Re: [Qemu-block] [PATCH 07/10] aio-posix: remove walking_handlers, protecting AioHandler list with list_lock

[Paolo Bonzini]

On 21/12/2016 15:03, Paolo Bonzini wrote:
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
>  aio-posix.c | 64 
> ++++++++++++++++++++++++++++++++++++-------------------------
>  1 file changed, 38 insertions(+), 26 deletions(-)
> 
> diff --git a/aio-posix.c b/aio-posix.c
> index f83b7af..9b13182 100644
> --- a/aio-posix.c
> +++ b/aio-posix.c
> @@ -16,7 +16,7 @@
>  #include "qemu/osdep.h"
>  #include "qemu-common.h"
>  #include "block/block.h"
> -#include "qemu/queue.h"
> +#include "qemu/rcu_queue.h"
>  #include "qemu/sockets.h"
>  #include "qemu/cutils.h"
>  #include "trace.h"
> @@ -66,7 +66,7 @@ static bool aio_epoll_try_enable(AioContext *ctx)
>      AioHandler *node;
>      struct epoll_event event;
>  
> -    QLIST_FOREACH(node, &ctx->aio_handlers, node) {
> +    QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
>          int r;
>          if (node->deleted || !node->pfd.events) {
>              continue;
> @@ -212,6 +212,8 @@ void aio_set_fd_handler(AioContext *ctx,
>      bool is_new = false;
>      bool deleted = false;
>  
> +    qemu_lockcnt_lock(&ctx->list_lock);
> +
>      node = find_aio_handler(ctx, fd);
>  
>      /* Are we deleting the fd handler? */

Oops, there's a return between this hunks that doesn't release the lock.
 Will resend.

Paolo

> @@ -223,13 +225,13 @@ void aio_set_fd_handler(AioContext *ctx,
>          g_source_remove_poll(&ctx->source, &node->pfd);
>  
>          /* If the lock is held, just mark the node as deleted */
> -        if (ctx->walking_handlers) {
> +        if (qemu_lockcnt_count(&ctx->list_lock)) {
>              node->deleted = 1;
>              node->pfd.revents = 0;
>          } else {
>              /* Otherwise, delete it for real.  We can't just mark it as
> -             * deleted because deleted nodes are only cleaned up after
> -             * releasing the walking_handlers lock.
> +             * deleted because deleted nodes are only cleaned up while
> +             * no one is walking the handlers list.
>               */
>              QLIST_REMOVE(node, node);
>              deleted = true;
> @@ -243,7 +245,7 @@ void aio_set_fd_handler(AioContext *ctx,
>              /* Alloc and insert if it's not already there */
>              node = g_new0(AioHandler, 1);
>              node->pfd.fd = fd;
> -            QLIST_INSERT_HEAD(&ctx->aio_handlers, node, node);
> +            QLIST_INSERT_HEAD_RCU(&ctx->aio_handlers, node, node);
>  
>              g_source_add_poll(&ctx->source, &node->pfd);
>              is_new = true;
> @@ -265,6 +267,7 @@ void aio_set_fd_handler(AioContext *ctx,
>      }
>  
>      aio_epoll_update(ctx, node, is_new);
> +    qemu_lockcnt_unlock(&ctx->list_lock);
>      aio_notify(ctx);
>  
>      if (deleted) {
> @@ -316,8 +319,8 @@ static void poll_set_started(AioContext *ctx, bool 
> started)
>  
>      ctx->poll_started = started;
>  
> -    ctx->walking_handlers++;
> -    QLIST_FOREACH(node, &ctx->aio_handlers, node) {
> +    qemu_lockcnt_inc(&ctx->list_lock);
> +    QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
>          IOHandler *fn;
>  
>          if (node->deleted) {
> @@ -334,7 +337,7 @@ static void poll_set_started(AioContext *ctx, bool 
> started)
>              fn(node->opaque);
>          }
>      }
> -    ctx->walking_handlers--;
> +    qemu_lockcnt_dec(&ctx->list_lock);
>  }
>  
>  
> @@ -349,22 +352,32 @@ bool aio_prepare(AioContext *ctx)
>  bool aio_pending(AioContext *ctx)
>  {
>      AioHandler *node;
> +    bool result = false;
>  
> -    QLIST_FOREACH(node, &ctx->aio_handlers, node) {
> +    /*
> +     * We have to walk very carefully in case aio_set_fd_handler is
> +     * called while we're walking.
> +     */
> +    qemu_lockcnt_inc(&ctx->list_lock);
> +
> +    QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
>          int revents;
>  
>          revents = node->pfd.revents & node->pfd.events;
>          if (revents & (G_IO_IN | G_IO_HUP | G_IO_ERR) && node->io_read &&
>              aio_node_check(ctx, node->is_external)) {
> -            return true;
> +            result = true;
> +            break;
>          }
>          if (revents & (G_IO_OUT | G_IO_ERR) && node->io_write &&
>              aio_node_check(ctx, node->is_external)) {
> -            return true;
> +            result = true;
> +            break;
>          }
>      }
> +    qemu_lockcnt_dec(&ctx->list_lock);
>  
> -    return false;
> +    return result;
>  }
>  
>  static bool aio_dispatch_handlers(AioContext *ctx)
> @@ -376,9 +389,9 @@ static bool aio_dispatch_handlers(AioContext *ctx)
>       * We have to walk very carefully in case aio_set_fd_handler is
>       * called while we're walking.
>       */
> -    ctx->walking_handlers++;
> +    qemu_lockcnt_inc(&ctx->list_lock);
>  
> -    QLIST_FOREACH_SAFE(node, &ctx->aio_handlers, node, tmp) {
> +    QLIST_FOREACH_SAFE_RCU(node, &ctx->aio_handlers, node, tmp) {
>          int revents;
>  
>          revents = node->pfd.revents & node->pfd.events;
> @@ -404,16 +417,15 @@ static bool aio_dispatch_handlers(AioContext *ctx)
>          }
>  
>          if (node->deleted) {
> -            ctx->walking_handlers--;
> -            if (!ctx->walking_handlers) {
> +            if (qemu_lockcnt_dec_if_lock(&ctx->list_lock)) {
>                  QLIST_REMOVE(node, node);
>                  g_free(node);
> +                qemu_lockcnt_inc_and_unlock(&ctx->list_lock);
>              }
> -            ctx->walking_handlers++;
>          }
>      }
>  
> -    ctx->walking_handlers--;
> +    qemu_lockcnt_dec(&ctx->list_lock);
>      return progress;
>  }
>  
> @@ -493,7 +505,7 @@ static bool run_poll_handlers_once(AioContext *ctx)
>      bool progress = false;
>      AioHandler *node;
>  
> -    QLIST_FOREACH(node, &ctx->aio_handlers, node) {
> +    QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
>          if (!node->deleted && node->io_poll &&
>                  node->io_poll(node->opaque)) {
>              progress = true;
> @@ -514,7 +526,7 @@ static bool run_poll_handlers_once(AioContext *ctx)
>   * Note that ctx->notify_me must be non-zero so this function can detect
>   * aio_notify().
>   *
> - * Note that the caller must have incremented ctx->walking_handlers.
> + * Note that the caller must have incremented ctx->list_lock.
>   *
>   * Returns: true if progress was made, false otherwise
>   */
> @@ -524,7 +536,7 @@ static bool run_poll_handlers(AioContext *ctx, int64_t 
> max_ns)
>      int64_t end_time;
>  
>      assert(ctx->notify_me);
> -    assert(ctx->walking_handlers > 0);
> +    assert(qemu_lockcnt_count(&ctx->list_lock) > 0);
>      assert(ctx->poll_disable_cnt == 0);
>  
>      trace_run_poll_handlers_begin(ctx, max_ns);
> @@ -546,7 +558,7 @@ static bool run_poll_handlers(AioContext *ctx, int64_t 
> max_ns)
>   *
>   * ctx->notify_me must be non-zero so this function can detect aio_notify().
>   *
> - * Note that the caller must have incremented ctx->walking_handlers.
> + * Note that the caller must have incremented ctx->list_lock.
>   *
>   * Returns: true if progress was made, false otherwise
>   */
> @@ -597,7 +609,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
>          atomic_add(&ctx->notify_me, 2);
>      }
>  
> -    ctx->walking_handlers++;
> +    qemu_lockcnt_inc(&ctx->list_lock);
>  
>      if (ctx->poll_max_ns) {
>          start = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
> @@ -611,7 +623,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
>          /* fill pollfds */
>  
>          if (!aio_epoll_enabled(ctx)) {
> -            QLIST_FOREACH(node, &ctx->aio_handlers, node) {
> +            QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
>                  if (!node->deleted && node->pfd.events
>                      && aio_node_check(ctx, node->is_external)) {
>                      add_pollfd(node);
> @@ -696,7 +708,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
>      }
>  
>      npfd = 0;
> -    ctx->walking_handlers--;
> +    qemu_lockcnt_dec(&ctx->list_lock);
>  
>      /* Run dispatch even if there were no readable fds to run timers */
>      if (aio_dispatch(ctx, ret > 0)) {
>