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The enclosed change adds a locking semaphore around the previously non-atomic compare_and_swap implementation. The implementation is derived from the one used in libstdc++. I've had this patch under test on and off for several months on hppa-unknown-linux-gnu. It fixes various process related testsuite fails. There was a contending implementation using a kernel light-weight syscall. I finally decided to go with this implementation because 1) it is usable with hpux, 2) the compare_and_swap light-weight syscall isn't available in all linux kernel versions, 3) I wasn't fully convinced that the light-weight syscall was completely reliable under heavy contention. Committed to 4.0, 4.1 and 4.2. Dave -- J. David Anglin dave.anglin@nrc-cnrc.gc.ca National Research Council of Canada (613) 990-0752 (FAX: 952-6602) 2005-12-28 John David Anglin <dave.anglin@nrc-cnrc.gc.ca> * sysdep/pa/locks.h (compare_and_swap): Add ldcw semaphore to make operation atomic. Index: sysdep/pa/locks.h =================================================================== --- sysdep/pa/locks.h (revision 109064) +++ sysdep/pa/locks.h (working copy) @@ -1,6 +1,6 @@ -// locks.h - Thread synchronization primitives. PARISC implementation. +// locks.h - Thread synchronization primitives. PA-RISC implementation. -/* Copyright (C) 2002 Free Software Foundation +/* Copyright (C) 2002, 2005 Free Software Foundation This file is part of libgcj. @@ -11,30 +11,62 @@ #ifndef __SYSDEP_LOCKS_H__ #define __SYSDEP_LOCKS_H__ -typedef size_t obj_addr_t; /* Integer type big enough for object */ - /* address. */ +// Integer type big enough for object address. +typedef size_t obj_addr_t; -// Atomically replace *addr by new_val if it was initially equal to old. -// Return true if the comparison succeeded. +template<int _Inst> + struct _pa_jv_cas_lock + { + static volatile int _S_pa_jv_cas_lock; + }; + +template<int _Inst> +volatile int +_pa_jv_cas_lock<_Inst>::_S_pa_jv_cas_lock __attribute__ ((aligned (16))) = 1; + +// Because of the lack of weak support when using the hpux som +// linker, we explicitly instantiate the atomicity lock. +template volatile int _pa_jv_cas_lock<0>::_S_pa_jv_cas_lock; + +// Atomically replace *addr by new_val if it was initially equal to old_val. +// Return true if the comparison is successful. // Assumed to have acquire semantics, i.e. later memory operations // cannot execute before the compare_and_swap finishes. +// The following implementation is atomic but it can deadlock +// (e.g., if a thread dies holding the lock). inline static bool +__attribute__ ((__unused__)) compare_and_swap(volatile obj_addr_t *addr, - obj_addr_t old, + obj_addr_t old_val, obj_addr_t new_val) { - /* FIXME: not atomic */ - obj_addr_t prev; + bool result; + int tmp; + volatile int& lock = _pa_jv_cas_lock<0>::_S_pa_jv_cas_lock; + + __asm__ __volatile__ ("ldcw 0(%1),%0\n\t" + "cmpib,<>,n 0,%0,.+20\n\t" + "ldw 0(%1),%0\n\t" + "cmpib,= 0,%0,.-4\n\t" + "nop\n\t" + "b,n .-20" + : "=&r" (tmp) + : "r" (&lock) + : "memory"); - if ((prev = *addr) == old) - { - *addr = new_val; - return true; - } + if (*addr != old_val) + result = false; else { - return false; + *addr = new_val; + result = true; } + + /* Reset lock with PA 2.0 "ordered" store. */ + __asm__ __volatile__ ("stw,ma %1,0(%0)" + : : "r" (&lock), "r" (tmp) : "memory"); + + return result; } // Set *addr to new_val with release semantics, i.e. making sure
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