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Re: [Qemu-devel] [PATCH] cpus: make qemu_mutex_iothread_locked() underst

From: Alex Bennée
Subject: Re: [Qemu-devel] [PATCH] cpus: make qemu_mutex_iothread_locked() understand co-routines
Date: Wed, 02 Nov 2016 14:55:37 +0000
User-agent: mu4e 0.9.17; emacs

Stefan Hajnoczi <address@hidden> writes:

> On Tue, Nov 01, 2016 at 04:21:36PM +0000, Alex Bennée wrote:
>> Paolo Bonzini <address@hidden> writes:
>> > On 21/10/2016 13:54, Alex Bennée wrote:
>> >> There is a slight wart when checking for the state of the BQL when using
>> >> GThread base co-routines (which we keep for ThreadSanitizer runs). While
>> >> the main-loop holds the BQL it is suspended until the co-routine
>> >> completes however the co-routines run in a separate thread so checking
>> >> the TLS variable could be wrong.
>> >>
>> >> We fix this by expanding the check to include qemu_in_coroutine() for
>> >> GThread based builds. As it is not used for production builds I'm not
>> >> overly worried about any performance impact which should be negligible
>> >> anyway.
>> >>
>> >> Signed-off-by: Alex Bennée <address@hidden>
>> >> Cc: Stefan Hajnoczi <address@hidden>
>> >
>> > This is wrong unfortunately.  It is possible to run coroutines outside
>> > the BQL (e.g. with -device virtio-blk,iothread=foo).
>> >
>> > Do you know exactly why TSAN has no love for coroutines?
>> The current production stuff is due to missing support for new stacks
>> with setcontext. However I have built the latest tsan support library
>> and that seems happy without the gthread co-routines.
>> Currently I'm dealing with glib's racy gthread support however.
> I think Paolo suggested we drop the GThread backend on IRC.  I agree
> that we should do that since GThread co-routines break code that uses
> thread-local variables and have never truly worked.

Indeed I have pulled your patch into my current series of sanitizer
fixes. Once I can fix the setcontext/sigjmp confusion for a
__SANITIZER__ builds I'll post the series.

Alex Bennée

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