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Replace all setjmp()/longjmp() with sigsetjmp()/siglongjmp()

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The setjmp() function doesn't specify whether signal masks are saved and
restored; on Linux they are not, but on BSD (including MacOSX) they are.
We want to have consistent behaviour across platforms, so we should
always use "don't save/restore signal mask" (this is also generally
going to be faster). This also works around a bug in MacOSX where the
signal-restoration on longjmp() affects the signal mask for a completely
different thread, not just the mask for the thread which did the longjmp.
The most visible effect of this was that ctrl-C was ignored on MacOSX
because the CPU thread did a longjmp which resulted in its signal mask
being applied to every thread, so that all threads had SIGINT and SIGTERM
blocked.

The POSIX-sanctioned portable way to do a jump without affecting signal
masks is to siglongjmp() to a sigjmp_buf which was created by calling
sigsetjmp() with a zero savemask parameter, so change all uses of
setjmp()/longjmp() accordingly. [Technically POSIX allows sigsetjmp(buf, 0)
to save the signal mask; however the following siglongjmp() must not
restore the signal mask, so the pair can be effectively considered as
"sigjmp/longjmp which don't touch the mask".]

For Windows we provide a trivial sigsetjmp/siglongjmp in terms of
setjmp/longjmp -- this is OK because no user will ever pass a non-zero
savemask.

The setjmp() uses in tests/tcg/test-i386.c and tests/tcg/linux-test.c
are left untouched because these are self-contained singlethreaded
test programs intended to be run under QEMU's Linux emulation, so they
have neither the portability nor the multithreading issues to deal with.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Tested-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
This commit is contained in:
Peter Maydell 2013-02-20 15:21:09 +00:00 • committed by Blue Swirl
parent d1c36ba707
commit 6ab7e5465a
9 changed files with 52 additions and 42 deletions
Download patch file Download diff file Expand all files Collapse all files

27
coroutine-ucontext.c
View file

@ -46,7 +46,7 @@ static unsigned int pool_size;
typedef struct {
Coroutine base;
void *stack;
jmp_buf env;
sigjmp_buf env;
#ifdef CONFIG_VALGRIND_H
unsigned int valgrind_stack_id;
@ -130,8 +130,8 @@ static void coroutine_trampoline(int i0, int i1)
co = &self->base;
/* Initialize longjmp environment and switch back the caller */
if (!setjmp(self->env)) {
longjmp(*(jmp_buf *)co->entry_arg, 1);
if (!sigsetjmp(self->env, 0)) {
siglongjmp(*(sigjmp_buf *)co->entry_arg, 1);
}
while (true) {
@ -145,14 +145,15 @@ static Coroutine *coroutine_new(void)
const size_t stack_size = 1 << 20;
CoroutineUContext *co;
ucontext_t old_uc, uc;
jmp_buf old_env;
sigjmp_buf old_env;
union cc_arg arg = {0};
/* The ucontext functions preserve signal masks which incurs a system call
* overhead. setjmp()/longjmp() does not preserve signal masks but only
* works on the current stack. Since we need a way to create and switch to
* a new stack, use the ucontext functions for that but setjmp()/longjmp()
* for everything else.
/* The ucontext functions preserve signal masks which incurs a
* system call overhead. sigsetjmp(buf, 0)/siglongjmp() does not
* preserve signal masks but only works on the current stack.
* Since we need a way to create and switch to a new stack, use
* the ucontext functions for that but sigsetjmp()/siglongjmp() for
* everything else.
*/
if (getcontext(&uc) == -1) {
@ -178,8 +179,8 @@ static Coroutine *coroutine_new(void)
makecontext(&uc, (void (*)(void))coroutine_trampoline,
2, arg.i[0], arg.i[1]);
/* swapcontext() in, longjmp() back out */
if (!setjmp(old_env)) {
/* swapcontext() in, siglongjmp() back out */
if (!sigsetjmp(old_env, 0)) {
swapcontext(&old_uc, &uc);
}
return &co->base;
@ -242,9 +243,9 @@ CoroutineAction qemu_coroutine_switch(Coroutine *from_, Coroutine *to_,
s->current = to_;
ret = setjmp(from->env);
ret = sigsetjmp(from->env, 0);
if (ret == 0) {
longjmp(to->env, action);
siglongjmp(to->env, action);
}
return ret;
}