[PATCH v2] elf: Use C11 atomics on _dl_mcount

[H.J. Lu]

On Mon, Sep 5, 2022 at 12:14 PM Adhemerval Zanella via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> All atomic operation are counters, so relaxed MO should be suffice.
>
> Checked on x86_64-linux-gnu.
> ---
>  elf/dl-profile.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/elf/dl-profile.c b/elf/dl-profile.c
> index ec57e3a965..67a356bf66 100644
> --- a/elf/dl-profile.c
> +++ b/elf/dl-profile.c
> @@ -548,7 +548,7 @@ _dl_mcount (ElfW(Addr) frompc, ElfW(Addr) selfpc)
>               size_t newfromidx;
>               to_index = (data[narcs].self_pc
>                           / (HASHFRACTION * sizeof (*tos)));
> -             newfromidx = catomic_exchange_and_add (&fromidx, 1) + 1;
> +             newfromidx = atomic_fetch_add_relaxed (&fromidx, 1) + 1;
>               froms[newfromidx].here = &data[narcs];
>               froms[newfromidx].link = tos[to_index];
>               tos[to_index] = newfromidx;
> @@ -558,14 +558,14 @@ _dl_mcount (ElfW(Addr) frompc, ElfW(Addr) selfpc)
>           /* If we still have no entry stop searching and insert.  */
>           if (*topcindex == 0)
>             {
> -             unsigned int newarc = catomic_exchange_and_add (narcsp, 1);
> +             unsigned int newarc = atomic_fetch_add_relaxed (narcsp, 1) + 1;

Shouldn't it be

unsigned int newarc = atomic_fetch_add_relaxed (narcsp, 1);

>
>               /* In rare cases it could happen that all entries in FROMS are
>                  occupied.  So we cannot count this anymore.  */
>               if (newarc >= fromlimit)
>                 goto done;
>
> -             *topcindex = catomic_exchange_and_add (&fromidx, 1) + 1;
> +             *topcindex = atomic_fetch_add_acquire (&fromidx, 1) + 1;
>               fromp = &froms[*topcindex];
>
>               fromp->here = &data[newarc];
> @@ -573,7 +573,7 @@ _dl_mcount (ElfW(Addr) frompc, ElfW(Addr) selfpc)
>               data[newarc].self_pc = selfpc;
>               data[newarc].count = 0;
>               fromp->link = 0;
> -             catomic_increment (&narcs);
> +             atomic_fetch_add_relaxed (&narcs, 1);
>
>               break;
>             }
> @@ -586,7 +586,7 @@ _dl_mcount (ElfW(Addr) frompc, ElfW(Addr) selfpc)
>      }
>
>    /* Increment the counter.  */
> -  catomic_increment (&fromp->here->count);
> +  atomic_fetch_add_relaxed (&fromp->here->count, 1);
>
>   done:
>    ;
> --
> 2.34.1
>


-- 
H.J.