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SubjectRe: [PATCH] mm/swap: piggyback lru_add_drain_all() calls
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On 04/10/2019 15.10, Matthew Wilcox wrote:
> On Fri, Oct 04, 2019 at 01:11:06PM +0300, Konstantin Khlebnikov wrote:
>> This is very slow operation. There is no reason to do it again if somebody
>> else already drained all per-cpu vectors after we waited for lock.
>> + seq = raw_read_seqcount_latch(&seqcount);
>> +
>> mutex_lock(&lock);
>> +
>> + /* Piggyback on drain done by somebody else. */
>> + if (__read_seqcount_retry(&seqcount, seq))
>> + goto done;
>> +
>> + raw_write_seqcount_latch(&seqcount);
>> +
>
> Do we really need the seqcount to do this? Wouldn't a mutex_trylock()
> have the same effect?
>

No, this is completely different semantics.

Operation could be safely skipped only if somebody else started and
finished drain after current task called this function.

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