Aw: Re: [RFC PATCH urcu on mips, parisc] Fix: compat_futex should work-around futex signal-restart kernel bug

[Helge Deller]

Hello Mathieu,

> > When testing liburcu on a 3.18 Linux kernel, 2-core MIPS (cpu model :
> > Ingenic JZRISC V4.15  FPU V0.0), we notice that a blocked sys_futex
> > FUTEX_WAIT returns -1, errno=ENOSYS when interrupted by a SA_RESTART
> > signal handler. This spurious ENOSYS behavior causes hangs in liburcu
> > 0.9.x. Running a MIPS 3.18 kernel under a QEMU emulator exhibits the
> > same behavior. This might affect earlier kernels.
> > 
> > This issue appears to be fixed in 3.18.y stable kernels and 3.19, but
> > nevertheless, we should try to handle this kernel bug more gracefully
> > than a user-space hang due to unexpected spurious ENOSYS return value.
> 
> It's actually fixed in 3.19, but not in 3.18.y stable kernels. The
> Linux kernel upstream fix commit is:
> e967ef02 "MIPS: Fix restart of indirect syscalls"

But that patch fixes mips only.
 
> I've created a small test program that could also be used on parisc
> to check if it suffers from the same issue (see attached).
> 
> On bogus mips kernels, we see the following output:
> [OK] Test program with pid: 5748 SIGUSR1 handler
> [FAIL] futex returns -1, Function not implemented

I tested it on a recent 4.2 kernel on parisc.
It fails as you describe:

Testing futex sigrestart. Stop with CTRL-c.
[OK] Test program with pid: 1361 SIGUSR1 handler
[OK] Test program with pid: 1361 SIGUSR1 handler
[FAIL] futex returns -1, Function not implemented
[OK] Test program with pid: 1361 SIGUSR1 handler
[FAIL] futex returns -1, Function not implemented

strace gives:
[pid  1329] futex(0x1210c, FUTEX_WAIT, -1, NULL <unfinished ...>
[pid  1328] nanosleep({1, 0},  <unfinished ...>
[pid  1329] <... futex resumed> )       = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
[pid  1329] write(2, "[FAIL] futex returns -1, Functio"..., 50[FAIL] futex returns -1, Function not implemented)


> > Therefore, fallback on the "async-safe" version of compat_futex in those
> > situations where FUTEX_WAIT returns ENOSYS. This async-safe fallback has
> > the nice property of being OK to use concurrently with other FUTEX_WAKE
> > and FUTEX_WAIT futex() calls, because it's simply a busy-wait scheme.
> > 
> > We suspect that parisc might be affected by a similar issue (Debian
> > build bots reported a similar hang on both mips and parisc), but we do
> > not have access to the hardware required to test this hypothesis.

If you want access to a machine, let me know.
I'll try the patch below as well..

> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > CC: Michael Jeanson <mjeanson@xxxxxxxxxxxx>
> > CC: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > CC: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
> > CC: linux-mips@xxxxxxxxxxxxxx
> > CC: linux-kernel@xxxxxxxxxxxxxxx
> > CC: "James E.J. Bottomley" <jejb@xxxxxxxxxxxxxxxx>
> > CC: Helge Deller <deller@xxxxxx>
> > CC: linux-parisc@xxxxxxxxxxxxxxx
> > ---
> > compat_futex.c |  2 ++
> > urcu/futex.h   | 12 +++++++++++-
> > 2 files changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/compat_futex.c b/compat_futex.c
> > index b7f78f0..9e918fe 100644
> > --- a/compat_futex.c
> > +++ b/compat_futex.c
> > @@ -111,6 +111,8 @@ end:
> >  * _ASYNC SIGNAL-SAFE_.
> >  * For now, timeout, uaddr2 and val3 are unused.
> >  * Waiter will busy-loop trying to read the condition.
> > + * It is OK to use compat_futex_async() on a futex address on which
> > + * futex() WAKE operations are also performed.
> >  */
> > 
> > int compat_futex_async(int32_t *uaddr, int op, int32_t val,
> > diff --git a/urcu/futex.h b/urcu/futex.h
> > index 4d16cfa..a17eda8 100644
> > --- a/urcu/futex.h
> > +++ b/urcu/futex.h
> > @@ -73,7 +73,17 @@ static inline int futex_noasync(int32_t *uaddr, int op,
> > int32_t val,
> > 
> > 	ret = futex(uaddr, op, val, timeout, uaddr2, val3);
> > 	if (caa_unlikely(ret < 0 && errno == ENOSYS)) {
> > -		return compat_futex_noasync(uaddr, op, val, timeout,
> > +		/*
> > +		 * The fallback on ENOSYS is the async-safe version of
> > +		 * the compat futex implementation, because the
> > +		 * async-safe compat implementation allows being used
> > +		 * concurrently with calls to futex(). Indeed, sys_futex
> > +		 * FUTEX_WAIT, on some architectures (e.g. mips), within
> > +		 * a given process, spuriously return ENOSYS due to
> > +		 * signal restart bugs on some kernel versions (e.g.
> > +		 * Linux kernel 3.18 and possibly earlier).
> > +		 */
> > +		return compat_futex_async(uaddr, op, val, timeout,
> > 				uaddr2, val3);
> > 	}
> > 	return ret;
--
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