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[v4,BZ,18034,AArch64] Lazy TLSDESC relocation data race fix

Message ID 557EB75E.6090002@arm.com
State Committed
Headers show

Commit Message

Szabolcs Nagy June 15, 2015, 11:30 a.m. UTC
On 03/06/15 11:40, Torvald Riegel wrote:
> On Mon, 2015-06-01 at 11:25 +0100, Szabolcs Nagy wrote:
>> i added a comment to the _dl_tlsdesc_resolve_early_return_p
>> call in aarch64 tlsdesc.c about the retry loop.
> 
> That's good, but it would have been better if you could have briefly
> pointed out that this relates to mo_relaxed loads in
> _dl_tlsdesc_resolve_early_return_p.  And/or added a comment there saying
> that the mo_relaxed loads are fine because of this retry loop in the
> caller(s).
> 
>> -  const ElfW(Rela) *reloc = td->arg;
>> +  const ElfW(Rela) *reloc = atomic_load_relaxed (&td->arg);
> 
> Good change.  Can you add a brief comment saying why the mo_relaxed load
> is sufficient?  IIRC, this is because of the acquire loads done by the
> caller.
> 
> OK with those changes.
> 

updated the comments in the code and the description:


Lazy TLSDESC initialization needs to be synchronized with concurrent TLS
accesses.  The TLS descriptor contains a function pointer (entry) and an
argument that is accessed from the entry function.  With lazy initialization
the first call to the entry function updates the entry and the argument to
their final value.  A final entry function must make sure that it accesses an
initialized argument, this needs synchronization on systems with weak memory
ordering otherwise the writes of the first call can be observed out of order.

There are at least two issues with the current code:

tlsdesc.c (i386, x86_64, arm, aarch64) uses volatile memory accesses on the
write side (in the initial entry function) instead of C11 atomics.

And on systems with weak memory ordering (arm, aarch64) the read side
synchronization is missing from the final entry functions (dl-tlsdesc.S).

This patch only deals with aarch64.

* Write side:

Volatile accesses were replaced with C11 relaxed atomics, and a release
store was used for the initialization of entry so the read side can
synchronize with it.

* Read side:

TLS access generated by the compiler and an entry function code is roughly

  ldr x1, [x0]    // load the entry
  blr x1          // call it

entryfunc:
  ldr x0, [x0,#8] // load the arg
  ret

Various alternatives were considered to force the ordering in the entry
function between the two loads:

(1) barrier

entryfunc:
  dmb ishld
  ldr x0, [x0,#8]

(2) address dependency (if the address of the second load depends on the
result of the first one the ordering is guaranteed):

entryfunc:
  ldr x1,[x0]
  and x1,x1,#8
  orr x1,x1,#8
  ldr x0,[x0,x1]

(3) load-acquire (ARMv8 instruction that is ordered before subsequent
loads and stores)

entryfunc:
  ldar xzr,[x0]
  ldr x0,[x0,#8]

Option (1) is the simplest but slowest (note: this runs at every TLS
access), options (2) and (3) do one extra load from [x0] (same address
loads are ordered so it happens-after the load on the call site),
option (2) clobbers x1 which is problematic because existing gcc does
not expect that, so approach (3) was chosen.

A new _dl_tlsdesc_return_lazy entry function was introduced for lazily
relocated static TLS, so non-lazy static TLS can avoid the synchronization
cost.

Changelog:

2015-06-15  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	[BZ #18034]
	* sysdeps/aarch64/dl-tlsdesc.h (_dl_tlsdesc_return_lazy): Declare.
	* sysdeps/aarch64/dl-tlsdesc.S (_dl_tlsdesc_return_lazy): Define.
	(_dl_tlsdesc_undefweak): Guarantee TLSDESC entry and argument load-load
	ordering using ldar.
	(_dl_tlsdesc_dynamic): Likewise.
	(_dl_tlsdesc_return_lazy): Likewise.
	* sysdeps/aarch64/tlsdesc.c (_dl_tlsdesc_resolve_rela_fixup): Use
	relaxed atomics instead of volatile and synchronize with release store.
	(_dl_tlsdesc_resolve_hold_fixup): Use relaxed atomics instead of
	volatile.
	* elf/tlsdeschtab.h (_dl_tlsdesc_resolve_early_return_p): Likewise.

Comments

Torvald Riegel June 15, 2015, 5:35 p.m. UTC | #1
On Mon, 2015-06-15 at 12:30 +0100, Szabolcs Nagy wrote:
> On 03/06/15 11:40, Torvald Riegel wrote:
> > On Mon, 2015-06-01 at 11:25 +0100, Szabolcs Nagy wrote:
> >> i added a comment to the _dl_tlsdesc_resolve_early_return_p
> >> call in aarch64 tlsdesc.c about the retry loop.
> > 
> > That's good, but it would have been better if you could have briefly
> > pointed out that this relates to mo_relaxed loads in
> > _dl_tlsdesc_resolve_early_return_p.  And/or added a comment there saying
> > that the mo_relaxed loads are fine because of this retry loop in the
> > caller(s).
> > 
> >> -  const ElfW(Rela) *reloc = td->arg;
> >> +  const ElfW(Rela) *reloc = atomic_load_relaxed (&td->arg);
> > 
> > Good change.  Can you add a brief comment saying why the mo_relaxed load
> > is sufficient?  IIRC, this is because of the acquire loads done by the
> > caller.
> > 
> > OK with those changes.
> > 
> 
> updated the comments in the code and the description:

OK.  Thanks!
Joseph Myers June 17, 2015, 5:21 p.m. UTC | #2
If your commit fully fixes bug 18034, please close it as fixed (as well as 
updating the patchwork state for your patches).
Szabolcs Nagy June 22, 2015, 8:29 a.m. UTC | #3
On 17/06/15 18:21, Joseph Myers wrote:
> If your commit fully fixes bug 18034, please close it as fixed (as well as 
> updating the patchwork state for your patches).
> 

done

patchwork feels a bit redundant manual work..
especially when it cant follow the various iterations of the same patch
diff mbox

Patch

diff --git a/elf/tlsdeschtab.h b/elf/tlsdeschtab.h
index d13b4e5..fb0eb88 100644
--- a/elf/tlsdeschtab.h
+++ b/elf/tlsdeschtab.h
@@ -20,6 +20,8 @@ 
 #ifndef TLSDESCHTAB_H
 # define TLSDESCHTAB_H 1
 
+#include <atomic.h>
+
 # ifdef SHARED
 
 #  include <inline-hashtab.h>
@@ -138,17 +140,17 @@  _dl_make_tlsdesc_dynamic (struct link_map *map, size_t ti_offset)
 static int
 _dl_tlsdesc_resolve_early_return_p (struct tlsdesc volatile *td, void *caller)
 {
-  if (caller != td->entry)
+  if (caller != atomic_load_relaxed (&td->entry))
     return 1;
 
   __rtld_lock_lock_recursive (GL(dl_load_lock));
-  if (caller != td->entry)
+  if (caller != atomic_load_relaxed (&td->entry))
     {
       __rtld_lock_unlock_recursive (GL(dl_load_lock));
       return 1;
     }
 
-  td->entry = _dl_tlsdesc_resolve_hold;
+  atomic_store_relaxed (&td->entry, _dl_tlsdesc_resolve_hold);
 
   return 0;
 }
diff --git a/sysdeps/aarch64/dl-tlsdesc.S b/sysdeps/aarch64/dl-tlsdesc.S
index be9b9b3..c7adf79 100644
--- a/sysdeps/aarch64/dl-tlsdesc.S
+++ b/sysdeps/aarch64/dl-tlsdesc.S
@@ -79,6 +79,29 @@  _dl_tlsdesc_return:
 	cfi_endproc
 	.size	_dl_tlsdesc_return, .-_dl_tlsdesc_return
 
+	/* Same as _dl_tlsdesc_return but with synchronization for
+	   lazy relocation.
+	   Prototype:
+	   _dl_tlsdesc_return_lazy (tlsdesc *) ;
+	 */
+	.hidden _dl_tlsdesc_return_lazy
+	.global	_dl_tlsdesc_return_lazy
+	.type	_dl_tlsdesc_return_lazy,%function
+	cfi_startproc
+	.align 2
+_dl_tlsdesc_return_lazy:
+	/* The ldar here happens after the load from [x0] at the call site
+	   (that is generated by the compiler as part of the TLS access ABI),
+	   so it reads the same value (this function is the final value of
+	   td->entry) and thus it synchronizes with the release store to
+	   td->entry in _dl_tlsdesc_resolve_rela_fixup ensuring that the load
+	   from [x0,#8] here happens after the initialization of td->arg.  */
+	ldar	xzr, [x0]
+	ldr	x0, [x0, #8]
+	RET
+	cfi_endproc
+	.size	_dl_tlsdesc_return_lazy, .-_dl_tlsdesc_return_lazy
+
 	/* Handler for undefined weak TLS symbols.
 	   Prototype:
 	   _dl_tlsdesc_undefweak (tlsdesc *);
@@ -96,6 +119,13 @@  _dl_tlsdesc_return:
 _dl_tlsdesc_undefweak:
 	str	x1, [sp, #-16]!
 	cfi_adjust_cfa_offset(16)
+	/* The ldar here happens after the load from [x0] at the call site
+	   (that is generated by the compiler as part of the TLS access ABI),
+	   so it reads the same value (this function is the final value of
+	   td->entry) and thus it synchronizes with the release store to
+	   td->entry in _dl_tlsdesc_resolve_rela_fixup ensuring that the load
+	   from [x0,#8] here happens after the initialization of td->arg.  */
+	ldar	xzr, [x0]
 	ldr	x0, [x0, #8]
 	mrs	x1, tpidr_el0
 	sub	x0, x0, x1
@@ -152,6 +182,13 @@  _dl_tlsdesc_dynamic:
 	stp	x3,  x4, [sp, #32+16*1]
 
 	mrs	x4, tpidr_el0
+	/* The ldar here happens after the load from [x0] at the call site
+	   (that is generated by the compiler as part of the TLS access ABI),
+	   so it reads the same value (this function is the final value of
+	   td->entry) and thus it synchronizes with the release store to
+	   td->entry in _dl_tlsdesc_resolve_rela_fixup ensuring that the load
+	   from [x0,#8] here happens after the initialization of td->arg.  */
+	ldar	xzr, [x0]
 	ldr	x1, [x0,#8]
 	ldr	x0, [x4]
 	ldr	x3, [x1,#16]
diff --git a/sysdeps/aarch64/dl-tlsdesc.h b/sysdeps/aarch64/dl-tlsdesc.h
index 7a1285e..e6c0078 100644
--- a/sysdeps/aarch64/dl-tlsdesc.h
+++ b/sysdeps/aarch64/dl-tlsdesc.h
@@ -46,6 +46,9 @@  extern ptrdiff_t attribute_hidden
 _dl_tlsdesc_return (struct tlsdesc *);
 
 extern ptrdiff_t attribute_hidden
+_dl_tlsdesc_return_lazy (struct tlsdesc *);
+
+extern ptrdiff_t attribute_hidden
 _dl_tlsdesc_undefweak (struct tlsdesc *);
 
 extern ptrdiff_t attribute_hidden
diff --git a/sysdeps/aarch64/tlsdesc.c b/sysdeps/aarch64/tlsdesc.c
index 4821f8c..9f3ff9b 100644
--- a/sysdeps/aarch64/tlsdesc.c
+++ b/sysdeps/aarch64/tlsdesc.c
@@ -25,6 +25,7 @@ 
 #include <dl-tlsdesc.h>
 #include <dl-unmap-segments.h>
 #include <tlsdeschtab.h>
+#include <atomic.h>
 
 /* The following functions take an entry_check_offset argument.  It's
    computed by the caller as an offset between its entry point and the
@@ -39,11 +40,15 @@ 
 
 void
 attribute_hidden
-_dl_tlsdesc_resolve_rela_fixup (struct tlsdesc volatile *td,
-				struct link_map *l)
+_dl_tlsdesc_resolve_rela_fixup (struct tlsdesc *td, struct link_map *l)
 {
-  const ElfW(Rela) *reloc = td->arg;
+  const ElfW(Rela) *reloc = atomic_load_relaxed (&td->arg);
 
+  /* After GL(dl_load_lock) is grabbed only one caller can see td->entry in
+     initial state in _dl_tlsdesc_resolve_early_return_p, other concurrent
+     callers will return and retry calling td->entry.  The updated td->entry
+     synchronizes with the single writer so all read accesses here can use
+     relaxed order.  */
   if (_dl_tlsdesc_resolve_early_return_p
       (td, (void*)(D_PTR (l, l_info[ADDRIDX (DT_TLSDESC_PLT)]) + l->l_addr)))
     return;
@@ -86,8 +91,10 @@  _dl_tlsdesc_resolve_rela_fixup (struct tlsdesc volatile *td,
 
   if (!sym)
     {
-      td->arg = (void*) reloc->r_addend;
-      td->entry = _dl_tlsdesc_undefweak;
+      atomic_store_relaxed (&td->arg, (void *) reloc->r_addend);
+      /* This release store synchronizes with the ldar acquire load
+	 instruction in _dl_tlsdesc_undefweak.  */
+      atomic_store_release (&td->entry, _dl_tlsdesc_undefweak);
     }
   else
     {
@@ -96,16 +103,22 @@  _dl_tlsdesc_resolve_rela_fixup (struct tlsdesc volatile *td,
 #  else
       if (!TRY_STATIC_TLS (l, result))
 	{
-	  td->arg = _dl_make_tlsdesc_dynamic (result, sym->st_value
+	  void *p = _dl_make_tlsdesc_dynamic (result, sym->st_value
 					      + reloc->r_addend);
-	  td->entry = _dl_tlsdesc_dynamic;
+	  atomic_store_relaxed (&td->arg, p);
+	  /* This release store synchronizes with the ldar acquire load
+	     instruction in _dl_tlsdesc_dynamic.  */
+	  atomic_store_release (&td->entry, _dl_tlsdesc_dynamic);
 	}
       else
 #  endif
 	{
-	  td->arg = (void*) (sym->st_value + result->l_tls_offset
+	  void *p = (void*) (sym->st_value + result->l_tls_offset
 			     + reloc->r_addend);
-	  td->entry = _dl_tlsdesc_return;
+	  atomic_store_relaxed (&td->arg, p);
+	  /* This release store synchronizes with the ldar acquire load
+	     instruction in _dl_tlsdesc_return_lazy.  */
+	  atomic_store_release (&td->entry, _dl_tlsdesc_return_lazy);
 	}
     }
 
@@ -120,11 +133,10 @@  _dl_tlsdesc_resolve_rela_fixup (struct tlsdesc volatile *td,
 
 void
 attribute_hidden
-_dl_tlsdesc_resolve_hold_fixup (struct tlsdesc volatile *td,
-				void *caller)
+_dl_tlsdesc_resolve_hold_fixup (struct tlsdesc *td, void *caller)
 {
   /* Maybe we're lucky and can return early.  */
-  if (caller != td->entry)
+  if (caller != atomic_load_relaxed (&td->entry))
     return;
 
   /* Locking here will stop execution until the running resolver runs