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.
commit 02357c5f11138f512e714d1740491abc86c61388 Author: Torvald Riegel <trie...@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; +}