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From: Thomas Huth <thuth@redhat.com>
To: Richard Henderson <richard.henderson@linaro.org>,
	Stefan Hajnoczi <stefanha@redhat.com>
Cc: fweimer@redhat.com, Kevin Wolf <kwolf@redhat.com>,
	"open list:Block layer core" <qemu-block@nongnu.org>,
	QEMU Developers <qemu-devel@nongnu.org>,
	Fam Zheng <fam@euphon.net>, Warner Losh <imp@bsdimp.com>,
	sguelton@redhat.com
Subject: Re: [RFC 0/2] tls: add macros for coroutine-safe TLS variables
Date: Tue, 26 Oct 2021 19:26:16 +0200	[thread overview]
Message-ID: <d1bbc5ee-53ba-dce2-9eb8-9320f03e9734@redhat.com> (raw)
In-Reply-To: <4b8a5b71-0ec2-7c2c-6929-de303ec29a53@linaro.org>

On 26/10/2021 19.10, Richard Henderson wrote:
> On 10/26/21 9:34 AM, Stefan Hajnoczi wrote:
>> On Tue, Oct 26, 2021 at 08:10:16AM -0700, Richard Henderson wrote:
>>> On 10/26/21 6:22 AM, Stefan Hajnoczi wrote:
>>>> If "safe" TLS variables are opt-in then we'll likely have obscure bugs
>>>> when code changes to access a TLS variable that was previously never
>>>> accessed from a coroutine. There is no compiler error and no way to
>>>> detect this. When it happens debugging it is painful.
>>>
>>> Co-routines are never used in user-only builds.
>>
>> If developers have the choice of using __thread then bugs can slip
>> through.
> 
> Huh?  How.  No, really.
> 
>> Are you concerned about performance, the awkwardness of calling
>> getters/setters, or something else for qemu-user?
> 
> Awkwardness first, performance second.
> 
> I'll also note that coroutines never run on vcpu threads, only io threads.  
> So I'll resist any use of these interfaces in TCG as well.

Would it maybe make sense to tweak check_patch.pl to forbid __thread in 
certain folders only, e.g. block/ and util/ (i.e. where we know that there 
might be code that the iothreads are using)?

  Thomas



  reply	other threads:[~2021-10-26 17:27 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-25 14:07 [RFC 0/2] tls: add macros for coroutine-safe TLS variables Stefan Hajnoczi
2021-10-25 14:07 ` [RFC 1/2] " Stefan Hajnoczi
2021-10-25 14:14   ` Daniel P. Berrangé
2021-10-26 13:36     ` Stefan Hajnoczi
2021-10-26 13:41     ` Stefan Hajnoczi
2021-10-26 14:10       ` Kevin Wolf
2021-10-26 16:26         ` Stefan Hajnoczi
2021-10-25 17:19   ` Richard Henderson
2021-10-26 13:30     ` Stefan Hajnoczi
2021-10-26 15:32       ` Richard Henderson
2021-10-26 16:27         ` Stefan Hajnoczi
2021-10-25 14:07 ` [RFC 2/2] util/async: replace __thread with QEMU TLS macros Stefan Hajnoczi
2021-10-25 14:20 ` [RFC 0/2] tls: add macros for coroutine-safe TLS variables Philippe Mathieu-Daudé
2021-10-25 16:16 ` Richard Henderson
2021-10-25 23:27   ` Warner Losh
2021-10-26 13:22     ` Stefan Hajnoczi
2021-10-26 15:10       ` Richard Henderson
2021-10-26 16:34         ` Stefan Hajnoczi
2021-10-26 17:10           ` Richard Henderson
2021-10-26 17:26             ` Thomas Huth [this message]
2021-10-26 18:03               ` Richard Henderson
2021-10-27 10:38             ` Kevin Wolf
2021-10-27 12:34             ` Stefan Hajnoczi

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