Re: [Qemu-devel] [PATCH 1/2] qemu-thread: always keep the posix wrapper layer

[Peter Xu]

On Tue, Apr 10, 2018 at 08:35:40AM -0500, Eric Blake wrote:
> On 04/10/2018 07:49 AM, Peter Xu wrote:
> > We will conditionally have a wrapper layer depending on whether the host
> > has the PTHREAD_SETNAME capability.  It complicates stuff.  Let's just
> > keep the wrapper there, meanwhile we opt out the pthread_setname_np()
> > call only.  The layer can be helpful in future patches to pass data from
> > the parent thread to the child thread.
> > 
> > Signed-off-by: Peter Xu 
> > ---
> >  util/qemu-thread-posix.c | 33 +
> >  1 file changed, 13 insertions(+), 20 deletions(-)
> > 
> > diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
> > index b789cf32e9..3ae96210d6 100644
> > --- a/util/qemu-thread-posix.c
> > +++ b/util/qemu-thread-posix.c
> > @@ -482,7 +482,6 @@ static void __attribute__((constructor)) 
> > qemu_thread_atexit_init(void)
> >  }
> >  
> 
> More context:
> 
> static bool name_threads;
> 
> void qemu_thread_naming(bool enable)
> {
> name_threads = enable;
> 
> #ifndef CONFIG_THREAD_SETNAME_BYTHREAD
> /* This is a debugging option, not fatal */
> if (enable) {
> fprintf(stderr, "qemu: thread naming not supported on this host\n");
> }
> #endif
> }
> 
> 
> >  
> > -#ifdef CONFIG_PTHREAD_SETNAME_NP
> 
> Why are we using CONFIG_THREAD_SETNAME_BYTHREAD in one place, and
> CONFIG_PTHREAD_SETNAME_NP in another?
> 
> /me checks configure - oh:
> 
> # Hold two types of flag:
> #   CONFIG_THREAD_SETNAME_BYTHREAD  - we've got a way of setting the name on
> # a thread we have a handle to
> #   CONFIG_PTHREAD_SETNAME_NP   - A way of doing it on a particular
> # platform
> 
> even though, right now, we only either set both flags at once or leave
> both clear, since we don't (yet?) have any other platform-specific ways
> to do it.

It seems so.  I'm not sure whether they could be useful in the future
and I'm fine with them, hence I keep it as is.

> 
> >  typedef struct {
> >  void *(*start_routine)(void *);
> >  void *arg;
> > @@ -498,13 +497,15 @@ static void *qemu_thread_start(void *args)
> >  /* Attempt to set the threads name; note that this is for debug, so
> >   * we're not going to fail if we can't set it.
> >   */
> > -pthread_setname_np(pthread_self(), qemu_thread_args->name);
> > +#ifdef CONFIG_PTHREAD_SETNAME_NP
> > +if (qemu_thread_args->name) {
> > +pthread_setname_np(pthread_self(), qemu_thread_args->name);
> 
> Post-patch, this (attempts to) set the thread name if a non-NULL name is
> present...
> 
> 
> >  
> > -#ifdef CONFIG_PTHREAD_SETNAME_NP
> > -if (name_threads) {
> > -QemuThreadArgs *qemu_thread_args;
> > -qemu_thread_args = g_new0(QemuThreadArgs, 1);
> > -qemu_thread_args->name = g_strdup(name);
> 
> ...but pre-patch, qemu_thread_args->name was left NULL unless
> name_threads was true, because someone had called
> qemu_thread_naming(true)...
> 
> > -qemu_thread_args->start_routine = start_routine;
> > -qemu_thread_args->arg = arg;
> > -
> > -err = pthread_create(>thread, ,
> > - qemu_thread_start, qemu_thread_args);
> > -} else
> > -#endif
> > -{
> > -err = pthread_create(>thread, ,
> > - start_routine, arg);
> > -}
> > +qemu_thread_args = g_new0(QemuThreadArgs, 1);
> > +qemu_thread_args->name = g_strdup(name);
> 
> ...so you have changed semantics - you are now unconditionally trying to
> set the thread name, instead of honoring qemu_thread_naming().  Do we
> still need qemu_thread_naming() (tied to opt debug-threads)?
> 
> You need to either fix your code to remain conditional on whether
> name_threads is set, or document the semantic change as intentional in
> the commit message.

Indeed, thanks for catching that. What I really wanted is probably
this:

static void *qemu_thread_start(void *args)
{
...
#ifdef CONFIG_PTHREAD_SETNAME_NP
if (name_threads && qemu_thread_args->name) {
pthread_setname_np(pthread_self(), qemu_thread_args->name);
}
#endif
...
}

I'll fix that up in next version.

> 
> However, the idea for refactoring to always use the shim makes sense.

Thanks,

-- 
Peter Xu

Re: [Qemu-devel] [PATCH 1/2] qemu-thread: always keep the posix wrapper layer

[Eric Blake]

On 04/10/2018 07:49 AM, Peter Xu wrote:
> We will conditionally have a wrapper layer depending on whether the host
> has the PTHREAD_SETNAME capability.  It complicates stuff.  Let's just
> keep the wrapper there, meanwhile we opt out the pthread_setname_np()
> call only.  The layer can be helpful in future patches to pass data from
> the parent thread to the child thread.
> 
> Signed-off-by: Peter Xu 
> ---
>  util/qemu-thread-posix.c | 33 +
>  1 file changed, 13 insertions(+), 20 deletions(-)
> 
> diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
> index b789cf32e9..3ae96210d6 100644
> --- a/util/qemu-thread-posix.c
> +++ b/util/qemu-thread-posix.c
> @@ -482,7 +482,6 @@ static void __attribute__((constructor)) 
> qemu_thread_atexit_init(void)
>  }
>  

More context:

static bool name_threads;

void qemu_thread_naming(bool enable)
{
name_threads = enable;

#ifndef CONFIG_THREAD_SETNAME_BYTHREAD
/* This is a debugging option, not fatal */
if (enable) {
fprintf(stderr, "qemu: thread naming not supported on this host\n");
}
#endif
}


>  
> -#ifdef CONFIG_PTHREAD_SETNAME_NP

Why are we using CONFIG_THREAD_SETNAME_BYTHREAD in one place, and
CONFIG_PTHREAD_SETNAME_NP in another?

/me checks configure - oh:

# Hold two types of flag:
#   CONFIG_THREAD_SETNAME_BYTHREAD  - we've got a way of setting the name on
# a thread we have a handle to
#   CONFIG_PTHREAD_SETNAME_NP   - A way of doing it on a particular
# platform

even though, right now, we only either set both flags at once or leave
both clear, since we don't (yet?) have any other platform-specific ways
to do it.

>  typedef struct {
>  void *(*start_routine)(void *);
>  void *arg;
> @@ -498,13 +497,15 @@ static void *qemu_thread_start(void *args)
>  /* Attempt to set the threads name; note that this is for debug, so
>   * we're not going to fail if we can't set it.
>   */
> -pthread_setname_np(pthread_self(), qemu_thread_args->name);
> +#ifdef CONFIG_PTHREAD_SETNAME_NP
> +if (qemu_thread_args->name) {
> +pthread_setname_np(pthread_self(), qemu_thread_args->name);

Post-patch, this (attempts to) set the thread name if a non-NULL name is
present...


>  
> -#ifdef CONFIG_PTHREAD_SETNAME_NP
> -if (name_threads) {
> -QemuThreadArgs *qemu_thread_args;
> -qemu_thread_args = g_new0(QemuThreadArgs, 1);
> -qemu_thread_args->name = g_strdup(name);

...but pre-patch, qemu_thread_args->name was left NULL unless
name_threads was true, because someone had called
qemu_thread_naming(true)...

> -qemu_thread_args->start_routine = start_routine;
> -qemu_thread_args->arg = arg;
> -
> -err = pthread_create(>thread, ,
> - qemu_thread_start, qemu_thread_args);
> -} else
> -#endif
> -{
> -err = pthread_create(>thread, ,
> - start_routine, arg);
> -}
> +qemu_thread_args = g_new0(QemuThreadArgs, 1);
> +qemu_thread_args->name = g_strdup(name);

...so you have changed semantics - you are now unconditionally trying to
set the thread name, instead of honoring qemu_thread_naming().  Do we
still need qemu_thread_naming() (tied to opt debug-threads)?

You need to either fix your code to remain conditional on whether
name_threads is set, or document the semantic change as intentional in
the commit message.

However, the idea for refactoring to always use the shim makes sense.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.   +1-919-301-3266
Virtualization:  qemu.org | libvirt.org



signature.asc
Description: OpenPGP digital signature

[Qemu-devel] [PATCH 1/2] qemu-thread: always keep the posix wrapper layer

[Peter Xu]

We will conditionally have a wrapper layer depending on whether the host
has the PTHREAD_SETNAME capability.  It complicates stuff.  Let's just
keep the wrapper there, meanwhile we opt out the pthread_setname_np()
call only.  The layer can be helpful in future patches to pass data from
the parent thread to the child thread.

Signed-off-by: Peter Xu 
---
 util/qemu-thread-posix.c | 33 +
 1 file changed, 13 insertions(+), 20 deletions(-)

diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index b789cf32e9..3ae96210d6 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -482,7 +482,6 @@ static void __attribute__((constructor)) 
qemu_thread_atexit_init(void)
 }
 
 
-#ifdef CONFIG_PTHREAD_SETNAME_NP
 typedef struct {
 void *(*start_routine)(void *);
 void *arg;
@@ -498,13 +497,15 @@ static void *qemu_thread_start(void *args)
 /* Attempt to set the threads name; note that this is for debug, so
  * we're not going to fail if we can't set it.
  */
-pthread_setname_np(pthread_self(), qemu_thread_args->name);
+#ifdef CONFIG_PTHREAD_SETNAME_NP
+if (qemu_thread_args->name) {
+pthread_setname_np(pthread_self(), qemu_thread_args->name);
+}
+#endif
 g_free(qemu_thread_args->name);
 g_free(qemu_thread_args);
 return start_routine(arg);
 }
-#endif
-
 
 void qemu_thread_create(QemuThread *thread, const char *name,
void *(*start_routine)(void*),
@@ -513,6 +514,7 @@ void qemu_thread_create(QemuThread *thread, const char 
*name,
 sigset_t set, oldset;
 int err;
 pthread_attr_t attr;
+QemuThreadArgs *qemu_thread_args;
 
 err = pthread_attr_init();
 if (err) {
@@ -527,22 +529,13 @@ void qemu_thread_create(QemuThread *thread, const char 
*name,
 sigfillset();
 pthread_sigmask(SIG_SETMASK, , );
 
-#ifdef CONFIG_PTHREAD_SETNAME_NP
-if (name_threads) {
-QemuThreadArgs *qemu_thread_args;
-qemu_thread_args = g_new0(QemuThreadArgs, 1);
-qemu_thread_args->name = g_strdup(name);
-qemu_thread_args->start_routine = start_routine;
-qemu_thread_args->arg = arg;
-
-err = pthread_create(>thread, ,
- qemu_thread_start, qemu_thread_args);
-} else
-#endif
-{
-err = pthread_create(>thread, ,
- start_routine, arg);
-}
+qemu_thread_args = g_new0(QemuThreadArgs, 1);
+qemu_thread_args->name = g_strdup(name);
+qemu_thread_args->start_routine = start_routine;
+qemu_thread_args->arg = arg;
+
+err = pthread_create(>thread, ,
+ qemu_thread_start, qemu_thread_args);
 
 if (err)
 error_exit(err, __func__);
-- 
2.14.3