linux-kernel - Re: arc_usr_cmpxchg and preemption

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Message-ID: <b0283507-5ab7-e40b-74a7-3ddebafb076d@synopsys.com>
Date:   Fri, 16 Mar 2018 10:54:52 -0700
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        "peterz@...radead.org" <peterz@...radead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
Subject: Re: arc_usr_cmpxchg and preemption

On 03/16/2018 10:33 AM, Alexey Brodkin wrote:
> Hi Peter, Vineet,
>
> On Wed, 2018-03-14 at 18:53 +0100, Peter Zijlstra wrote:
>> On Wed, Mar 14, 2018 at 09:58:19AM -0700, Vineet Gupta wrote:
>>
>>> Well it is broken wrt the semantics the syscall is supposed to provide.
>>> Preemption disabling is what prevents a concurrent thread from coming in and
>>> modifying the same location (Imagine a variable which is being cmpxchg
>>> concurrently by 2 threads).
>>>
>>> One approach is to do it the MIPS way, emulate the llsc flag - set it under
>>> preemption disabled section and clear it in switch_to
>> *shudder*... just catch the -EFAULT, force the write fault and retry.
> More I look at this initially quite simple thing more it looks like
> a can of worms...
>

I'd say just bite the bullet, write the patch and we can refine it there !

-Vineet