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The Go programming language
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The g->sched.sp saved stack pointer and the g->stackbase and g->stackguard stack bounds can change even while "the world is stopped", because a goroutine has to call functions (and therefore might split its stack) when exiting a system call to check whether the world is stopped (and if so, wait until the world continues). That means the garbage collector cannot access those values safely (without a race) for goroutines executing system calls. Instead, save a consistent triple in g->gcsp, g->gcstack, g->gcguard during entersyscall and have the garbage collector refer to those. The old code was occasionally seeing (because of the race) an sp and stk that did not correspond to each other, so that stk - sp was not the number of stack bytes following sp. In that case, if sp < stk then the call scanblock(sp, stk - sp) scanned too many bytes (anything between the two pointers, which pointed into different allocation blocks). If sp > stk then stk - sp wrapped around. On 32-bit, stk - sp is a uintptr (uint32) converted to int64 in the call to scanblock, so a large (~4G) but positive number. Scanblock would try to scan that many bytes and eventually fault accessing unmapped memory. On 64-bit, stk - sp is a uintptr (uint64) promoted to int64 in the call to scanblock, so a negative number. Scanblock would not scan anything, possibly causing in-use blocks to be freed. In short, 32-bit platforms would have seen either ineffective garbage collection or crashes during garbage collection, while 64-bit platforms would have seen either ineffective or incorrect garbage collection. You can see the invalid arguments to scanblock in the stack traces in issue 1620. Fixes #1620. Fixes #1746. R=iant, r CC=golang-dev https://golang.org/cl/4437075 |
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This is the source code repository for the Go programming language.
For documentation about how to install and use Go,
visit http://golang.org/ or load doc/install.html in your web browser.
After installing Go, you can view a nicely formatted
doc/install.html by running godoc --http=:6060
and then visiting http://localhost:6060/doc/install.html.
Unless otherwise noted, the Go source files are distributed
under the BSD-style license found in the LICENSE file.
--
Binary Distribution Notes
If you have just untarred a binary Go distribution, you need to set
the environment variable $GOROOT to the full path of the go
directory (the one containing this README). You can omit the
variable if you unpack it into /usr/local/go, or if you rebuild
from sources by running all.bash (see doc/install.html).
You should also add the Go binary directory $GOROOT/bin
to your shell's path.
For example, if you extracted the tar file into $HOME/go, you might
put the following in your .profile:
export GOROOT=$HOME/go
export PATH=$PATH:$GOROOT/bin
See doc/install.html for more details.