svn commit: r357641 - head/sys/kern

[Michael Tuexen]

> On 6. Feb 2020, at 21:51, Jeff Roberson <jeff at freebsd.org> wrote:
> 
> Author: jeff
> Date: Thu Feb  6 20:51:46 2020
> New Revision: 357641
> URL: https://svnweb.freebsd.org/changeset/base/357641
> 
> Log:
>  Fix a race in smr_advance() that could result in unnecessary poll calls.
> 
>  This was relatively harmless but surprising to see in counters.  The
>  race occurred when rd_seq was read after the goal was updated and we
>  incorrectly calculated the delta between them.
> 
>  Reviewed by:	rlibby
>  Differential Revision:	https://reviews.freebsd.org/D23464
> 
> Modified:
>  head/sys/kern/subr_smr.c
> 
> Modified: head/sys/kern/subr_smr.c
> ==============================================================================
> --- head/sys/kern/subr_smr.c	Thu Feb  6 20:47:50 2020	(r357640)
> +++ head/sys/kern/subr_smr.c	Thu Feb  6 20:51:46 2020	(r357641)
> @@ -160,7 +160,7 @@ static uma_zone_t smr_zone;
> #define	SMR_SEQ_INCR	(UINT_MAX / 10000)
> #define	SMR_SEQ_INIT	(UINT_MAX - 100000)
> /* Force extra polls to test the integer overflow detection. */
> -#define	SMR_SEQ_MAX_DELTA	(1000)
> +#define	SMR_SEQ_MAX_DELTA	(SMR_SEQ_INCR * 32)
> #define	SMR_SEQ_MAX_ADVANCE	SMR_SEQ_MAX_DELTA / 2
> #endif
> 
> @@ -188,7 +188,7 @@ smr_seq_t
> smr_advance(smr_t smr)
> {
> 	smr_shared_t s;
> -	smr_seq_t goal;
> +	smr_seq_t goal, s_rd_seq;
> 
> 	/*
> 	 * It is illegal to enter while in an smr section.
> @@ -203,12 +203,18 @@ smr_advance(smr_t smr)
> 	atomic_thread_fence_rel();
> 
> 	/*
> +	 * Load the current read seq before incrementing the goal so
> +	 * we are guaranteed it is always < goal.
> +	 */
> +	s = zpcpu_get(smr)->c_shared;
> +	s_rd_seq = atomic_load_acq_int(&s->s_rd_seq);
> +
> +	/*
> 	 * Increment the shared write sequence by 2.  Since it is
> 	 * initialized to 1 this means the only valid values are
> 	 * odd and an observed value of 0 in a particular CPU means
> 	 * it is not currently in a read section.
> 	 */
> -	s = zpcpu_get(smr)->c_shared;
> 	goal = atomic_fetchadd_int(&s->s_wr_seq, SMR_SEQ_INCR) + SMR_SEQ_INCR;
> 	counter_u64_add(advance, 1);
> 
> @@ -217,7 +223,7 @@ smr_advance(smr_t smr)
> 	 * far ahead of the read sequence number.  This keeps the
> 	 * wrap detecting arithmetic working in pathological cases.
> 	 */
> -	if (goal - atomic_load_int(&s->s_rd_seq) >= SMR_SEQ_MAX_DELTA) {
> +	if (SMR_SEQ_DELTA(goal, s_rd_seq) >= SMR_SEQ_MAX_DELTA) {
SMR_SEQ_DELTA is not defined, therefore compilation fails.

Best regards
Michael
> 		counter_u64_add(advance_wait, 1);
> 		smr_wait(smr, goal - SMR_SEQ_MAX_ADVANCE);
> 	}