svn commit: r357641 - head/sys/kern

[Jung-uk Kim]

On 2/6/20, Michael Tuexen wrote:
> 
> 
>> 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.

https://reviews.freebsd.org/D23464#516866
https://reviews.freebsd.org/D23464#516881

Jung-uk Kim

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

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