| Message ID | 20161021115442.18420-1-alex.bennee@linaro.org |
|---|---|
| State | New |
| Headers | show |
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 <alex.bennee@linaro.org> > Cc: Stefan Hajnoczi <stefanha@redhat.com> 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? Paolo > --- > configure | 3 +++ > cpus.c | 13 +++++++++++++ > 2 files changed, 16 insertions(+) > > diff --git a/configure b/configure > index 91a14c1..97b89fb 100755 > --- a/configure > +++ b/configure > @@ -5461,6 +5461,9 @@ if test "$rbd" = "yes" ; then > fi > > echo "CONFIG_COROUTINE_BACKEND=$coroutine" >> $config_host_mak > +if test "$coroutine" = "gthread" ; then > + echo "CONFIG_COROUTINE_GTHREAD=1" >> $config_host_mak > +fi > if test "$coroutine_pool" = "yes" ; then > echo "CONFIG_COROUTINE_POOL=1" >> $config_host_mak > else > diff --git a/cpus.c b/cpus.c > index 0c18a9f..a3e189a 100644 > --- a/cpus.c > +++ b/cpus.c > @@ -49,6 +49,10 @@ > #include "hw/nmi.h" > #include "sysemu/replay.h" > > +#ifdef CONFIG_COROUTINE_GTHREAD > +#include "qemu/coroutine.h" > +#endif > + > #ifndef _WIN32 > #include "qemu/compatfd.h" > #endif > @@ -1422,9 +1426,18 @@ bool qemu_in_vcpu_thread(void) > > static __thread bool iothread_locked = false; > > +/* > + * There is a slight wart when using gthread based co-routines. Here > + * the BQL is held by the main-thread which is suspended until the > + * co-routines complete. > + */ > bool qemu_mutex_iothread_locked(void) > { > +#ifdef CONFIG_COROUTINE_GTHREAD > + return iothread_locked || qemu_in_coroutine(); > +#else > return iothread_locked; > +#endif > } > > void qemu_mutex_lock_iothread(void) >
Paolo Bonzini <pbonzini@redhat.com> 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 <alex.bennee@linaro.org> >> Cc: Stefan Hajnoczi <stefanha@redhat.com> > > 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. > > Paolo > >> --- >> configure | 3 +++ >> cpus.c | 13 +++++++++++++ >> 2 files changed, 16 insertions(+) >> >> diff --git a/configure b/configure >> index 91a14c1..97b89fb 100755 >> --- a/configure >> +++ b/configure >> @@ -5461,6 +5461,9 @@ if test "$rbd" = "yes" ; then >> fi >> >> echo "CONFIG_COROUTINE_BACKEND=$coroutine" >> $config_host_mak >> +if test "$coroutine" = "gthread" ; then >> + echo "CONFIG_COROUTINE_GTHREAD=1" >> $config_host_mak >> +fi >> if test "$coroutine_pool" = "yes" ; then >> echo "CONFIG_COROUTINE_POOL=1" >> $config_host_mak >> else >> diff --git a/cpus.c b/cpus.c >> index 0c18a9f..a3e189a 100644 >> --- a/cpus.c >> +++ b/cpus.c >> @@ -49,6 +49,10 @@ >> #include "hw/nmi.h" >> #include "sysemu/replay.h" >> >> +#ifdef CONFIG_COROUTINE_GTHREAD >> +#include "qemu/coroutine.h" >> +#endif >> + >> #ifndef _WIN32 >> #include "qemu/compatfd.h" >> #endif >> @@ -1422,9 +1426,18 @@ bool qemu_in_vcpu_thread(void) >> >> static __thread bool iothread_locked = false; >> >> +/* >> + * There is a slight wart when using gthread based co-routines. Here >> + * the BQL is held by the main-thread which is suspended until the >> + * co-routines complete. >> + */ >> bool qemu_mutex_iothread_locked(void) >> { >> +#ifdef CONFIG_COROUTINE_GTHREAD >> + return iothread_locked || qemu_in_coroutine(); >> +#else >> return iothread_locked; >> +#endif >> } >> >> void qemu_mutex_lock_iothread(void) >> -- Alex Bennée
On Tue, Nov 01, 2016 at 04:21:36PM +0000, Alex Bennée wrote: > > Paolo Bonzini <pbonzini@redhat.com> 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 <alex.bennee@linaro.org> > >> Cc: Stefan Hajnoczi <stefanha@redhat.com> > > > > 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. Stefan
Stefan Hajnoczi <stefanha@redhat.com> writes: > On Tue, Nov 01, 2016 at 04:21:36PM +0000, Alex Bennée wrote: >> >> Paolo Bonzini <pbonzini@redhat.com> 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 <alex.bennee@linaro.org> >> >> Cc: Stefan Hajnoczi <stefanha@redhat.com> >> > >> > 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
diff --git a/configure b/configure index 91a14c1..97b89fb 100755 --- a/configure +++ b/configure @@ -5461,6 +5461,9 @@ if test "$rbd" = "yes" ; then fi echo "CONFIG_COROUTINE_BACKEND=$coroutine" >> $config_host_mak +if test "$coroutine" = "gthread" ; then + echo "CONFIG_COROUTINE_GTHREAD=1" >> $config_host_mak +fi if test "$coroutine_pool" = "yes" ; then echo "CONFIG_COROUTINE_POOL=1" >> $config_host_mak else diff --git a/cpus.c b/cpus.c index 0c18a9f..a3e189a 100644 --- a/cpus.c +++ b/cpus.c @@ -49,6 +49,10 @@ #include "hw/nmi.h" #include "sysemu/replay.h" +#ifdef CONFIG_COROUTINE_GTHREAD +#include "qemu/coroutine.h" +#endif + #ifndef _WIN32 #include "qemu/compatfd.h" #endif @@ -1422,9 +1426,18 @@ bool qemu_in_vcpu_thread(void) static __thread bool iothread_locked = false; +/* + * There is a slight wart when using gthread based co-routines. Here + * the BQL is held by the main-thread which is suspended until the + * co-routines complete. + */ bool qemu_mutex_iothread_locked(void) { +#ifdef CONFIG_COROUTINE_GTHREAD + return iothread_locked || qemu_in_coroutine(); +#else return iothread_locked; +#endif } void qemu_mutex_lock_iothread(void)
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 <alex.bennee@linaro.org> Cc: Stefan Hajnoczi <stefanha@redhat.com> --- configure | 3 +++ cpus.c | 13 +++++++++++++ 2 files changed, 16 insertions(+) -- 2.9.3