[PATCH] linux-user: Remove stale "not threadsafe" comments

Peter Maydell posted 1 patch 1 week, 6 days ago
Test checkpatch passed
Patches applied successfully (tree, apply log)
git fetch https://github.com/patchew-project/qemu tags/patchew/20220114155015.3767671-1-peter.maydell@linaro.org
Maintainers: Laurent Vivier <laurent@vivier.eu>
linux-user/signal.c | 2 --
1 file changed, 2 deletions(-)

[PATCH] linux-user: Remove stale "not threadsafe" comments

Posted by Peter Maydell 1 week, 6 days ago
In linux-user/signal.c we have two FIXME comments claiming that
parts of the signal-handling code are not threadsafe. These are
very old, as they were first introduced in commit 624f7979058
in 2008. Since then we've radically overhauled the signal-handling
logic, while carefully preserving these FIXME comments.

It's unclear exactly what thread-safety issue the original
author was trying to point out -- the relevant data structures
are in the TaskStruct, which makes them per-thread and only
operated on by that thread. The old code at the time of that
commit did have various races involving signal handlers being
invoked at awkward times; possibly this was what was meant.

Delete these FIXME comments:
 * they were written at a time when the way we handled
   signals was completely different
 * the code today appears to us to not have thread-safety issues
 * nobody knows what the problem the comments were trying to
   point out was
so they are serving no useful purpose for us today.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
Marked "RFC" because I'm a bit uneasy with deleting FIXMEs
simply because I can't personally figure out why they're
there. This patch is more to start a discussion to see
if anybody does understand the issue -- in which case we
can instead augment the comments to describe it.
---
 linux-user/signal.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/linux-user/signal.c b/linux-user/signal.c
index 32854bb3752..e7410776e21 100644
--- a/linux-user/signal.c
+++ b/linux-user/signal.c
@@ -1001,7 +1001,6 @@ int do_sigaction(int sig, const struct target_sigaction *act,
         oact->sa_mask = k->sa_mask;
     }
     if (act) {
-        /* FIXME: This is not threadsafe.  */
         __get_user(k->_sa_handler, &act->_sa_handler);
         __get_user(k->sa_flags, &act->sa_flags);
 #ifdef TARGET_ARCH_HAS_SA_RESTORER
@@ -1151,7 +1150,6 @@ void process_pending_signals(CPUArchState *cpu_env)
     sigset_t *blocked_set;
 
     while (qatomic_read(&ts->signal_pending)) {
-        /* FIXME: This is not threadsafe.  */
         sigfillset(&set);
         sigprocmask(SIG_SETMASK, &set, 0);
 
-- 
2.25.1


Re: [PATCH] linux-user: Remove stale "not threadsafe" comments

Posted by Peter Maydell 1 week, 6 days ago
On Fri, 14 Jan 2022 at 15:50, Peter Maydell <peter.maydell@linaro.org> wrote:
>
> In linux-user/signal.c we have two FIXME comments claiming that
> parts of the signal-handling code are not threadsafe. These are
> very old, as they were first introduced in commit 624f7979058
> in 2008. Since then we've radically overhauled the signal-handling
> logic, while carefully preserving these FIXME comments.

Oops, I meant to send this as RFC, not PATCH -- I hit ^C
on the send but obviously not quite in time to stop it getting
out of the door. Ignore this one, I've sent it again with
the right tag.

thanks
-- PMM