Re: [Qemu-devel] [PATCH 06/23] block: Eliminate bdrv_states, use block_next() instead

["Benoît Canet" <benoit.canet@irqsave.net>]

The Wednesday 10 Sep 2014 à 10:13:35 (+0200), Markus Armbruster wrote :
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>  block.c                   | 43 +++++++++++++++++++------------------------
>  block/block-backend.c     |  4 ++++
>  include/block/block_int.h |  2 --
>  3 files changed, 23 insertions(+), 26 deletions(-)
> 
> diff --git a/block.c b/block.c
> index a6c03da..89f9cf0 100644
> --- a/block.c
> +++ b/block.c
> @@ -24,6 +24,7 @@
>  #include "config-host.h"
>  #include "qemu-common.h"
>  #include "trace.h"
> +#include "sysemu/block-backend.h"
>  #include "block/block_int.h"
>  #include "block/blockjob.h"
>  #include "qemu/module.h"
> @@ -90,9 +91,6 @@ static void coroutine_fn bdrv_co_do_rw(void *opaque);
>  static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,
>      int64_t sector_num, int nb_sectors, BdrvRequestFlags flags);
>  
> -static QTAILQ_HEAD(, BlockDriverState) bdrv_states =
> -    QTAILQ_HEAD_INITIALIZER(bdrv_states);
> -
>  static QTAILQ_HEAD(, BlockDriverState) graph_bdrv_states =
>      QTAILQ_HEAD_INITIALIZER(graph_bdrv_states);
>  
> @@ -355,9 +353,6 @@ BlockDriverState *bdrv_new_named(const char *device_name, Error **errp)
>      bs = bdrv_new();
>  
>      pstrcpy(bs->device_name, sizeof(bs->device_name), device_name);
> -    if (device_name[0] != '\0') {
> -        QTAILQ_INSERT_TAIL(&bdrv_states, bs, device_list);
> -    }
>  
>      return bs;
>  }
> @@ -1888,7 +1883,7 @@ void bdrv_close_all(void)
>  {
>      BlockDriverState *bs;
>  
> -    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> +    for (bs = bdrv_next(NULL); bs; bs = bdrv_next(bs)) {
>          AioContext *aio_context = bdrv_get_aio_context(bs);
>  
>          aio_context_acquire(aio_context);
> @@ -1939,7 +1934,7 @@ void bdrv_drain_all(void)
>      while (busy) {
>          busy = false;
>  
> -        QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> +        for (bs = bdrv_next(NULL); bs; bs = bdrv_next(bs)) {
>              AioContext *aio_context = bdrv_get_aio_context(bs);
>              bool bs_busy;
>  
> @@ -1960,9 +1955,6 @@ void bdrv_drain_all(void)
>     Also, NULL terminate the device_name to prevent double remove */
>  void bdrv_make_anon(BlockDriverState *bs)
>  {
> -    if (bs->device_name[0] != '\0') {
> -        QTAILQ_REMOVE(&bdrv_states, bs, device_list);
> -    }
>      bs->device_name[0] = '\0';
>      if (bs->node_name[0] != '\0') {
>          QTAILQ_REMOVE(&graph_bdrv_states, bs, node_list);
> @@ -2016,10 +2008,9 @@ static void bdrv_move_feature_fields(BlockDriverState *bs_dest,
>      /* job */
>      bs_dest->job                = bs_src->job;
>  
> -    /* keep the same entry in bdrv_states */
>      pstrcpy(bs_dest->device_name, sizeof(bs_dest->device_name),
>              bs_src->device_name);
> -    bs_dest->device_list = bs_src->device_list;
> +
>      memcpy(bs_dest->op_blockers, bs_src->op_blockers,
>             sizeof(bs_dest->op_blockers));
>  }
> @@ -2363,7 +2354,7 @@ int bdrv_commit_all(void)
>  {
>      BlockDriverState *bs;
>  
> -    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> +    for (bs = bdrv_next(NULL); bs; bs = bdrv_next(bs)) {
>          AioContext *aio_context = bdrv_get_aio_context(bs);
>  
>          aio_context_acquire(aio_context);
> @@ -3807,7 +3798,7 @@ BlockDriverState *bdrv_find(const char *name)
>  {
>      BlockDriverState *bs;
>  
> -    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> +    for (bs = bdrv_next(NULL); bs; bs = bdrv_next(bs)) {
>          if (!strcmp(name, bs->device_name)) {
>              return bs;
>          }
> @@ -3888,17 +3879,21 @@ bool bdrv_chain_contains(BlockDriverState *top, BlockDriverState *base)
>  
>  BlockDriverState *bdrv_next(BlockDriverState *bs)
>  {
> -    if (!bs) {
> -        return QTAILQ_FIRST(&bdrv_states);
> -    }
> -    return QTAILQ_NEXT(bs, device_list);

> +    BlockBackend *blk;
> +
> +    for (blk = blk_next(bs ? bs->blk : NULL);
> +         blk && !blk_bs(blk);
> +         blk = blk_next(blk))
> +        ;
> +
> +    return blk ? blk_bs(blk) : NULL;

I find ?: hard to read and I know at least one maintainer
prefers simples ifs to it.

BlockDriverState *bdrv_next(BlockDriverState *bs)
{
    BlockBackend *blk = NULL;

    /* if a BDS is provided use it's block backend as a starting point */
    if (bs) {
        blk = bl->blk;
    }

    /* looking for the next block backend having a BDS attached */
    for (blk = blk_next(blk);
         blk && !blk_bs(blk);
         blk = blk_next(blk))
         ;

   /* the search was successfull */
   if (blk) {
       return blk_bs(blk);
   }

   return NULL;
}

I think getting rid of ?: spread the brain load sequentially by being less compact
but anyway your code is correct so it's up to you.

>  }
>  
>  void bdrv_iterate(void (*it)(void *opaque, BlockDriverState *bs), void *opaque)
>  {
>      BlockDriverState *bs;
>  
> -    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> +    for (bs = bdrv_next(NULL); bs; bs = bdrv_next(bs)) {
>          it(opaque, bs);
>      }
>  }
> @@ -3918,7 +3913,7 @@ int bdrv_flush_all(void)
>      BlockDriverState *bs;
>      int result = 0;
>  
> -    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> +    for (bs = bdrv_next(NULL); bs; bs = bdrv_next(bs)) {
>          AioContext *aio_context = bdrv_get_aio_context(bs);
>          int ret;
>  
> @@ -5065,7 +5060,7 @@ void bdrv_invalidate_cache_all(Error **errp)
>      BlockDriverState *bs;
>      Error *local_err = NULL;
>  
> -    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> +    for (bs = bdrv_next(NULL); bs; bs = bdrv_next(bs)) {
>          AioContext *aio_context = bdrv_get_aio_context(bs);
>  
>          aio_context_acquire(aio_context);
> @@ -5082,7 +5077,7 @@ void bdrv_clear_incoming_migration_all(void)
>  {
>      BlockDriverState *bs;
>  
> -    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> +    for (bs = bdrv_next(NULL); bs; bs = bdrv_next(bs)) {
>          AioContext *aio_context = bdrv_get_aio_context(bs);
>  
>          aio_context_acquire(aio_context);
> @@ -5918,7 +5913,7 @@ bool bdrv_is_first_non_filter(BlockDriverState *candidate)
>      BlockDriverState *bs;
>  
>      /* walk down the bs forest recursively */
> -    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> +    for (bs = bdrv_next(NULL); bs; bs = bdrv_next(bs)) {
>          bool perm;
>  
>          /* try to recurse in this top level bs */
> diff --git a/block/block-backend.c b/block/block-backend.c
> index ae51f7f..c0876fa 100644
> --- a/block/block-backend.c
> +++ b/block/block-backend.c
> @@ -191,6 +191,10 @@ BlockDriverState *blk_detach_bs(BlockBackend *blk)
>  {
>      BlockDriverState *bs = blk->bs;
>  
> +    /*
> +     * Must bdrv_unref() before zapping bs->blk, or else
> +     * bdrv_drain_all() will miss bs.
> +     */
>      if (bs) {
>          bs->blk = NULL;
>          blk->bs = NULL;
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index a04c852..d5de08b 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -396,8 +396,6 @@ struct BlockDriverState {
>      QTAILQ_ENTRY(BlockDriverState) node_list;
>      /* Device name is the name associated with the "drive" the guest sees */
>      char device_name[32];
> -    /* element of the list of "drives" the guest sees */
> -    QTAILQ_ENTRY(BlockDriverState) device_list;
>      QLIST_HEAD(, BdrvDirtyBitmap) dirty_bitmaps;
>      int refcnt;
>  
> -- 
> 1.9.3
>