[RFC, patch] libitm: Filter out undo writes that overlap with the libitm stack.
Torvald Riegel
triegel@redhat.com
Tue Jan 10 14:33:00 GMT 2012
On Tue, 2012-01-10 at 16:35 +1100, Richard Henderson wrote:
> On 01/09/2012 06:26 AM, Torvald Riegel wrote:
> > libitm: Filter out undo writes that overlap with the libitm stack.
> >
> > libitm/
> > * config/generic/tls.h (GTM::mask_stack_top,
> > GTM::mask_stack_bottom): New.
> > * local.cc (gtm_undolog::rollback): Filter out any updates that
> > overlap the libitm stack. Add current transaction as parameter.
> > * libitm_i.h (GTM::gtm_undolog::rollback): Adapt.
> > * beginend.cc (GTM::gtm_thread::rollback): Adapt.
> > * testsuite/libitm.c/stackundo.c: New test.
>
> One could steal code from bohem-gc for this.
> See GC_get_stack_base in os_dep.c.
Thanks for the pointer. I looked at this code, and it seems fairly
complex given the dependencies on OS/libc and OS/libc behavior. From a
maintenance point-of-view, does it make sense to copy that complexity
into libitm? boehm-gc is used in GCC, so perhaps that's not much of a
problem, however. I also looked at glibc's memcpy implementations, and
copying those plus a simple byte-wise copy for the generic case could be
also a fairly clean solution.
Also, is the license compatible with the GPL wrt. mixing sources?
What about keeping the patch/hack that I posted for now, creating a PR,
and looking at this again for another release?
Attached a slightly updated version with just comments in local.cc
changed.
-------------- next part --------------
commit 02357c5f11138f512e714d1740491abc86c61388
Author: Torvald Riegel <triegel@redhat.com>
Date: Sun Jan 8 20:12:33 2012 +0100
libitm: Filter out undo writes that overlap with the libitm stack.
libitm/
* config/generic/tls.h (GTM::mask_stack_top,
GTM::mask_stack_bottom): New.
* local.cc (gtm_undolog::rollback): Filter out any updates that
overlap the libitm stack. Add current transaction as parameter.
* libitm_i.h (GTM::gtm_undolog::rollback): Adapt.
* beginend.cc (GTM::gtm_thread::rollback): Adapt.
* testsuite/libitm.c/stackundo.c: New test.
diff --git a/libitm/beginend.cc b/libitm/beginend.cc
index fe14f32..08c2174 100644
--- a/libitm/beginend.cc
+++ b/libitm/beginend.cc
@@ -327,7 +327,7 @@ GTM::gtm_thread::rollback (gtm_transaction_cp *cp, bool aborting)
// data. Because of the latter, we have to roll it back before any
// dispatch-specific rollback (which handles synchronization with other
// transactions).
- undolog.rollback (cp ? cp->undolog_size : 0);
+ undolog.rollback (this, cp ? cp->undolog_size : 0);
// Perform dispatch-specific rollback.
abi_disp()->rollback (cp);
diff --git a/libitm/config/generic/tls.h b/libitm/config/generic/tls.h
index 6bbdccf..07efef3 100644
--- a/libitm/config/generic/tls.h
+++ b/libitm/config/generic/tls.h
@@ -60,6 +60,25 @@ static inline abi_dispatch * abi_disp() { return _gtm_thr_tls.disp; }
static inline void set_abi_disp(abi_dispatch *x) { _gtm_thr_tls.disp = x; }
#endif
+#ifndef HAVE_ARCH_GTM_MASK_STACK
+// To filter out any updates that overlap the libitm stack, we define
+// gtm_mask_stack_top to the entry point to the library and
+// gtm_mask_stack_bottom to below current function. This
+// definition should be fine for all stack-grows-down architectures.
+// FIXME We fake the bottom to be lower so that we are safe even if we might
+// call further functions (compared to where we called gtm_mask_stack_bottom
+// in the call hierarchy) to actually undo or redo writes (e.g., memcpy).
+// This is a completely arbitrary value; can we instead ensure that there are
+// no such calls, or can we determine a future-proof value otherwise?
+static inline void *
+mask_stack_top(gtm_thread *tx) { return tx->jb.cfa; }
+static inline void *
+mask_stack_bottom(gtm_thread *tx)
+{
+ return (uint8_t*)__builtin_dwarf_cfa() - 128;
+}
+#endif
+
} // namespace GTM
#endif // LIBITM_TLS_H
diff --git a/libitm/libitm_i.h b/libitm/libitm_i.h
index f922d22..f849654 100644
--- a/libitm/libitm_i.h
+++ b/libitm/libitm_i.h
@@ -138,7 +138,7 @@ struct gtm_undolog
size_t size() const { return undolog.size(); }
// In local.cc
- void rollback (size_t until_size = 0);
+ void rollback (gtm_thread* tx, size_t until_size = 0);
};
// Contains all thread-specific data required by the entire library.
diff --git a/libitm/local.cc b/libitm/local.cc
index 39b6da3..8123063 100644
--- a/libitm/local.cc
+++ b/libitm/local.cc
@@ -26,11 +26,20 @@
namespace GTM HIDDEN {
-
-void
-gtm_undolog::rollback (size_t until_size)
+// This function needs to be noinline because we need to prevent that it gets
+// inlined into another function that calls further functions. This could
+// break our assumption that we only call memcpy and thus only need to
+// additionally protect the memcpy stack (see the hack in mask_stack_bottom()).
+// Even if that isn't an issue because those other calls don't happen during
+// copying, we still need mask_stack_bottom() to be called "close" to the
+// memcpy in terms of stack frames, so just ensure that for now using the
+// noinline.
+void __attribute__((noinline))
+gtm_undolog::rollback (gtm_thread* tx, size_t until_size)
{
size_t i, n = undolog.size();
+ void *top = mask_stack_top(tx);
+ void *bot = mask_stack_bottom(tx);
if (n > 0)
{
@@ -40,7 +49,17 @@ gtm_undolog::rollback (size_t until_size)
size_t len = undolog[i];
size_t words = (len + sizeof(gtm_word) - 1) / sizeof(gtm_word);
i -= words;
- __builtin_memcpy (ptr, &undolog[i], len);
+ // Filter out any updates that overlap the libitm stack. We don't
+ // bother filtering out just the overlapping bytes because we don't
+ // merge writes and thus any overlapping write is either bogus or
+ // would restore data on stack frames that are not in use anymore.
+ // FIXME The memcpy can/will end up as another call but we
+ // calculated BOT based on the current function. Can we inline or
+ // reimplement this without too much trouble due to unaligned calls
+ // and still have good performance, so that we can remove the hack
+ // in mask_stack_bottom()?
+ if (likely(ptr > top || (uint8_t*)ptr + len <=bot))
+ __builtin_memcpy (ptr, &undolog[i], len);
}
}
}
diff --git a/libitm/testsuite/libitm.c/stackundo.c b/libitm/testsuite/libitm.c/stackundo.c
new file mode 100644
index 0000000..02759d7
--- /dev/null
+++ b/libitm/testsuite/libitm.c/stackundo.c
@@ -0,0 +1,23 @@
+int __attribute__((noinline)) test2(int x[1000])
+{
+ int i;
+ return x[12];
+}
+
+int __attribute__((noinline)) test1()
+{
+ int x[1000], i;
+
+ for (i = 0; i < 1000; i++)
+ x[i] = i;
+ return test2(x);
+}
+
+int main()
+{
+ __transaction_atomic {
+ if (test1() !=0)
+ __transaction_cancel;
+ }
+ return 0;
+}
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