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Re: [Qemu-devel] [PATCH 2/3] Make paio subsystem use threadlets


From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH 2/3] Make paio subsystem use threadlets
Date: Wed, 20 Oct 2010 10:30:38 +0100

On Tue, Oct 19, 2010 at 6:43 PM, Arun R Bharadwaj
<address@hidden> wrote:
> From: Gautham R Shenoy <address@hidden>
>
> This patch makes the paio subsystem use the threadlet framework thereby
> decoupling asynchronous threading framework portion out of
> posix-aio-compat.c
>
> The patch has been tested with fstress.
>
> Signed-off-by: Gautham R Shenoy <address@hidden>
> Signed-off-by: Sripathi Kodi <address@hidden>
> ---
>  posix-aio-compat.c |  166 
> +++++++++-------------------------------------------
>  1 files changed, 30 insertions(+), 136 deletions(-)
>
> diff --git a/posix-aio-compat.c b/posix-aio-compat.c
> index 7b862b5..6977c18 100644
> --- a/posix-aio-compat.c
> +++ b/posix-aio-compat.c
> @@ -29,6 +29,7 @@
>  #include "block_int.h"
>
>  #include "block/raw-posix-aio.h"
> +#include "qemu-threadlets.h"
>
>
>  struct qemu_paiocb {
> @@ -51,6 +52,7 @@ struct qemu_paiocb {
>     struct qemu_paiocb *next;
>
>     int async_context_id;
> +    ThreadletWork work;

The QTAILQ_ENTRY(qemu_paiocb) node field is no longer used, please remove it.

>  };
>
>  typedef struct PosixAioState {
> @@ -59,15 +61,6 @@ typedef struct PosixAioState {
>  } PosixAioState;
>
>
> -static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
> -static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
> -static pthread_t thread_id;
> -static pthread_attr_t attr;
> -static int max_threads = 64;
> -static int cur_threads = 0;
> -static int idle_threads = 0;
> -static QTAILQ_HEAD(, qemu_paiocb) request_list;
> -
>  #ifdef CONFIG_PREADV
>  static int preadv_present = 1;
>  #else
> @@ -85,39 +78,6 @@ static void die(const char *what)
>     die2(errno, what);
>  }
>
> -static void mutex_lock(pthread_mutex_t *mutex)
> -{
> -    int ret = pthread_mutex_lock(mutex);
> -    if (ret) die2(ret, "pthread_mutex_lock");
> -}
> -
> -static void mutex_unlock(pthread_mutex_t *mutex)
> -{
> -    int ret = pthread_mutex_unlock(mutex);
> -    if (ret) die2(ret, "pthread_mutex_unlock");
> -}
> -
> -static int cond_timedwait(pthread_cond_t *cond, pthread_mutex_t *mutex,
> -                           struct timespec *ts)
> -{
> -    int ret = pthread_cond_timedwait(cond, mutex, ts);
> -    if (ret && ret != ETIMEDOUT) die2(ret, "pthread_cond_timedwait");
> -    return ret;
> -}
> -
> -static void cond_signal(pthread_cond_t *cond)
> -{
> -    int ret = pthread_cond_signal(cond);
> -    if (ret) die2(ret, "pthread_cond_signal");
> -}
> -
> -static void thread_create(pthread_t *thread, pthread_attr_t *attr,
> -                          void *(*start_routine)(void*), void *arg)
> -{
> -    int ret = pthread_create(thread, attr, start_routine, arg);
> -    if (ret) die2(ret, "pthread_create");
> -}
> -
>  static ssize_t handle_aiocb_ioctl(struct qemu_paiocb *aiocb)
>  {
>     int ret;
> @@ -301,106 +261,51 @@ static ssize_t handle_aiocb_rw(struct qemu_paiocb 
> *aiocb)
>     return nbytes;
>  }
>
> -static void *aio_thread(void *unused)
> +static void aio_thread(ThreadletWork *work)
>  {
>     pid_t pid;
> +    struct qemu_paiocb *aiocb = container_of(work, struct qemu_paiocb, work);
> +    ssize_t ret = 0;
>
>     pid = getpid();
> +    aiocb->active = 1;
>
> -    while (1) {
> -        struct qemu_paiocb *aiocb;
> -        ssize_t ret = 0;
> -        qemu_timeval tv;
> -        struct timespec ts;
> -
> -        qemu_gettimeofday(&tv);
> -        ts.tv_sec = tv.tv_sec + 10;
> -        ts.tv_nsec = 0;
> -
> -        mutex_lock(&lock);
> -
> -        while (QTAILQ_EMPTY(&request_list) &&
> -               !(ret == ETIMEDOUT)) {
> -            ret = cond_timedwait(&cond, &lock, &ts);
> -        }
> -
> -        if (QTAILQ_EMPTY(&request_list))
> -            break;
> -
> -        aiocb = QTAILQ_FIRST(&request_list);
> -        QTAILQ_REMOVE(&request_list, aiocb, node);
> -        aiocb->active = 1;
> -        idle_threads--;
> -        mutex_unlock(&lock);
> -
> -        switch (aiocb->aio_type & QEMU_AIO_TYPE_MASK) {
> -        case QEMU_AIO_READ:
> -        case QEMU_AIO_WRITE:
> -            ret = handle_aiocb_rw(aiocb);
> -            break;
> -        case QEMU_AIO_FLUSH:
> -            ret = handle_aiocb_flush(aiocb);
> -            break;
> -        case QEMU_AIO_IOCTL:
> -            ret = handle_aiocb_ioctl(aiocb);
> -            break;
> -        default:
> -            fprintf(stderr, "invalid aio request (0x%x)\n", aiocb->aio_type);
> -            ret = -EINVAL;
> -            break;
> -        }
> -
> -        mutex_lock(&lock);
> -        aiocb->ret = ret;
> -        idle_threads++;
> -        mutex_unlock(&lock);
> -
> -        if (kill(pid, aiocb->ev_signo)) die("kill failed");
> +    switch (aiocb->aio_type & QEMU_AIO_TYPE_MASK) {
> +    case QEMU_AIO_READ:
> +    case QEMU_AIO_WRITE:
> +        ret = handle_aiocb_rw(aiocb);
> +        break;
> +    case QEMU_AIO_FLUSH:
> +        ret = handle_aiocb_flush(aiocb);
> +        break;
> +    case QEMU_AIO_IOCTL:
> +        ret = handle_aiocb_ioctl(aiocb);
> +        break;
> +    default:
> +        fprintf(stderr, "invalid aio request (0x%x)\n", aiocb->aio_type);
> +        ret = -EINVAL;
> +        break;
>     }
>
> -    idle_threads--;
> -    cur_threads--;
> -    mutex_unlock(&lock);
> +    aiocb->ret = ret;
>
> -    return NULL;
> -}
> -
> -static void spawn_thread(void)
> -{
> -    sigset_t set, oldset;
> -
> -    cur_threads++;
> -    idle_threads++;
> -
> -    /* block all signals */
> -    if (sigfillset(&set)) die("sigfillset");
> -    if (sigprocmask(SIG_SETMASK, &set, &oldset)) die("sigprocmask");
> -
> -    thread_create(&thread_id, &attr, aio_thread, NULL);
> -
> -    if (sigprocmask(SIG_SETMASK, &oldset, NULL)) die("sigprocmask restore");
> +    if (kill(pid, aiocb->ev_signo)) die("kill failed");
>  }
>
>  static void qemu_paio_submit(struct qemu_paiocb *aiocb)
>  {
>     aiocb->ret = -EINPROGRESS;
>     aiocb->active = 0;
> -    mutex_lock(&lock);
> -    if (idle_threads == 0 && cur_threads < max_threads)
> -        spawn_thread();
> -    QTAILQ_INSERT_TAIL(&request_list, aiocb, node);
> -    mutex_unlock(&lock);
> -    cond_signal(&cond);
> +
> +    aiocb->work.func = aio_thread;
> +    submit_threadletwork(&aiocb->work);
>  }
>
>  static ssize_t qemu_paio_return(struct qemu_paiocb *aiocb)
>  {
>     ssize_t ret;
>
> -    mutex_lock(&lock);
>     ret = aiocb->ret;
> -    mutex_unlock(&lock);
> -
>     return ret;
>  }
>
> @@ -536,14 +441,14 @@ static void paio_cancel(BlockDriverAIOCB *blockacb)
>     struct qemu_paiocb *acb = (struct qemu_paiocb *)blockacb;
>     int active = 0;
>
> -    mutex_lock(&lock);
>     if (!acb->active) {

I'm not sure the active field serves any purpose.  No memory barriers
are used so the value of active is 0 before the work is executed and 0
*or* 1 while the work is executed.

The cancel_threadletwork() function already indicates whether
cancellation succeeded.  Why not just try to cancel instead of using
the active field?

> -        QTAILQ_REMOVE(&request_list, acb, node);
> -        acb->ret = -ECANCELED;
> +        if (!cancel_threadletwork(&acb->work))
> +            acb->ret = -ECANCELED;
> +         else
> +            active = 1;

The 0 and 1 return value from cancel_threadletwork() is inverted.  See
also my comment on patch 1/3 in this series.

>     } else if (acb->ret == -EINPROGRESS) {
>         active = 1;
>     }
> -    mutex_unlock(&lock);
>
>     if (active) {
>         /* fail safe: if the aio could not be canceled, we wait for

while (qemu_paio_error(acb) == EINPROGRESS)
    ;

Tight loop with no memory barrier reading a memory location that is
updated by another thread.  We shouldn't communicate between threads
without barriers.

Stefan



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