Re: [Qemu-devel] [PATCH 4/7] address_space_read: address_space_to_flatview needs RCU lock

[Alexey Kardashevskiy]

On 05/03/18 19:36, Paolo Bonzini wrote:
> address_space_read is calling address_space_to_flatview but it can
> be called outside the RCU lock.  To fix it, push the rcu_read_lock/unlock
> pair up from flatview_read_full to address_space_read's constant size
> fast path and address_space_read_full.
> 
> Cc: address@hidden
> Signed-off-by: Paolo Bonzini <address@hidden>

Reviewed-by: Alexey Kardashevskiy <address@hidden>

> ---
>  exec.c                | 44 ++++++++++++++++++++++++++++----------------
>  include/exec/memory.h | 25 ++++++++++---------------
>  2 files changed, 38 insertions(+), 31 deletions(-)
> 
> diff --git a/exec.c b/exec.c
> index 0b74b58d45..55b7452bd7 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -2612,6 +2612,8 @@ static const MemoryRegionOps watch_mem_ops = {
>      },
>  };
>  
> +static MemTxResult flatview_read(FlatView *fv, hwaddr addr,
> +                                      MemTxAttrs attrs, uint8_t *buf, int 
> len);
>  static MemTxResult flatview_write(FlatView *fv, hwaddr addr, MemTxAttrs 
> attrs,
>                                    const uint8_t *buf, int len);
>  static bool flatview_access_valid(FlatView *fv, hwaddr addr, int len,
> @@ -3161,24 +3163,18 @@ MemTxResult flatview_read_continue(FlatView *fv, 
> hwaddr addr,
>      return result;
>  }
>  
> -MemTxResult flatview_read_full(FlatView *fv, hwaddr addr,
> -                               MemTxAttrs attrs, uint8_t *buf, int len)
> +/* Called from RCU critical section.  */
> +static MemTxResult flatview_read(FlatView *fv, hwaddr addr,
> +                                 MemTxAttrs attrs, uint8_t *buf, int len)
>  {
>      hwaddr l;
>      hwaddr addr1;
>      MemoryRegion *mr;
> -    MemTxResult result = MEMTX_OK;
> -
> -    if (len > 0) {
> -        rcu_read_lock();
> -        l = len;
> -        mr = flatview_translate(fv, addr, &addr1, &l, false);
> -        result = flatview_read_continue(fv, addr, attrs, buf, len,
> -                                        addr1, l, mr);
> -        rcu_read_unlock();
> -    }
>  
> -    return result;
> +    l = len;
> +    mr = flatview_translate(fv, addr, &addr1, &l, false);
> +    return flatview_read_continue(fv, addr, attrs, buf, len,
> +                                  addr1, l, mr);
>  }
>  
>  static MemTxResult flatview_rw(FlatView *fv, hwaddr addr, MemTxAttrs attrs,
> @@ -3199,6 +3195,22 @@ MemTxResult address_space_rw(AddressSpace *as, hwaddr 
> addr,
>                         addr, attrs, buf, len, is_write);
>  }
>  
> +MemTxResult address_space_read_full(AddressSpace *as, hwaddr addr,
> +                                    MemTxAttrs attrs, uint8_t *buf, int len)
> +{
> +    MemTxResult result = MEMTX_OK;
> +    FlatView *fv;
> +
> +    if (len > 0) {
> +        rcu_read_lock();
> +        fv = address_space_to_flatview(as);
> +        result = flatview_read(fv, addr, attrs, buf, len);
> +        rcu_read_unlock();
> +    }
> +
> +    return result;
> +}
> +
>  MemTxResult address_space_write(AddressSpace *as, hwaddr addr,
>                                  MemTxAttrs attrs,
>                                  const uint8_t *buf, int len)
> diff --git a/include/exec/memory.h b/include/exec/memory.h
> index 6c8e394675..54ff81fbd1 100644
> --- a/include/exec/memory.h
> +++ b/include/exec/memory.h
> @@ -1894,13 +1894,12 @@ void address_space_unmap(AddressSpace *as, void 
> *buffer, hwaddr len,
>  
>  
>  /* Internal functions, part of the implementation of address_space_read.  */
> +MemTxResult address_space_read_full(AddressSpace *as, hwaddr addr,
> +                                    MemTxAttrs attrs, uint8_t *buf, int len);
>  MemTxResult flatview_read_continue(FlatView *fv, hwaddr addr,
>                                     MemTxAttrs attrs, uint8_t *buf,
>                                     int len, hwaddr addr1, hwaddr l,
>                                     MemoryRegion *mr);
> -
> -MemTxResult flatview_read_full(FlatView *fv, hwaddr addr,
> -                               MemTxAttrs attrs, uint8_t *buf, int len);
>  void *qemu_map_ram_ptr(RAMBlock *ram_block, ram_addr_t addr);
>  
>  static inline bool memory_access_is_direct(MemoryRegion *mr, bool is_write)
> @@ -1919,25 +1918,28 @@ static inline bool 
> memory_access_is_direct(MemoryRegion *mr, bool is_write)
>   *
>   * Return a MemTxResult indicating whether the operation succeeded
>   * or failed (eg unassigned memory, device rejected the transaction,
> - * IOMMU fault).
> + * IOMMU fault).  Called within RCU critical section.
>   *
> - * @fv: #FlatView to be accessed
> + * @as: #AddressSpace to be accessed
>   * @addr: address within that address space
>   * @attrs: memory transaction attributes
>   * @buf: buffer with the data transferred
>   */
>  static inline __attribute__((__always_inline__))
> -MemTxResult flatview_read(FlatView *fv, hwaddr addr, MemTxAttrs attrs,
> -                          uint8_t *buf, int len)
> +MemTxResult address_space_read(AddressSpace *as, hwaddr addr,
> +                               MemTxAttrs attrs, uint8_t *buf,
> +                               int len)
>  {
>      MemTxResult result = MEMTX_OK;
>      hwaddr l, addr1;
>      void *ptr;
>      MemoryRegion *mr;
> +    FlatView *fv;
>  
>      if (__builtin_constant_p(len)) {
>          if (len) {
>              rcu_read_lock();
> +            fv = address_space_to_flatview(as);
>              l = len;
>              mr = flatview_translate(fv, addr, &addr1, &l, false);
>              if (len == l && memory_access_is_direct(mr, false)) {
> @@ -1950,18 +1952,11 @@ MemTxResult flatview_read(FlatView *fv, hwaddr addr, 
> MemTxAttrs attrs,
>              rcu_read_unlock();
>          }
>      } else {
> -        result = flatview_read_full(fv, addr, attrs, buf, len);
> +        result = address_space_read_full(as, addr, attrs, buf, len);
>      }
>      return result;
>  }
>  
> -static inline MemTxResult address_space_read(AddressSpace *as, hwaddr addr,
> -                                             MemTxAttrs attrs, uint8_t *buf,
> -                                             int len)
> -{
> -    return flatview_read(address_space_to_flatview(as), addr, attrs, buf, 
> len);
> -}
> -
>  /**
>   * address_space_read_cached: read from a cached RAM region
>   *
> 


-- 
Alexey