diff options
Diffstat (limited to 'toolchain/musl/patches/500-0002-don-t-use-libc.threads_minus_1-as-relaxed-atomic-for.patch')
| -rw-r--r-- | toolchain/musl/patches/500-0002-don-t-use-libc.threads_minus_1-as-relaxed-atomic-for.patch | 69 |
1 files changed, 69 insertions, 0 deletions
diff --git a/toolchain/musl/patches/500-0002-don-t-use-libc.threads_minus_1-as-relaxed-atomic-for.patch b/toolchain/musl/patches/500-0002-don-t-use-libc.threads_minus_1-as-relaxed-atomic-for.patch new file mode 100644 index 0000000000..4ca51b0be0 --- /dev/null +++ b/toolchain/musl/patches/500-0002-don-t-use-libc.threads_minus_1-as-relaxed-atomic-for.patch @@ -0,0 +1,69 @@ +From e01b5939b38aea5ecbe41670643199825874b26c Mon Sep 17 00:00:00 2001 +From: Rich Felker <dalias@aerifal.cx> +Date: Thu, 21 May 2020 23:32:45 -0400 +Subject: [PATCH 2/4] don't use libc.threads_minus_1 as relaxed atomic for + skipping locks + +after all but the last thread exits, the next thread to observe +libc.threads_minus_1==0 and conclude that it can skip locking fails to +synchronize with any changes to memory that were made by the +last-exiting thread. this can produce data races. + +on some archs, at least x86, memory synchronization is unlikely to be +a problem; however, with the inline locks in malloc, skipping the lock +also eliminated the compiler barrier, and caused code that needed to +re-check chunk in-use bits after obtaining the lock to reuse a stale +value, possibly from before the process became single-threaded. this +in turn produced corruption of the heap state. + +some uses of libc.threads_minus_1 remain, especially for allocation of +new TLS in the dynamic linker; otherwise, it could be removed +entirely. it's made non-volatile to reflect that the remaining +accesses are only made under lock on the thread list. + +instead of libc.threads_minus_1, libc.threaded is now used for +skipping locks. the difference is that libc.threaded is permanently +true once an additional thread has been created. this will produce +some performance regression in processes that are mostly +single-threaded but occasionally creating threads. in the future it +may be possible to bring back the full lock-skipping, but more care +needs to be taken to produce a safe design. +--- + src/internal/libc.h | 2 +- + src/malloc/malloc.c | 2 +- + src/thread/__lock.c | 2 +- + 3 files changed, 3 insertions(+), 3 deletions(-) + +--- a/src/internal/libc.h ++++ b/src/internal/libc.h +@@ -21,7 +21,7 @@ struct __libc { + int can_do_threads; + int threaded; + int secure; +- volatile int threads_minus_1; ++ int threads_minus_1; + size_t *auxv; + struct tls_module *tls_head; + size_t tls_size, tls_align, tls_cnt; +--- a/src/malloc/malloc.c ++++ b/src/malloc/malloc.c +@@ -26,7 +26,7 @@ int __malloc_replaced; + + static inline void lock(volatile int *lk) + { +- if (libc.threads_minus_1) ++ if (libc.threaded) + while(a_swap(lk, 1)) __wait(lk, lk+1, 1, 1); + } + +--- a/src/thread/__lock.c ++++ b/src/thread/__lock.c +@@ -18,7 +18,7 @@ + + void __lock(volatile int *l) + { +- if (!libc.threads_minus_1) return; ++ if (!libc.threaded) return; + /* fast path: INT_MIN for the lock, +1 for the congestion */ + int current = a_cas(l, 0, INT_MIN + 1); + if (!current) return; |