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Re: [PATCH v3 1/1] waitid: Add support for waiting for the current process group
- From: Rich Felker <dalias at libc dot org>
- To: Christian Brauner <christian dot brauner at ubuntu dot com>
- Cc: Oleg Nesterov <oleg at redhat dot com>, linux-kernel at vger dot kernel dot org, libc-alpha at sourceware dot org, alistair23 at gmail dot com, ebiederm at xmission dot com, arnd at arndb dot de, torvalds at linux-foundation dot org, adhemerval dot zanella at linaro dot org, fweimer at redhat dot com, palmer at sifive dot com, macro at wdc dot com, zongbox at gmail dot com, akpm at linux-foundation dot org, viro at zeniv dot linux dot org dot uk, hpa at zytor dot com
- Date: Wed, 14 Aug 2019 12:55:01 -0400
- Subject: Re: [PATCH v3 1/1] waitid: Add support for waiting for the current process group
- References: <CAKmqyKMJPQAOKn11xepzAwXOd4e9dU0Cyz=A0T-uMEgUp5yJjA@mail.gmail.com> <20190814154400.6371-1-christian.brauner@ubuntu.com> <20190814154400.6371-2-christian.brauner@ubuntu.com> <20190814160917.GG11595@redhat.com> <20190814161517.ldbn62mulk2pmqo5@wittgenstein> <20190814163443.6odsksff4jbta7be@wittgenstein>
On Wed, Aug 14, 2019 at 06:34:44PM +0200, Christian Brauner wrote:
> On Wed, Aug 14, 2019 at 06:15:17PM +0200, Christian Brauner wrote:
> > On Wed, Aug 14, 2019 at 06:09:17PM +0200, Oleg Nesterov wrote:
> > > On 08/14, Christian Brauner wrote:
> > > >
> > > > and a signal could come in between the system call that
> > > > retrieved the process gorup and the call to waitid that changes the
> > > ^^^^^
> > > > current process group.
> > >
> > > I noticed this typo only because I spent 2 minutes or more trying to
> > > understand this sentence ;) But yes, a signal handler or another thread
> >
> > I'll try to rewrite it. :)
>
> Ok, here's what I changed it to:
>
> It was recently discovered that the linux version of waitid is not a
> superset of the other wait functions because it does not include
> support for waiting for the current process group. This has two
> downsides:
> 1. An extra system call is needed to get the current process group.
> 2. After the current process group is received and before it is passed
> to waitid a signal could arrive causing the current process group to change.
I don't think "downsides" sufficiently conveys that this is hard
breakage of a requirement for waitpid. How about something like the
following?
"It was recently discovered that the linux version of waitid is not a
superset of the other wait functions because it does not include
support for waiting for the current process group. Userspace cannot
simply emulate this functionality with an additional getpgid syscall
due to inherent race conditions that violate the async-signal safety
requirements for waitpid."
Rich