From: Eric Wong <e@80x24.org>
To: David Laight <David.Laight@ACULAB.COM>
Cc: 'Oleg Nesterov' <oleg@redhat.com>,
Deepa Dinamani <deepa.kernel@gmail.com>,
Al Viro <viro@ZenIV.linux.org.uk>,
"Eric W. Biederman" <ebiederm@xmission.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
"arnd@arndb.de" <arnd@arndb.de>,
"dbueso@suse.de" <dbueso@suse.de>,
"axboe@kernel.dk" <axboe@kernel.dk>,
"dave@stgolabs.net" <dave@stgolabs.net>,
"jbaron@akamai.com" <jbaron@akamai.com>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
"linux-aio@kvack.org" <linux-aio@kvack.org>,
"omar.kilani@gmail.com" <omar.kilani@gmail.com>,
"tglx@linutronix.de" <tglx@linutronix.de>,
"stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: Re: pselect/etc semantics (Was: [PATCH v2] signal: Adjust error codes according to restore_user_sigmask())
Date: Wed, 29 May 2019 18:50:12 +0000 [thread overview]
Message-ID: <20190529185012.qqeqq4fsolprknrz@dcvr> (raw)
In-Reply-To: <b05cec7f9e8f457281e689576a7a360f@AcuMS.aculab.com>
David Laight <David.Laight@ACULAB.COM> wrote:
> From: Oleg Nesterov
> > Sent: 29 May 2019 17:12
> > Al, Linus, Eric, please help.
> >
> > The previous discussion was very confusing, we simply can not understand each
> > other.
> >
> > To me everything looks very simple and clear, but perhaps I missed something
> > obvious? Please correct me.
> >
> > I think that the following code is correct
> >
> > int interrupted = 0;
> >
> > void sigint_handler(int sig)
> > {
> > interrupted = 1;
> > }
> >
> > int main(void)
> > {
> > sigset_t sigint, empty;
> >
> > sigemptyset(&sigint);
> > sigaddset(&sigint, SIGINT);
> > sigprocmask(SIG_BLOCK, &sigint, NULL);
> >
> > signal(SIGINT, sigint_handler);
> >
> > sigemptyset(&empty); // so pselect() unblocks SIGINT
> >
> > ret = pselect(..., &empty);
> ^^^^^ sigint
> >
> > if (ret >= 0) // sucess or timeout
> > assert(!interrupted);
> >
> > if (interrupted)
> > assert(ret == -EINTR);
> > }
> >
> > IOW, if pselect(sigmask) temporary unblocks SIGINT according to sigmask, this
> > signal should not be delivered if a ready fd was found or timeout. The signal
> > handle should only run if ret == -EINTR.
>
> Personally I think that is wrong.
> Given code like the above that has:
> while (!interrupted) {
> pselect(..., &sigint);
> // process available data
> }
>
> You want the signal handler to be executed even if one of the fds
> always has available data.
> Otherwise you can't interrupt a process that is always busy.
Agreed... I believe cmogstored has always had a bug in the way
it uses epoll_pwait because it failed to check interrupts if:
a) an FD is ready + interrupt
b) epoll_pwait returns 0 on interrupt
The bug remains in userspace for a), which I will fix by adding
an interrupt check when an FD is ready. The window is very
small for a) and difficult to trigger, and also in a rare code
path.
The b) case is the kernel bug introduced in 854a6ed56839a40f
("signal: Add restore_user_sigmask()").
I don't think there's any disagreement that b) is a kernel bug.
So the confusion is for a), and POSIX is not clear w.r.t. how
pselect/poll works when there's both FD readiness and an
interrupt.
Thus I'm inclined to believe *select/*poll/epoll_*wait should
follow POSIX read() semantics:
If a read() is interrupted by a signal before it reads any data, it shall
return −1 with errno set to [EINTR].
If a read() is interrupted by a signal after it has successfully read
some data, it shall return the number of bytes read.
> One option is to return -EINTR if a signal is pending when the mask
> is updated - before even looking at anything else.
>
> Signals that happen later on (eg after a timeout) need not be reported
> (until the next time around the loop).
I'm not sure that's necessary and it would cause delays in
signal handling.
next prev parent reply other threads:[~2019-05-29 18:50 UTC|newest]
Thread overview: 95+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-22 3:21 [PATCH v2] signal: Adjust error codes according to restore_user_sigmask() Deepa Dinamani
2019-05-22 15:05 ` Oleg Nesterov
2019-05-22 15:55 ` Deepa Dinamani
2019-05-22 16:14 ` Oleg Nesterov
2019-05-22 16:33 ` Deepa Dinamani
2019-05-23 9:03 ` David Laight
2019-05-23 14:59 ` Oleg Nesterov
2019-05-23 16:18 ` David Laight
2019-05-23 16:36 ` Oleg Nesterov
2019-05-23 16:56 ` David Laight
2019-05-23 18:06 ` Deepa Dinamani
2019-05-23 20:41 ` Deepa Dinamani
2019-05-23 21:06 ` Deepa Dinamani
2019-05-24 9:58 ` David Laight
2019-05-24 14:10 ` Oleg Nesterov
2019-05-24 15:16 ` Deepa Dinamani
2019-05-24 16:33 ` Oleg Nesterov
2019-05-24 17:01 ` Deepa Dinamani
2019-05-27 15:04 ` Oleg Nesterov
2019-05-28 20:47 ` Deepa Dinamani
2019-05-29 16:57 ` Oleg Nesterov
2019-05-29 18:42 ` Deepa Dinamani
2019-05-28 9:02 ` David Laight
2019-05-28 9:12 ` David Laight
2019-05-28 11:37 ` Deepa Dinamani
2019-05-28 12:04 ` David Laight
2019-05-24 14:19 ` Oleg Nesterov
2019-05-24 14:29 ` Deepa Dinamani
2019-05-24 14:51 ` Oleg Nesterov
2019-05-24 13:29 ` Oleg Nesterov
2019-05-24 14:59 ` David Laight
2019-05-24 15:09 ` David Laight
2019-05-24 15:46 ` Oleg Nesterov
2019-05-24 15:44 ` Oleg Nesterov
2019-05-24 16:40 ` David Laight
2019-05-23 14:33 ` Oleg Nesterov
2019-05-22 22:18 ` Chris Down
2019-05-22 22:52 ` Deepa Dinamani
2019-05-29 16:11 ` pselect/etc semantics (Was: [PATCH v2] signal: Adjust error codes according to restore_user_sigmask()) Oleg Nesterov
2019-05-29 16:54 ` David Laight
2019-05-29 18:50 ` Eric Wong [this message]
2019-05-30 9:34 ` David Laight
2019-05-30 13:04 ` pselect/etc semantics Eric W. Biederman
2019-05-29 16:56 ` pselect/etc semantics (Was: [PATCH v2] signal: Adjust error codes according to restore_user_sigmask()) Deepa Dinamani
2019-05-29 18:26 ` Deepa Dinamani
2019-05-29 22:32 ` Arnd Bergmann
2019-05-30 1:54 ` pselect/etc semantics Eric W. Biederman
2019-05-30 18:28 ` Arnd Bergmann
2019-05-30 14:40 ` pselect/etc semantics (Was: [PATCH v2] signal: Adjust error codes according to restore_user_sigmask()) Oleg Nesterov
2019-05-30 18:37 ` Arnd Bergmann
2019-05-30 13:01 ` pselect/etc semantics Eric W. Biederman
2019-05-30 15:18 ` David Laight
2019-05-30 16:13 ` Oleg Nesterov
2019-05-30 15:38 ` Eric W. Biederman
2019-05-30 15:48 ` Deepa Dinamani
2019-05-30 16:59 ` Deepa Dinamani
2019-05-30 16:08 ` Oleg Nesterov
2019-05-30 17:20 ` Eric W. Biederman
2019-05-30 16:22 ` David Laight
2019-05-30 15:57 ` Oleg Nesterov
2019-05-30 21:03 ` Eric Wong
2019-06-04 13:41 ` [PATCH] signal: remove the wrong signal_pending() check in restore_user_sigmask() Oleg Nesterov
2019-06-04 15:31 ` Eric W. Biederman
2019-06-04 15:57 ` David Laight
2019-06-04 16:37 ` Arnd Bergmann
2019-06-04 18:14 ` Deepa Dinamani
2019-06-04 18:35 ` Eric Wong
2019-06-04 21:26 ` Linus Torvalds
2019-06-04 22:24 ` Eric Wong
2019-06-04 23:51 ` Eric W. Biederman
2019-06-05 9:04 ` Oleg Nesterov
2019-06-05 8:56 ` Oleg Nesterov
2019-06-05 9:02 ` David Laight
2019-06-05 9:25 ` Oleg Nesterov
2019-06-05 9:58 ` David Laight
2019-06-05 15:58 ` [PATCH -mm 0/1] signal: simplify set_user_sigmask/restore_user_sigmask Oleg Nesterov
2019-06-05 15:58 ` [PATCH -mm 1/1] " Oleg Nesterov
2019-06-06 0:14 ` kbuild test robot
2019-06-06 1:06 ` kbuild test robot
2019-06-06 7:25 ` Oleg Nesterov
2019-06-06 7:30 ` Sedat Dilek
2019-06-05 17:24 ` [PATCH -mm 0/1] " Linus Torvalds
2019-06-06 9:05 ` David Laight
2019-06-06 11:05 ` Oleg Nesterov
2019-06-06 11:29 ` David Laight
2019-06-06 12:41 ` Oleg Nesterov
2019-06-06 13:23 ` David Laight
2019-06-06 10:22 ` Oleg Nesterov
2019-06-06 11:32 ` [PATCH -mm V2 1/1] " Oleg Nesterov
2019-06-06 14:08 ` [PATCH 0/2] select: simplify the usage of restore_saved_sigmask_unless() Oleg Nesterov
2019-06-06 14:08 ` [PATCH 1/2] select: change do_poll() to return -ERESTARTNOHAND rather than -EINTR Oleg Nesterov
2019-06-07 18:05 ` Linus Torvalds
2019-06-06 14:09 ` [PATCH 2/2] select: shift restore_saved_sigmask_unless() into poll_select_copy_remaining() Oleg Nesterov
2019-06-07 21:39 ` [RFC PATCH 0/5]: Removing saved_sigmask Eric W. Biederman
2019-06-11 18:58 ` Oleg Nesterov
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