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70 lines
2.5 KiB
Diff
70 lines
2.5 KiB
Diff
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From e01b5939b38aea5ecbe41670643199825874b26c Mon Sep 17 00:00:00 2001
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From: Rich Felker <dalias@aerifal.cx>
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Date: Thu, 21 May 2020 23:32:45 -0400
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Subject: [PATCH 2/4] don't use libc.threads_minus_1 as relaxed atomic for
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skipping locks
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after all but the last thread exits, the next thread to observe
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libc.threads_minus_1==0 and conclude that it can skip locking fails to
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synchronize with any changes to memory that were made by the
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last-exiting thread. this can produce data races.
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on some archs, at least x86, memory synchronization is unlikely to be
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a problem; however, with the inline locks in malloc, skipping the lock
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also eliminated the compiler barrier, and caused code that needed to
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re-check chunk in-use bits after obtaining the lock to reuse a stale
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value, possibly from before the process became single-threaded. this
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in turn produced corruption of the heap state.
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some uses of libc.threads_minus_1 remain, especially for allocation of
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new TLS in the dynamic linker; otherwise, it could be removed
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entirely. it's made non-volatile to reflect that the remaining
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accesses are only made under lock on the thread list.
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instead of libc.threads_minus_1, libc.threaded is now used for
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skipping locks. the difference is that libc.threaded is permanently
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true once an additional thread has been created. this will produce
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some performance regression in processes that are mostly
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single-threaded but occasionally creating threads. in the future it
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may be possible to bring back the full lock-skipping, but more care
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needs to be taken to produce a safe design.
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---
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src/internal/libc.h | 2 +-
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src/malloc/malloc.c | 2 +-
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src/thread/__lock.c | 2 +-
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3 files changed, 3 insertions(+), 3 deletions(-)
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--- a/src/internal/libc.h
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+++ b/src/internal/libc.h
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@@ -21,7 +21,7 @@ struct __libc {
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int can_do_threads;
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int threaded;
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int secure;
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- volatile int threads_minus_1;
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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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size_t tls_size, tls_align, tls_cnt;
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--- a/src/malloc/malloc.c
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+++ b/src/malloc/malloc.c
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@@ -26,7 +26,7 @@ int __malloc_replaced;
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static inline void lock(volatile int *lk)
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{
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- if (libc.threads_minus_1)
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+ if (libc.threaded)
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while(a_swap(lk, 1)) __wait(lk, lk+1, 1, 1);
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}
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--- a/src/thread/__lock.c
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+++ b/src/thread/__lock.c
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@@ -18,7 +18,7 @@
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void __lock(volatile int *l)
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{
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- if (!libc.threads_minus_1) return;
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+ if (!libc.threaded) 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 (!current) return;
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