Re: [PATCH v4 sched_ext/for-7.3 04/40] sched_ext: Use READ_ONCE/WRITE_ONCE in cmask word ops and drop _RACY variants

[Andrea Righi]

On Wed, Jul 08, 2026 at 11:23:53AM -1000, Tejun Heo wrote:
> The cmask ops can operate on BPF-arena cmasks which BPF programs may read
> and write concurrently. The _RACY op variants existed to make such lockless
> reads explicit but this turned out to be too restrictive. Mark the word
> accesses in all the two-cmask ops with READ_ONCE/WRITE_ONCE instead and drop
> the _RACY variants.
> 
> v2: READ_ONCE the cmask_word_op1() ANY_SET read. (sashiko AI)
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>

Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>

Thanks,
-Andrea

> ---
>  kernel/sched/ext/cid.c | 55 ++++++++----------------------------------
>  kernel/sched/ext/cid.h |  2 --
>  2 files changed, 10 insertions(+), 47 deletions(-)
> 
> diff --git a/kernel/sched/ext/cid.c b/kernel/sched/ext/cid.c
> index af83084ec740..f31113b080e9 100644
> --- a/kernel/sched/ext/cid.c
> +++ b/kernel/sched/ext/cid.c
> @@ -395,17 +395,15 @@ __bpf_kfunc s32 scx_bpf_cpu_to_cid(s32 cpu, const struct bpf_prog_aux *aux)
>   * bits outside stay untouched. In particular, scx_cmask_copy() does NOT zero
>   * @dst bits that lie outside @src's range.
>   *
> - * The _RACY variants are otherwise identical to their non-racy counterpart but
> - * read @src word-by-word via data_race(). Memory ordering with concurrent
> - * writers is the caller's responsibility.
> + * Word accesses use READ_ONCE/WRITE_ONCE so a caller may read @src
> + * locklessly. Memory ordering against concurrent writers is the caller's
> + * responsibility.
>   */
>  enum cmask_op2 {
>  	/* mutating */
>  	CMASK_OP2_AND,
>  	CMASK_OP2_OR,
> -	CMASK_OP2_OR_RACY,
>  	CMASK_OP2_COPY,
> -	CMASK_OP2_COPY_RACY,
>  	CMASK_OP2_ANDNOT,
>  	/* predicates - short-circuit when the per-word result is true */
>  	CMASK_OP2_SUBSET,
> @@ -422,28 +420,22 @@ static __always_inline bool cmask_word_op2(u64 *av, const u64 *bp, u64 mask,
>  {
>  	switch (op) {
>  	case CMASK_OP2_AND:
> -		*av &= ~mask | *bp;
> +		WRITE_ONCE(*av, *av & (~mask | READ_ONCE(*bp)));
>  		return false;
>  	case CMASK_OP2_OR:
> -		*av |= *bp & mask;
> -		return false;
> -	case CMASK_OP2_OR_RACY:
> -		*av |= data_race(*bp) & mask;
> +		WRITE_ONCE(*av, *av | (READ_ONCE(*bp) & mask));
>  		return false;
>  	case CMASK_OP2_COPY:
> -		*av = (*av & ~mask) | (*bp & mask);
> -		return false;
> -	case CMASK_OP2_COPY_RACY:
> -		*av = (*av & ~mask) | (data_race(*bp) & mask);
> +		WRITE_ONCE(*av, (*av & ~mask) | (READ_ONCE(*bp) & mask));
>  		return false;
>  	case CMASK_OP2_ANDNOT:
> -		*av &= ~(*bp & mask);
> +		WRITE_ONCE(*av, *av & ~(READ_ONCE(*bp) & mask));
>  		return false;
>  	case CMASK_OP2_SUBSET:
>  		/* stop on the first bit in @sub not set in @super */
> -		return (*bp & ~*av) & mask;
> +		return (READ_ONCE(*bp) & ~READ_ONCE(*av)) & mask;
>  	case CMASK_OP2_INTERSECTS:
> -		return (*av & *bp) & mask;
> +		return (READ_ONCE(*av) & READ_ONCE(*bp)) & mask;
>  	}
>  	unreachable();
>  }
> @@ -504,7 +496,7 @@ static __always_inline bool cmask_word_op1(const u64 *ap, u64 mask,
>  {
>  	switch (op) {
>  	case CMASK_OP1_ANY_SET:
> -		return *ap & mask;
> +		return READ_ONCE(*ap) & mask;
>  	}
>  	unreachable();
>  }
> @@ -556,39 +548,12 @@ void scx_cmask_or(struct scx_cmask *dst, const struct scx_cmask *src)
>  		       src->bits, src->base, src->nr_cids, CMASK_OP2_OR);
>  }
>  
> -/**
> - * scx_cmask_or_racy - OR @src into @dst, reading @src without locking
> - *
> - * @src is read word-by-word through data_race(). Same per-bit independence
> - * rationale as scx_cmask_copy_racy(). Memory ordering with writers is the
> - * caller's responsibility.
> - */
> -void scx_cmask_or_racy(struct scx_cmask *dst, const struct scx_cmask *src)
> -{
> -	cmask_walk_op2(dst->bits, dst->base, dst->nr_cids,
> -		       src->bits, src->base, src->nr_cids, CMASK_OP2_OR_RACY);
> -}
> -
>  void scx_cmask_copy(struct scx_cmask *dst, const struct scx_cmask *src)
>  {
>  	cmask_walk_op2(dst->bits, dst->base, dst->nr_cids,
>  		       src->bits, src->base, src->nr_cids, CMASK_OP2_COPY);
>  }
>  
> -/**
> - * scx_cmask_copy_racy - Snapshot @src into @dst without locking
> - *
> - * @src is read word-by-word through data_race(). Head/tail masking matches
> - * scx_cmask_copy(). Each bit in a cmask is independent, so partial updates
> - * just leave some bits fresher than others. Memory ordering with writers is
> - * the caller's responsibility.
> - */
> -void scx_cmask_copy_racy(struct scx_cmask *dst, const struct scx_cmask *src)
> -{
> -	cmask_walk_op2(dst->bits, dst->base, dst->nr_cids,
> -		       src->bits, src->base, src->nr_cids, CMASK_OP2_COPY_RACY);
> -}
> -
>  void scx_cmask_andnot(struct scx_cmask *dst, const struct scx_cmask *src)
>  {
>  	cmask_walk_op2(dst->bits, dst->base, dst->nr_cids,
> diff --git a/kernel/sched/ext/cid.h b/kernel/sched/ext/cid.h
> index 9c4f4b907f12..54b10df32fd5 100644
> --- a/kernel/sched/ext/cid.h
> +++ b/kernel/sched/ext/cid.h
> @@ -57,9 +57,7 @@ void scx_cmask_clear(struct scx_cmask *m);
>  void scx_cmask_fill(struct scx_cmask *m);
>  void scx_cmask_and(struct scx_cmask *dst, const struct scx_cmask *src);
>  void scx_cmask_or(struct scx_cmask *dst, const struct scx_cmask *src);
> -void scx_cmask_or_racy(struct scx_cmask *dst, const struct scx_cmask *src);
>  void scx_cmask_copy(struct scx_cmask *dst, const struct scx_cmask *src);
> -void scx_cmask_copy_racy(struct scx_cmask *dst, const struct scx_cmask *src);
>  void scx_cmask_andnot(struct scx_cmask *dst, const struct scx_cmask *src);
>  bool scx_cmask_subset(const struct scx_cmask *sub, const struct scx_cmask *super);
>  bool scx_cmask_intersects(const struct scx_cmask *a, const struct scx_cmask *b);
> -- 
> 2.54.0
>