 100644
> >> >> > --- a/include/linux/bitops.h
> >> >> > +++ b/include/linux/bitops.h
> >> >> > @@ -4,6 +4,7 @@
> >> >> >
> >> >> >  #include <asm/types.h>
> >> >> >  #include <linux/bits.h>
> >> >> > +#include <linux/typecheck.h>
> >> >> >
> >> >> >  #include <uapi/linux/kernel.h>
> >> >> >
> >> >> > @@ -253,6 +254,24 @@ static __always_inline void __assign_bit(long nr, volatile unsigned long
> *addr,
> >> >> >  		__clear_bit(nr, addr);
> >> >> >  }
> >> >> >
> >> >> > +#define __ptr_set_bit(nr, addr)                         \
> >> >> > +	({                                              \
> >> >> > +		typecheck_pointer(*(addr));             \
> >> >> > +		__set_bit(nr, (unsigned long *)(addr)); \
> >> >> > +	})
> >> >> > +
> >> >> > +#define __ptr_clear_bit(nr, addr)                         \
> >> >> > +	({                                                \
> >> >> > +		typecheck_pointer(*(addr));               \
> >> >> > +		__clear_bit(nr, (unsigned long *)(addr)); \
> >> >> > +	})
> >> >> > +
> >> >> > +#define __ptr_test_bit(nr, addr)                       \
> >> >> > +	({                                             \
> >> >> > +		typecheck_pointer(*(addr));            \
> >> >> > +		test_bit(nr, (unsigned long *)(addr)); \
> >> >> > +	})
> >> >> > +
> >> >>
> >> >> Before these were functions that returned the modified values, now they
> >> >> are macros that modify in-place. Why the change? :)
> >> >>
> >> >
> >> > Given that we're exporting this to all kernel users now, it felt more
> >> > appropriate to follow the existing convention/argument order for the
> >> > functions/ops they are wrapping.
> >>
> >> I wasn't talking about the order of the arguments; swapping those is
> >> fine. But before, you had:
> >>
> >> static void *__ptr_set_bit(void *ptr, int bit)
> >>
> >> with usage (function return is the modified value):
> >> ret = ptr_ring_produce(rcpu->queue, __ptr_set_bit(skb, 0));
> >>
> >> now you have:
> >> #define __ptr_set_bit(nr, addr)
> >>
> >> with usage (modifies argument in-place):
> >> __ptr_set_bit(0, &skb);
> >> ret = ptr_ring_produce(rcpu->queue, skb);
> >>
> >> why change from function to macro?
> >>
> >
> > Earlier it just took the pointer value and returned one with the bit set. I
> > changed it to work similar to __set_bit.
> 
> Hmm, okay, fair enough I suppose there's something to be said for
> consistency, even though I personally prefer the function style. Let's
> keep it as macros, then :)

Passing the address of the pointer will trash a lot of optimisations.
You do really want to use the return address.
Or, even better, get the whole thing inlined.

So something like:
#define ptr_set_bit(ptr, val) ((typeof (ptr))((unsigned long)(ptr) | (1 << (val))))

	David

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