[lttng-dev] [PATCH lttng-tools 1/3] Fix: Filter bytecode alloc buffer size must be a power of 2

[Mathieu Desnoyers]

* Christian Babeux (christian.babeux at efficios.com) wrote:
> The current allocation policy for the filter bytecode buffer is to double
> the size each time the underlying buffer can no longer contain the entire
> bytecode plus padding.
> 
> In some cases, the initial allocation length is not a multiple of 2, thus
> possibly leading to odd-looking allocation size each time the buffer size
> is doubled.
> 
> Signed-off-by: Christian Babeux <christian.babeux at efficios.com>

Acked-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>

> ---
>  .../filter/filter-visitor-generate-bytecode.c      | 41 +++++++++++++++++++++-
>  1 file changed, 40 insertions(+), 1 deletion(-)
> 
> diff --git a/src/lib/lttng-ctl/filter/filter-visitor-generate-bytecode.c b/src/lib/lttng-ctl/filter/filter-visitor-generate-bytecode.c
> index 36d35c5..71da21c 100644
> --- a/src/lib/lttng-ctl/filter/filter-visitor-generate-bytecode.c
> +++ b/src/lib/lttng-ctl/filter/filter-visitor-generate-bytecode.c
> @@ -38,6 +38,45 @@ static
>  int recursive_visit_gen_bytecode(struct filter_parser_ctx *ctx,
>  		struct ir_op *node);
>  
> +static inline int fls(unsigned int x)
> +{
> +	int r = 32;
> +
> +	if (!x)
> +		return 0;
> +	if (!(x & 0xFFFF0000U)) {
> +		x <<= 16;
> +		r -= 16;
> +	}
> +	if (!(x & 0xFF000000U)) {
> +		x <<= 8;
> +		r -= 8;
> +	}
> +	if (!(x & 0xF0000000U)) {
> +		x <<= 4;
> +		r -= 4;
> +	}
> +	if (!(x & 0xC0000000U)) {
> +		x <<= 2;
> +		r -= 2;
> +	}
> +	if (!(x & 0x80000000U)) {
> +		x <<= 1;
> +		r -= 1;
> +	}
> +	return r;
> +}
> +
> +static inline int get_count_order(unsigned int count)
> +{
> +	int order;
> +
> +	order = fls(count) - 1;
> +	if (count & (count - 1))
> +		order++;
> +	return order;
> +}
> +
>  static
>  int bytecode_init(struct lttng_filter_bytecode_alloc **fb)
>  {
> @@ -58,7 +97,7 @@ int32_t bytecode_reserve(struct lttng_filter_bytecode_alloc **fb, uint32_t align
>  
>  	if ((*fb)->b.len + padding + len > (*fb)->alloc_len) {
>  		uint32_t new_len =
> -			max_t(uint32_t, (*fb)->b.len + padding + len,
> +			max_t(uint32_t, 1U << get_count_order((*fb)->b.len + padding + len),
>  				(*fb)->alloc_len << 1);
>  		uint32_t old_len = (*fb)->alloc_len;
>  
> -- 
> 1.7.11.4
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com