linux-kernel - Re: [PATCH 3/5] arm64: refactor delay() to enable polling for value

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Message-ID: <3156dcdbddc9b1f692bc45adba0893bcc9b58035.camel@amazon.com>
Date: Wed, 6 Nov 2024 17:38:55 +0000
From: "Okanovic, Haris" <harisokn@...zon.com>
To: "catalin.marinas@....com" <catalin.marinas@....com>
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Subject: Re: [PATCH 3/5] arm64: refactor delay() to enable polling for value

On Wed, 2024-11-06 at 09:18 +0000, Catalin Marinas wrote:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> 
> 
> 
> On Tue, Nov 05, 2024 at 12:30:39PM -0600, Haris Okanovic wrote:
> > +             do {
> > +                     cur = __READ_ONCE_EX(*addr);
> > +                     if ((cur & mask) == val) {
> > +                             break;
> > +                     }
> >                       wfet(end);
> 
> Constructs like this need to be entirely in assembly. The compiler may
> spill 'cur' onto the stack and the write could clear the exclusive
> monitor which makes the wfet return immediately. That's highly CPU
> implementation specific but it's the reason we have functions like
> __cmpwait() in assembly (or whatever else deals with exclusives). IOW,
> we can't have other memory accesses between the LDXR and whatever is
> consuming the exclusive monitor armed state (typically STXR but WFE/WFET
> can be another).

Agreed, will rewrite parts of delay() in asm.

> 
> --
> Catalin