On Wed, Mar 20, 2024 at 10:26:04PM +0000, Guilherme Janczak wrote:
> The standard says it's necessary.
> If you have a `volatile long long` on a 32-bit architecture, the
> compiler will have to compile it to some bignum code (meaning multiple
> instructions), and the signal can come in between them.
Agreed, because in this case the variable is not machine-word sized.
What happens if you instead use a 32-bit aligned volatile quantity?
(Or 16 or 8 bits, for that matter, at least on PowerPC [1].)
I would hope that these smaller sizes would work, otherwise, again,
device drivers become problematic on that platform.
Thanx, Paul
[1] There have been some platforms without 16-bit or 8-bit loads,
and on such platforms, you could also see failures of one sort
or another.
> I can quickly make it happen on a Nintendo Wii (32-bit powerpc) as
> well as an i386 laptop using this test program:
> ```
> #define _POSIX_C_SOURCE 200809L
>
> #include <sys/time.h>
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <signal.h>
> #include <string.h>
>
> static void catch_sigalrm(int);
>
> volatile unsigned long long longword;
> volatile sig_atomic_t partial_update;
>
> int
> main(void)
> {
> const struct sigaction act = { .sa_handler = catch_sigalrm };
> const struct itimerval timer = {
> .it_value.tv_usec = 1,
> .it_interval.tv_usec = 1,
> };
> unsigned long long n;
>
> sigaction(SIGALRM, &act, NULL);
> setitimer(ITIMER_REAL, &timer, NULL);
> for (unsigned char i = 0;; i++) {
> memset(&n, i, sizeof(n));
> longword = n;
> if (partial_update) {
> fprintf(stderr, "longword partially updated\n");
> exit(1);
> }
> }
> }
>
> static void
> catch_sigalrm(int unused)
> {
> union multibyte {
> long long n;
> unsigned char bytes[sizeof(long long)];
> } window;
> (void)unused;
> window.n = longword;
>
> for (size_t i = 1; i < sizeof(window.bytes); i++) {
> if (window.bytes[i] != window.bytes[0])
> partial_update = 1;
> }
> }
> ```
>
> Output:
> ```
> $ cc -O2 test.c && ./a.out
> longword partially updated
> ```
>
> The program runs (apparently) forever on my amd64 desktop.
>
> If you look at the powerpc assembly version of the program in Godbolt:
> https://godbolt.org/z/Pc8q7E5ej
> Lines 69 and 70 of the assembly use 2 STW instructions to store each
> 32-bit half of the bignum.
>
> On Wed, Mar 20, 2024 at 04:44:33PM -0400, Elad Lahav wrote:
> > Do you really need volatile?
> > There are two cases to consider. Either your code synchronizes updates
> > to the shared value with the signal handler (e.g., by blocking and
> > then unblocking the signal), in which case I believe the compiler
> > cannot ignore updates to the value; or you don't, and you can't depend
> > on the variable having any specific value in the signal handler. The
> > only thing you want to prevent in the latter case is the handler
> > observing a partial update to the variable, which I presume is where
> > the other requirements originate. (In practice, there should be little
> > or no concern with any primitive type on modern hardware).
> >
> > --Elad
> >
> > On Wed, Mar 20, 2024 at 4:32 PM Guilherme Janczak
> > <[email protected]> wrote:
> > >
> > > Variables shared with signal handlers must be of type `volatile
> > > sigatomic_t`, not `volatile` or `sigatomic_t` as the current text says,
> > > according to a C11 draft:
> > >
> > > When ... interrupted by ... a signal, values of objects that are
> > > neither lock-free atomic objects nor of type volatile sig_atomic_t
> > > are unspecified.
> > >
> > > Ref: https://www.iso-9899.info/n1570.html#5.1.2.3p5
> > > Signed-off-by: Guilherme Janczak <[email protected]>
> > > ---
> > > memorder/memorder.tex | 4 ++--
> > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/memorder/memorder.tex b/memorder/memorder.tex
> > > index 5c50d42d..873c3424 100644
> > > --- a/memorder/memorder.tex
> > > +++ b/memorder/memorder.tex
> > > @@ -1317,8 +1317,8 @@ from the viewpoint of the interrupted thread, at
> > > least at the
> > > assembly-language level.
> > > However, the C and C++ languages do not define the results of handlers
> > > and interrupted threads sharing plain variables.
> > > -Instead, such shared variables must be \co{sig_atomic_t}, lock-free
> > > -atomics, or \co{volatile}.
> > > +Instead, such shared variables must be \co{volatile sig_atomic_t} or
> > > +lock-free atomics.
> > >
> > > On the other hand, because the handler executes within the interrupted
> > > thread's context, the memory ordering used to synchronize communication
> > > --
> > > 2.42.0
> > >
> > >
>
