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elf: Fix slow tls access after dlopen [BZ #19924]

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In short: __tls_get_addr checks the global generation counter and if
the current dtv is older then _dl_update_slotinfo updates dtv up to the
generation of the accessed module. So if the global generation is newer
than generation of the module then __tls_get_addr keeps hitting the
slow dtv update path. The dtv update path includes a number of checks
to see if any update is needed and this already causes measurable tls
access slow down after dlopen.

It may be possible to detect up-to-date dtv faster.  But if there are
many modules loaded (> TLS_SLOTINFO_SURPLUS) then this requires at
least walking the slotinfo list.

This patch tries to update the dtv to the global generation instead, so
after a dlopen the tls access slow path is only hit once.  The modules
with larger generation than the accessed one were not necessarily
synchronized before, so additional synchronization is needed.

This patch uses acquire/release synchronization when accessing the
generation counter.

Note: in the x86_64 version of dl-tls.c the generation is only loaded
once, since relaxed mo is not faster than acquire mo load.

I have not benchmarked this. Tested by Adhemerval Zanella on aarch64,
powerpc, sparc, x86 who reported that it fixes the performance issue
of bug 19924.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
This commit is contained in:
Szabolcs Nagy 2021-02-16 12:55:13 +00:00
parent 1493622f4f
commit d2123d6827
6 changed files with 74 additions and 66 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
elf/dl-close.c
View File

@ -703,7 +703,7 @@ _dl_close_worker (struct link_map *map, bool force)
if (__glibc_unlikely (newgen == 0))
_dl_fatal_printf ("TLS generation counter wrapped! Please report as described in "REPORT_BUGS_TO".\n");
/* Can be read concurrently. */
atomic_store_relaxed (&GL(dl_tls_generation), newgen);
atomic_store_release (&GL(dl_tls_generation), newgen);
if (tls_free_end == GL(dl_tls_static_used))
GL(dl_tls_static_used) = tls_free_start;

8
elf/dl-open.c
View File

@ -405,7 +405,7 @@ update_tls_slotinfo (struct link_map *new)
_dl_fatal_printf (N_("\
TLS generation counter wrapped! Please report this."));
/* Can be read concurrently. */
atomic_store_relaxed (&GL(dl_tls_generation), newgen);
atomic_store_release (&GL(dl_tls_generation), newgen);
/* We need a second pass for static tls data, because
_dl_update_slotinfo must not be run while calls to
@ -422,8 +422,8 @@ TLS generation counter wrapped! Please report this."));
now, but we can delay updating the DTV. */
imap->l_need_tls_init = 0;
#ifdef SHARED
/* Update the slot information data for at least the
generation of the DSO we are allocating data for. */
/* Update the slot information data for the current
generation. */
/* FIXME: This can terminate the process on memory
allocation failure. It is not possible to raise
@ -431,7 +431,7 @@ TLS generation counter wrapped! Please report this."));
_dl_update_slotinfo would have to be split into two
operations, similar to resize_scopes and update_scopes
above. This is related to bug 16134. */
_dl_update_slotinfo (imap->l_tls_modid);
_dl_update_slotinfo (imap->l_tls_modid, newgen);
#endif
dl_init_static_tls (imap);

6
elf/dl-reloc.c
View File

@ -112,11 +112,11 @@ _dl_try_allocate_static_tls (struct link_map *map, bool optional)
if (map->l_real->l_relocated)
{
#ifdef SHARED
/* Update the DTV of the current thread. Note: GL(dl_load_tls_lock)
is held here so normal load of the generation counter is valid. */
if (__builtin_expect (THREAD_DTV()[0].counter != GL(dl_tls_generation),
0))
/* Update the slot information data for at least the generation of
the DSO we are allocating data for. */
(void) _dl_update_slotinfo (map->l_tls_modid);
(void) _dl_update_slotinfo (map->l_tls_modid, GL(dl_tls_generation));
#endif
dl_init_static_tls (map);

117
elf/dl-tls.c
View File

@ -715,57 +715,57 @@ allocate_and_init (struct link_map *map)
struct link_map *
_dl_update_slotinfo (unsigned long int req_modid)
_dl_update_slotinfo (unsigned long int req_modid, size_t new_gen)
{
struct link_map *the_map = NULL;
dtv_t *dtv = THREAD_DTV ();
/* The global dl_tls_dtv_slotinfo array contains for each module
index the generation counter current when the entry was created.
This array never shrinks so that all module indices which were
valid at some time can be used to access it. Before the first
use of a new module index in this function the array was extended
appropriately. Access also does not have to be guarded against
modifications of the array. It is assumed that pointer-size
values can be read atomically even in SMP environments. It is
possible that other threads at the same time dynamically load
code and therefore add to the slotinfo list. This is a problem
since we must not pick up any information about incomplete work.
The solution to this is to ignore all dtv slots which were
created after the one we are currently interested. We know that
dynamic loading for this module is completed and this is the last
load operation we know finished. */
unsigned long int idx = req_modid;
struct dtv_slotinfo_list *listp = GL(dl_tls_dtv_slotinfo_list);
while (idx >= listp->len)
{
idx -= listp->len;
listp = listp->next;
}
if (dtv[0].counter < listp->slotinfo[idx].gen)
{
/* CONCURRENCY NOTES:
Here the dtv needs to be updated to new_gen generation count.
The global dl_tls_dtv_slotinfo_list array contains for each module
index the generation counter current when that entry was updated.
This array never shrinks so that all module indices which were
valid at some time can be used to access it. Concurrent loading
and unloading of modules can update slotinfo entries or extend
the array. The updates happen under the GL(dl_load_tls_lock) and
finish with the release store of the generation counter to
GL(dl_tls_generation) which is synchronized with the load of
new_gen in the caller. So updates up to new_gen are synchronized
but updates for later generations may not be.
This code may be called during TLS access when GL(dl_load_tls_lock)
is not held. In that case the user code has to synchronize with
dlopen and dlclose calls of relevant modules. A module m is
relevant if the generation of m <= new_gen and dlclose of m is
synchronized: a memory access here happens after the dlopen and
before the dlclose of relevant modules. The dtv entries for
relevant modules need to be updated, other entries can be
arbitrary.
Here we update the thread dtv from old_gen (== dtv[0].counter) to
new_gen generation. For this, each dtv[i] entry is either set to
an unallocated state (set), or left unmodified (nop). Where (set)
may resize the dtv first if modid i >= dtv[-1].counter. The rules
for the decision between (set) and (nop) are
This e.g. means that the first part of the slotinfo list can be
accessed race free, but the tail may be concurrently extended.
Similarly relevant slotinfo entries can be read race free, but
other entries are racy. However updating a non-relevant dtv
entry does not affect correctness. For a relevant module m,
max_modid >= modid of m. */
size_t new_gen = listp->slotinfo[idx].gen;
(1) If slotinfo entry i is concurrently updated then either (set)
or (nop) is valid: TLS access cannot use dtv[i] unless it is
synchronized with a generation > new_gen.
Otherwise, if the generation of slotinfo entry i is gen and the
loaded module for this entry is map then
(2) If gen <= old_gen then do (nop).
(3) If old_gen < gen <= new_gen then
(3.1) if map != 0 then (set)
(3.2) if map == 0 then either (set) or (nop).
Note that (1) cannot be reliably detected, but since both actions
are valid it does not have to be. Only (2) and (3.1) cases need
to be distinguished for which relaxed mo access of gen and map is
enough: their value is synchronized when it matters.
Note that a relaxed mo load may give an out-of-thin-air value since
it is used in decisions that can affect concurrent stores. But this
should only happen if the OOTA value causes UB that justifies the
concurrent store of the value. This is not expected to be an issue
in practice. */
struct dtv_slotinfo_list *listp = GL(dl_tls_dtv_slotinfo_list);
if (dtv[0].counter < new_gen)
{
size_t total = 0;
size_t max_modid = atomic_load_relaxed (&GL(dl_tls_max_dtv_idx));
assert (max_modid >= req_modid);
@ -778,31 +778,33 @@ _dl_update_slotinfo (unsigned long int req_modid)
{
size_t modid = total + cnt;
/* Later entries are not relevant. */
/* Case (1) for all later modids. */
if (modid > max_modid)
break;
size_t gen = atomic_load_relaxed (&listp->slotinfo[cnt].gen);
/* Case (1). */
if (gen > new_gen)
/* Not relevant. */
continue;
/* If the entry is older than the current dtv layout we
know we don't have to handle it. */
/* Case (2) or (1). */
if (gen <= dtv[0].counter)
continue;
/* Case (3) or (1). */
/* If there is no map this means the entry is empty. */
struct link_map *map
= atomic_load_relaxed (&listp->slotinfo[cnt].map);
/* Check whether the current dtv array is large enough. */
if (dtv[-1].counter < modid)
{
/* Case (3.2) or (1). */
if (map == NULL)
continue;
/* Resize the dtv. */
/* Resizing the dtv aborts on failure: bug 16134. */
dtv = _dl_resize_dtv (dtv, max_modid);
assert (modid <= dtv[-1].counter);
@ -813,7 +815,7 @@ _dl_update_slotinfo (unsigned long int req_modid)
}
/* If there is currently memory allocate for this
dtv entry free it. */
dtv entry free it. Note: this is not AS-safe. */
/* XXX Ideally we will at some point create a memory
pool. */
free (dtv[modid].pointer.to_free);
@ -908,9 +910,9 @@ tls_get_addr_tail (GET_ADDR_ARGS, dtv_t *dtv, struct link_map *the_map)
static struct link_map *
__attribute_noinline__
update_get_addr (GET_ADDR_ARGS)
update_get_addr (GET_ADDR_ARGS, size_t gen)
{
struct link_map *the_map = _dl_update_slotinfo (GET_ADDR_MODULE);
struct link_map *the_map = _dl_update_slotinfo (GET_ADDR_MODULE, gen);
dtv_t *dtv = THREAD_DTV ();
void *p = dtv[GET_ADDR_MODULE].pointer.val;
@ -940,12 +942,17 @@ __tls_get_addr (GET_ADDR_ARGS)
dtv_t *dtv = THREAD_DTV ();
/* Update is needed if dtv[0].counter < the generation of the accessed
module. The global generation counter is used here as it is easier
to check. Synchronization for the relaxed MO access is guaranteed
by user code, see CONCURRENCY NOTES in _dl_update_slotinfo. */
module, but the global generation counter is easier to check (which
must be synchronized up to the generation of the accessed module by
user code doing the TLS access so relaxed mo read is enough). */
size_t gen = atomic_load_relaxed (&GL(dl_tls_generation));
if (__glibc_unlikely (dtv[0].counter != gen))
return update_get_addr (GET_ADDR_PARAM);
{
/* Update DTV up to the global generation, see CONCURRENCY NOTES
in _dl_update_slotinfo. */
gen = atomic_load_acquire (&GL(dl_tls_generation));
return update_get_addr (GET_ADDR_PARAM, gen);
}
void *p = dtv[GET_ADDR_MODULE].pointer.val;

3
sysdeps/generic/ldsodefs.h
View File

@ -1251,7 +1251,8 @@ extern void _dl_add_to_slotinfo (struct link_map *l, bool do_add)
/* Update slot information data for at least the generation of the
module with the given index. */
extern struct link_map *_dl_update_slotinfo (unsigned long int req_modid)
extern struct link_map *_dl_update_slotinfo (unsigned long int req_modid,
size_t gen)
attribute_hidden;
/* Look up the module's TLS block as for __tls_get_addr,

4
sysdeps/x86_64/dl-tls.c
View File

@ -40,9 +40,9 @@ __tls_get_addr_slow (GET_ADDR_ARGS)
{
dtv_t *dtv = THREAD_DTV ();
size_t gen = atomic_load_relaxed (&GL(dl_tls_generation));
size_t gen = atomic_load_acquire (&GL(dl_tls_generation));
if (__glibc_unlikely (dtv[0].counter != gen))
return update_get_addr (GET_ADDR_PARAM);
return update_get_addr (GET_ADDR_PARAM, gen);
return tls_get_addr_tail (GET_ADDR_PARAM, dtv, NULL);
}