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runtime: don't use runtime_lock in __go_get_backtrace_state

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If getSiginfo does not know how to determine the PC, it will call
    runtime_callers. That can happen in a thread that was started by
    non-Go code, in which case the TLS variable g will not be set, in
    which case runtime_lock will crash.
    
    Avoid the problem by using atomic operations for the lock. This is OK
    since creating a backtrace state is fast and never blocks.
    
    The test case is TestCgoExternalThreadSIGPROF in the runtime package
    on a system that getSiginfo doesn't handle specially.
    
    Updates golang/go#20931
    
    Reviewed-on: https://go-review.googlesource.com/50650

From-SVN: r250439
This commit is contained in:
Ian Lance Taylor 2017-07-21 18:27:35 +00:00
parent 2401ffc3fe
commit df206c6e77
2 changed files with 12 additions and 4 deletions
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2
gcc/go/gofrontend/MERGE
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@ -1,4 +1,4 @@
a9f1aeced86691de891fbf2a8c97e848faf1962e
b712bacd939466e66972337744983e180849c535
The first line of this file holds the git revision number of the last
merge done from the gofrontend repository.

14
libgo/runtime/go-caller.c
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@ -74,7 +74,7 @@ static void *back_state;
/* A lock to control creating back_state. */
static Lock back_state_lock;
static uint32 back_state_lock;
/* The program arguments. */
@ -85,7 +85,15 @@ extern Slice runtime_get_args(void);
struct backtrace_state *
__go_get_backtrace_state ()
{
runtime_lock (&back_state_lock);
uint32 set;
/* We may not have a g here, so we can't use runtime_lock. */
set = 0;
while (!__atomic_compare_exchange_n (&back_state_lock, &set, 1, false, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
{
runtime_osyield ();
set = 0;
}
if (back_state == NULL)
{
Slice args;
@ -113,7 +121,7 @@ __go_get_backtrace_state ()
back_state = backtrace_create_state (filename, 1, error_callback, NULL);
}
runtime_unlock (&back_state_lock);
__atomic_store_n (&back_state_lock, 0, __ATOMIC_RELEASE);
return back_state;
}