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Message-ID: <0c7d1d79-2372-4bae-ba9b-e7b6070af14c@linux.dev>
Date: Mon, 11 Mar 2024 06:11:58 +0100
From: Zhu Yanjun <yanjun.zhu@...ux.dev>
To: linke li <lilinke99@...com>, leon@...nel.org
Cc: bmt@...ich.ibm.com, jgg@...pe.ca, linux-kernel@...r.kernel.org,
linux-rdma@...r.kernel.org
Subject: Re: [PATCH] RDMA/siw: Reuse value read using READ_ONCE instead of
re-reading it
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/
>
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