linux-kernel - Is s390's new generic-using syscall code actually correct?

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Date:   Sat, 20 Mar 2021 20:48:34 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Sven Schnelle <svens@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>, X86 ML <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Is s390's new generic-using syscall code actually correct?

Hi all-

I'm working on my kentry patchset, and I encountered:

commit 56e62a73702836017564eaacd5212e4d0fa1c01d
Author: Sven Schnelle <svens@...ux.ibm.com>
Date:   Sat Nov 21 11:14:56 2020 +0100

    s390: convert to generic entry

As part of this work, I was cleaning up the generic syscall helpers,
and I encountered the goodies in do_syscall() and __do_syscall().

I'm trying to wrap my head around the current code, and I'm rather confused.

1. syscall_exit_to_user_mode_work() does *all* the exit work, not just
the syscall exit work.  So a do_syscall() that gets called twice will
do the loopy part of the exit work (e.g. signal handling) twice.  Is
this intentional?  If so, why?

2. I don't understand how this PIF_SYSCALL_RESTART thing is supposed
to work.  Looking at the code in Linus' tree, if a signal is pending
and a syscall returns -ERESTARTSYS, the syscall will return back to
do_syscall().  The work (as in (1)) gets run, calling do_signal(),
which will notice -ERESTARTSYS and set PIF_SYSCALL_RESTART.
Presumably it will also push the signal frame onto the stack and aim
the return address at the svc instruction mentioned in the commit
message from "s390: convert to generic entry".  Then __do_syscall()
will turn interrupts back on and loop right back into do_syscall().
That seems incorrect.

Can you enlighten me?  My WIP tree is here:
https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/log/?h=x86/kentry

Here are my changes to s390, and I don't think they're really correct:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/diff/arch/s390/kernel/syscall.c?h=x86/kentry&id=58a459922be0fb8e0f17aeaebcb0ac8d0575a62c