[Qemu-devel] Re: [PATCH v2 upstream 07/22] add assertions on the owner of a QemuMutex

[Jan Kiszka <jan.kiszka@web.de>]

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On 2011-02-26 16:40, Paolo Bonzini wrote:
> These are already present in the Win32 implementation, add them to
> the pthread wrappers as well.  Use PTHREAD_MUTEX_ERRORCHECK for mutex
> operations, and track the owner separately for cond_signal/broadcast.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  qemu-thread-posix.c |   23 +++++++++++++++++++++--
>  qemu-thread-posix.h |    1 +
>  2 files changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/qemu-thread-posix.c b/qemu-thread-posix.c
> index e307773..a4c6e25 100644
> --- a/qemu-thread-posix.c
> +++ b/qemu-thread-posix.c
> @@ -16,9 +16,12 @@
>  #include <time.h>
>  #include <signal.h>
>  #include <stdint.h>
> +#include <assert.h>
>  #include <string.h>
>  #include "qemu-thread.h"
>  
> +static pthread_t pthread_null;
> +
>  static void error_exit(int err, const char *msg)
>  {
>      fprintf(stderr, "qemu: %s: %s\n", msg, strerror(err));
> @@ -28,8 +31,13 @@ static void error_exit(int err, const char *msg)
>  void qemu_mutex_init(QemuMutex *mutex)
>  {
>      int err;
> +    pthread_mutexattr_t mutexattr;
>  
> -    err = pthread_mutex_init(&mutex->lock, NULL);
> +    mutex->owner = pthread_null;
> +    pthread_mutexattr_init(&mutexattr);
> +    pthread_mutexattr_settype(&mutexattr, PTHREAD_MUTEX_ERRORCHECK);
> +    err = pthread_mutex_init(&mutex->lock, &mutexattr);
> +    pthread_mutexattr_destroy(&mutexattr);
>      if (err)
>          error_exit(err, __func__);
>  }
> @@ -48,13 +56,20 @@ void qemu_mutex_lock(QemuMutex *mutex)
>      int err;
>  
>      err = pthread_mutex_lock(&mutex->lock);
> +    mutex->owner = pthread_self();
>      if (err)
>          error_exit(err, __func__);
>  }
>  
>  int qemu_mutex_trylock(QemuMutex *mutex)
>  {
> -    return pthread_mutex_trylock(&mutex->lock);
> +    int err;
> +    err = pthread_mutex_trylock(&mutex->lock);
> +    if (err == 0) {
> +        mutex->owner = pthread_self();
> +    }
> +
> +    return !!err;
>  }
>  
>  static void timespec_add_ms(struct timespec *ts, uint64_t msecs)
> @@ -85,6 +100,7 @@ void qemu_mutex_unlock(QemuMutex *mutex)
>  {
>      int err;
>  
> +    mutex->owner = pthread_null;
>      err = pthread_mutex_unlock(&mutex->lock);
>      if (err)
>          error_exit(err, __func__);
> @@ -130,7 +146,10 @@ void qemu_cond_wait(QemuCond *cond, QemuMutex *mutex)
>  {
>      int err;
>  
> +    assert(pthread_equal(mutex->owner, pthread_self()));
> +    mutex->owner = pthread_null;
>      err = pthread_cond_wait(&cond->cond, &mutex->lock);

Though POSIX is not 100% explicit on this, every sane pthread_cond_wait
implementation will apply the same error checking as on
pthread_mutex_unlock when the given mutex is of
PTHREAD_MUTEX_ERRORCHECK. So, this assert is actually redundant as well.

Now that we are left without any assertions, I start wondering about one
of the original missions: enforce qemu_cond_signal/broadcast to be
called under a mutex. What about extending those services with a mutex
argument and applying the assert there? Could become a static-inline
wrapper so that the argument is optimized away if assert() is inactive.

Jan


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