linux-kernel - Re: [PATCH] lightnvm: pblk: take write semaphore on metadata

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Date:   Fri, 3 Aug 2018 13:28:48 +0000
From:   Javier Gonzalez <javier@...xlabs.com>
To:     Matias Bjørling <mb@...htnvm.io>
CC:     "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lightnvm: pblk: take write semaphore on metadata


> On 3 Aug 2018, at 14.45, Matias Bjørling <mb@...htnvm.io> wrote:
> 
> On 08/03/2018 02:05 PM, Javier González wrote:
>> pblk guarantees write ordering at a chunk level through a per open chunk
>> semaphore. At this point, since we only have an open I/O stream for both
>> user and GC data, the semaphore is per parallel unit.
>> Since metadata I/O is synchronous, the semaphore is not needed as
>> ordering is guaranteed. However, if the metadata scheme changes or
>> multiple streams are open, this guarantee might not be preserved.
>> This patch makes sure that all writes go through the semaphore, even for
>> synchronous I/O. This is consistent with pblk's write I/O model. It also
>> simplifies maintenance since changes in the metdatada scheme could cause
>> ordering issues.
>> Signed-off-by: Javier González <javier@...xlabs.com>
>> ---
>>  drivers/lightnvm/pblk-core.c | 14 ++++++++++++--
>>  drivers/lightnvm/pblk.h      |  1 +
>>  2 files changed, 13 insertions(+), 2 deletions(-)
>> diff --git a/drivers/lightnvm/pblk-core.c b/drivers/lightnvm/pblk-core.c
>> index 00984b486fea..160b54d26bfa 100644
>> --- a/drivers/lightnvm/pblk-core.c
>> +++ b/drivers/lightnvm/pblk-core.c
>> @@ -493,6 +493,16 @@ int pblk_submit_io_sync(struct pblk *pblk, struct nvm_rq *rqd)
>>  	return nvm_submit_io_sync(dev, rqd);
>>  }
>>  +int pblk_submit_io_sync_sem(struct pblk *pblk, struct nvm_rq *rqd)
>> +{
>> +	if (rqd->opcode != NVM_OP_PWRITE)
>> +		pblk_submit_io_sync(pblk, rqd);
>> +
> 
> Why should the write be issued twice?
> 

It is the read that is sent twice, that's why it does not fail. Rebased
the patch manually and messed up... should be return... I'll send a V2.

Javier

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