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91 lines
3.1 KiB
Diff
91 lines
3.1 KiB
Diff
From 8d81ba8c0bc6fe31136cb15c9c82ef4c24965040 Mon Sep 17 00:00:00 2001
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From: Rich Felker <dalias@aerifal.cx>
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Date: Fri, 22 May 2020 17:45:47 -0400
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Subject: [PATCH 4/4] restore lock-skipping for processes that return to
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single-threaded state
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the design used here relies on the barrier provided by the first lock
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operation after the process returns to single-threaded state to
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synchronize with actions by the last thread that exited. by storing
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the intent to change modes in the same object used to detect whether
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locking is needed, it's possible to avoid an extra (possibly costly)
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memory load after the lock is taken.
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---
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src/internal/libc.h | 1 +
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src/malloc/malloc.c | 5 ++++-
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src/thread/__lock.c | 4 +++-
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src/thread/pthread_create.c | 8 ++++----
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4 files changed, 12 insertions(+), 6 deletions(-)
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--- a/src/internal/libc.h
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+++ b/src/internal/libc.h
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@@ -21,6 +21,7 @@ struct __libc {
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char can_do_threads;
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char threaded;
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char secure;
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+ volatile signed char need_locks;
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int threads_minus_1;
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size_t *auxv;
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struct tls_module *tls_head;
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--- a/src/malloc/malloc.c
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+++ b/src/malloc/malloc.c
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@@ -26,8 +26,11 @@ int __malloc_replaced;
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static inline void lock(volatile int *lk)
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{
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- if (libc.threaded)
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+ int need_locks = libc.need_locks;
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+ if (need_locks) {
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while(a_swap(lk, 1)) __wait(lk, lk+1, 1, 1);
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+ if (need_locks < 0) libc.need_locks = 0;
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+ }
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}
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static inline void unlock(volatile int *lk)
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--- a/src/thread/__lock.c
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+++ b/src/thread/__lock.c
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@@ -18,9 +18,11 @@
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void __lock(volatile int *l)
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{
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- if (!libc.threaded) return;
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+ int need_locks = libc.need_locks;
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+ if (!need_locks) return;
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/* fast path: INT_MIN for the lock, +1 for the congestion */
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int current = a_cas(l, 0, INT_MIN + 1);
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+ if (need_locks < 0) libc.need_locks = 0;
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if (!current) return;
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/* A first spin loop, for medium congestion. */
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for (unsigned i = 0; i < 10; ++i) {
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--- a/src/thread/pthread_create.c
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+++ b/src/thread/pthread_create.c
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@@ -118,8 +118,8 @@ _Noreturn void __pthread_exit(void *resu
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* until the lock is released, which only happens after SYS_exit
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* has been called, via the exit futex address pointing at the lock.
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* This needs to happen after any possible calls to LOCK() that might
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- * skip locking if libc.threads_minus_1 is zero. */
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- libc.threads_minus_1--;
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+ * skip locking if process appears single-threaded. */
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+ if (!--libc.threads_minus_1) libc.need_locks = -1;
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self->next->prev = self->prev;
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self->prev->next = self->next;
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self->prev = self->next = self;
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@@ -339,7 +339,7 @@ int __pthread_create(pthread_t *restrict
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~(1UL<<((SIGCANCEL-1)%(8*sizeof(long))));
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__tl_lock();
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- libc.threads_minus_1++;
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+ if (!libc.threads_minus_1++) libc.need_locks = 1;
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ret = __clone((c11 ? start_c11 : start), stack, flags, args, &new->tid, TP_ADJ(new), &__thread_list_lock);
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/* All clone failures translate to EAGAIN. If explicit scheduling
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@@ -363,7 +363,7 @@ int __pthread_create(pthread_t *restrict
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new->next->prev = new;
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new->prev->next = new;
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} else {
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- libc.threads_minus_1--;
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+ if (!--libc.threads_minus_1) libc.need_locks = 0;
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}
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__tl_unlock();
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__restore_sigs(&set);
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