linux-kernel - Re: [PATCH] RDMA/siw: Reuse value read using READ

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

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/
>