Re: [PATCH 13/16] libdw: Make libdw_findcu thread-safe

[Heather McIntyre <hsm2@rice.edu>]

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Hi Mark,

For the call to __libdw_intern_next_unit in __libdw_findcu, I have updated
the locking to the per struct drawf lock. Although I have not encountered a
data race report regarding the call to __libdw_intern_next_unit in
dwarf_formref_die, I have also placed the updated locking there as well.

On Thu, Oct 12, 2023 at 5:03 PM Mark Wielaard <mark@klomp.org> wrote:

> Hi Heather,
>
> On Tue, Oct 10, 2023 at 03:42:57PM +0200, Mark Wielaard wrote:
> > From: Heather McIntyre <hsm2@rice.edu>
> >
> >       * libdw/libdw_findcu.c (findcu_cb): Use eu_tsearch.
> >       (__libdw_findcu): Use eu_tfind and next_tucu_offset_lock.
> >       (__libdw_findcu_addr): Use eu_tfind.
> >       (__libdw_find_split_dbg_addr): Likewise.
> >
> > Signed-off-by: Heather S. McIntyre <hsm2@rice.edu>
> > Signed-off-by: Mark Wielaard <mark@klomp.org>
> > ---
> >  libdw/libdw_findcu.c | 54 ++++++++++++++++++++++++++++----------------
> >  1 file changed, 35 insertions(+), 19 deletions(-)
> >
> > diff --git a/libdw/libdw_findcu.c b/libdw/libdw_findcu.c
> > index ed744231..e546fb0f 100644
> > --- a/libdw/libdw_findcu.c
> > +++ b/libdw/libdw_findcu.c
> > @@ -32,9 +32,13 @@
> >  #endif
> >
> >  #include <assert.h>
> > -#include <search.h>
> > +#include <eu-search.h>
> >  #include "libdwP.h"
> >
> > +/* __libdw_findcu modifies "&dbg->next_tu_offset :
> &dbg->next_cu_offset".
> > +   May read or write, so mutual exclusion is enforced to prevent a
> race. */
> > +rwlock_define(static, next_tucu_offset_lock);
> > +
>
> Could this be a per struct Dwarf lock?
>
> >  static int
> >  findcu_cb (const void *arg1, const void *arg2)
> >  {
> > @@ -213,7 +217,7 @@ __libdw_intern_next_unit (Dwarf *dbg, bool
> debug_types)
> >
> >      Dwarf_Sig8_Hash_insert (&dbg->sig8_hash, unit_id8, newp);
> >
> >    /* Add the new entry to the search tree.  */
> > -  if (tsearch (newp, tree, findcu_cb) == NULL)
> > +  if (eu_tsearch (newp, tree, findcu_cb) == NULL)
> >      {
> >        /* Something went wrong.  Undo the operation.  */
> >        *offsetp = oldoff;
>
> This is OK since when __libdw_intern_next_unit is called from
> __libdw_findcu the next_tucu_offset_lock is held.
>
> But there is also a call to __libdw_intern_next_unit from
> dwarf_formref_die. So there should also be a lock there?
>
> > @@ -234,28 +238,40 @@ __libdw_findcu (Dwarf *dbg, Dwarf_Off start, bool
> v4_debug_types)
> >
> >    /* Maybe we already know that CU.  */
> >    struct Dwarf_CU fake = { .start = start, .end = 0 };
> > -  struct Dwarf_CU **found = tfind (&fake, tree, findcu_cb);
> > +  struct Dwarf_CU **found = eu_tfind (&fake, tree, findcu_cb);
> > +  struct Dwarf_CU *result = NULL;
> > +
> >    if (found != NULL)
> >      return *found;
> >
> > -  if (start < *next_offset)
> > -    {
> > -      __libdw_seterrno (DWARF_E_INVALID_DWARF);
> > -      return NULL;
> > -    }
> > +  rwlock_wrlock(next_tucu_offset_lock);
> >
> > -  /* No.  Then read more CUs.  */
> > -  while (1)
> > +  if (start < *next_offset)
> > +    __libdw_seterrno (DWARF_E_INVALID_DWARF);
> > +  else
> >      {
> > -      struct Dwarf_CU *newp = __libdw_intern_next_unit (dbg,
> v4_debug_types);
> > -      if (newp == NULL)
> > -     return NULL;
> > +      /* No.  Then read more CUs.  */
> > +      while (1)
> > +     {
> > +       struct Dwarf_CU *newp = __libdw_intern_next_unit (dbg,
> > +
>  v4_debug_types);
> > +       if (newp == NULL)
> > +         {
> > +           result = NULL;
> > +           break;
> > +         }
> >
> > -      /* Is this the one we are looking for?  */
> > -      if (start < *next_offset || start == newp->start)
> > -     return newp;
> > +       /* Is this the one we are looking for?  */
> > +       if (start < *next_offset || start == newp->start)
> > +         {
> > +           result = newp;
> > +           break;
> > +         }
> > +     }
> >      }
> > -  /* NOTREACHED */
> > +
> > +  rwlock_unlock(next_tucu_offset_lock);
> > +  return result;
> >  }
>
> This look OK.
>
> >  struct Dwarf_CU *
> > @@ -283,7 +299,7 @@ __libdw_findcu_addr (Dwarf *dbg, void *addr)
> >      return NULL;
> >
> >    struct Dwarf_CU fake = { .start = start, .end = 0 };
> > -  struct Dwarf_CU **found = tfind (&fake, tree, findcu_cb);
> > +  struct Dwarf_CU **found = eu_tfind (&fake, tree, findcu_cb);
> >
> >    if (found != NULL)
> >      return *found;
> > @@ -298,7 +314,7 @@ __libdw_find_split_dbg_addr (Dwarf *dbg, void *addr)
> >    /* XXX Assumes split DWARF only has CUs in main IDX_debug_info.  */
> >    Elf_Data fake_data = { .d_buf = addr, .d_size = 0 };
> >    Dwarf fake = { .sectiondata[IDX_debug_info] = &fake_data };
> > -  Dwarf **found = tfind (&fake, &dbg->split_tree, __libdw_finddbg_cb);
> > +  Dwarf **found = eu_tfind (&fake, &dbg->split_tree,
> __libdw_finddbg_cb);
> >
> >    if (found != NULL)
> >      return *found;
>
> OK.
>
> Thanks,
>
> Mark
>