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From: Brandon Williams <bmwill@google.com>
To: Jeff King <peff@peff.net>
Cc: Michael Haggerty <mhagger@alum.mit.edu>,
	Lars Schneider <larsxschneider@gmail.com>,
	Martin Ågren <martin.agren@gmail.com>,
	Git Users <git@vger.kernel.org>
Subject: Re: [PATCH] config: use a static lock_file struct
Date: Wed, 30 Aug 2017 14:06:02 -0700
Message-ID: <20170830210602.GC50018@google.com> (raw)
In-Reply-To: <20170830201132.kmtp5mfu6qotn5s7@sigill.intra.peff.net>

On 08/30, Jeff King wrote:
> On Wed, Aug 30, 2017 at 12:57:31PM -0700, Brandon Williams wrote:
> 
> > > And I think we're fine there even with a doubly-linked list. It's still
> > > the single update of the "next" pointer that controls that second
> > > traversal.
> > 
> > I know it was mentioned earlier but if this is a critical section, and
> > it would be bad if it was interrupted, then couldn't we turn off
> > interrupts before attempting to remove an item from the list?
> 
> If we have to, that's an option. I mostly didn't want to get into the
> portability headaches of signal-handling if we can avoid it.
> 
> Right now sigchain uses plain old signal(). We do use sigaction() in a
> few places, but in a vanilla way. So the portability questions are:
> 
>   - can we rely on using more interesting bits of sigaction like
>     sa_mask?  Probably.
> 
>   - which flags should we be setting in sa_flags to get the same
>     behavior we've been getting with signal()? Or alternatively, which
>     behavior do we want?
> 
> On Linux and BSD systems, at least, signal() defers repeat instances of
> the handled signal. So getting two quick SIGTERMs will not interrupt the
> handler to deliver the second. But getting SIGTERM followed by SIGINT would.
> 
> We could extend that protection by having sigchain_push_common() set
> sa_mask to cover all of the related signals. On Linux and BSD the
> current code using signal() also implies SA_RESTART. We could add that
> to our flags, though I suspect in practice it doesn't matter. Whenever
> we establish a handler like this our intent is to never return from it.
> 
> That just protects us from calling the _same_ handler from itself. But
> that's probably enough in practice, and means handlers don't have to
> worry about "critical sections". The other alternative is sigprocmask()
> to block signals entirely during a section. I'm not sure if there are
> portability questions there (it looks like we have a mingw wrapper
> there, but it's a complete noop).

Yeah there's a lot about signals that I'm not very clear on.  I do know
that Eric helped me out on the fork-exec series I worked on earlier in
the year and I believe it was to turn on/off signals during process
launches in 45afb1ca9 (run-command: block signals between fork and
execve, 2017-04-19).  Though that bit of code is strictly for unix so I
wouldn't know how that would work on windows machines.  Portability does
seem to always be a challenging problem.

-- 
Brandon Williams

  reply index

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-08-27  7:37 [PATCH] pkt-line: re-'static'-ify buffer in packet_write_fmt_1() Martin Ågren
2017-08-27 15:41 ` Jeff King
2017-08-27 18:25   ` Jeff King
2017-08-27 18:21 ` Lars Schneider
2017-08-27 19:09   ` Martin Ågren
2017-08-27 19:15     ` Jeff King
2017-08-27 20:04     ` Lars Schneider
2017-08-27 23:23       ` Jeff King
2017-08-28  4:11         ` Martin Ågren
2017-08-28 17:52           ` Stefan Beller
2017-08-28 17:58             ` Jeff King
2017-09-05 10:03               ` Junio C Hamano
2017-08-29 17:51           ` Lars Schneider
2017-08-29 18:53             ` Jeff King
2017-08-29 18:58               ` [PATCH] config: use a static lock_file struct Jeff King
2017-08-29 19:09                 ` Brandon Williams
2017-08-29 19:10                   ` Brandon Williams
2017-08-29 19:12                   ` Jeff King
2017-08-30  3:25                     ` Michael Haggerty
2017-08-30  4:31                       ` Jeff King
2017-08-30  4:55                         ` Michael Haggerty
2017-08-30  4:55                         ` Jeff King
2017-08-30  5:55                           ` Jeff King
2017-08-30  7:07                             ` Michael Haggerty
2017-09-02  8:44                               ` Jeff King
2017-09-02 13:50                                 ` Jeff King
2017-08-30  6:55                           ` Michael Haggerty
2017-08-30 19:53                             ` Jeff King
2017-08-30 19:57                               ` Brandon Williams
2017-08-30 20:11                                 ` Jeff King
2017-08-30 21:06                                   ` Brandon Williams [this message]
2017-08-31  4:09                                     ` Jeff King
2017-09-06  3:59                 ` Junio C Hamano
2017-09-06 12:41                   ` Jeff King
2017-08-29 19:22               ` [PATCH] pkt-line: re-'static'-ify buffer in packet_write_fmt_1() Martin Ågren
2017-08-29 21:48                 ` Jeff King
2017-08-30  5:31                   ` Jeff King
2017-09-05 10:03                     ` Junio C Hamano
2017-10-10  4:06                       ` [PATCH 0/2] Do not call cmd_*() as a subroutine Junio C Hamano
2017-10-10  4:06                         ` [PATCH 1/2] describe: do not use " Junio C Hamano
2017-10-10 13:43                           ` SZEDER Gábor
2017-10-11  6:00                             ` Junio C Hamano
2017-10-10  4:06                         ` [PATCH 2/2] merge-ours: " Junio C Hamano

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