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Message-ID: <0c7d1d79-2372-4bae-ba9b-e7b6070af14c@linux.dev>
Date: Mon, 11 Mar 2024 06:11:58 +0100
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MIME-Version: 1.0
Subject: Re: [PATCH] RDMA/siw: Reuse value read using READ_ONCE instead of
 re-reading it
To: linke li <lilinke99@qq.com>, leon@kernel.org
Cc: bmt@zurich.ibm.com, jgg@ziepe.ca, linux-kernel@vger.kernel.org,
 linux-rdma@vger.kernel.org
References: <20240310191910.GG12921@unreal>
 <tencent_03614198A34E56D038455012AA31022D9C06@qq.com>
From: Zhu Yanjun <yanjun.zhu@linux.dev>
In-Reply-To: <tencent_03614198A34E56D038455012AA31022D9C06@qq.com>
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在 2024/3/11 3:34, linke li 写道:
>> If value can change between subsequent reads, then you need to use locks
>> to make sure that it doesn't happen. Using READ_ONCE() doesn't solve the
>> concurrency issue, but makes sure that compiler doesn't reorder reads
>> and writes.
> 
> This code do not need to prevent other thread from writing on the flags.
> 
> This topic got quite a bit of discussion [1], quote from it:
> 
>      (READ_ONCE and WRITE_ONCE)
>      That's often useful - lots of code doesn't really care if you get the
>      old or the new value, but the code *does* care that it gets *one*
>      value, and not some random mix of "I tested one value for validity,
>      then it got reloaded due to register pressure, and I actually used
>      another value".
> 
>      And not some "I read one value, and it was a mix of two other values".
>   
>  From the original code, the first read seems to do the same things. So
> READ_ONCE is probably ok here.
> 
> I just want to make sure the flags stored to wqe->sqe.flags is consistent
> with the read used in the if condition.

Sure. Follow Leon's advice, to make this ("wqe->sqe.flags is consistent 
with the read used in the if condition") happen, you need a lock to 
ensure it. The lock can be spin lock or mutex lock depens on its 
sleeping or not.

 From the original source code, wqe->sqe.flags should be a volatile 
variable. It should be read from the original source, not from cache.

Zhu Yanjun

> 
> [1]https://lore.kernel.org/lkml/CAHk-=wgG6Dmt1JTXDbrbXh_6s2yLjL=9pHo7uv0==LHFD+aBtg@mail.gmail.com/
>