Date: Tue, 7 Jun 2016 19:01:55 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Jilles Tjoelker <jilles@stack.nl> Cc: Mark Johnston <markj@FreeBSD.org>, freebsd-current@FreeBSD.org, cem@FreeBSD.org Subject: Re: thread suspension when dumping core Message-ID: <20160607160155.GP38613@kib.kiev.ua> In-Reply-To: <20160607142452.GA48251@stack.nl> References: <20160604022347.GA1096@wkstn-mjohnston.west.isilon.com> <20160604093236.GA38613@kib.kiev.ua> <20160606171311.GC10101@wkstn-mjohnston.west.isilon.com> <20160607024610.GI38613@kib.kiev.ua> <20160607041741.GA29017@wkstn-mjohnston.west.isilon.com> <20160607042956.GM38613@kib.kiev.ua> <20160607142452.GA48251@stack.nl>
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On Tue, Jun 07, 2016 at 04:24:53PM +0200, Jilles Tjoelker wrote: > On Tue, Jun 07, 2016 at 07:29:56AM +0300, Konstantin Belousov wrote: > > This looks as if we should not ignore suspension requests in > > thread_suspend_check() completely in TDF_SBDRY case, but return either > > EINTR or ERESTART (most likely ERESTART). Note that the goal of > > TDF_SBDRY is to avoid suspending in the protected region, not to make an > > impression that the suspension does not occur at all. > > This looks like it would revert r246417 and re-introduce the bug fixed > by it (unexpected [EINTR] and short reads/writes after stop signals). Well, the patch returns ERESTART and not EINTR, so the syscall should be retried after all the unwinding. > > After r246417, TDF_SBDRY is intended for sleeps that occur while holding > resources such as vnode locks and are normally short but should be > interruptible by fatal signals because they may occasionally be > indefinitely long (such as a non-responsive NFS server). > > It looks like yet another kind of sleep may be required, since advisory > locks still hold some filesystem resources across the sleep (though not > vnode locks). I do not think that adv locks enter sleep with any resource held which would block other threads. But I agree with the statement because the lock might be granted and then the stopped thread would appear to own the blocking resource. > > We then have four kinds: > > * uninterruptible by signals, ignores stops (default) > * interruptible by signals, ignores stops (current TDF_SBDRY with > PCATCH) > * interruptible by signals, freezes in place on stops (avoids > unexpected short I/O) (current PCATCH, otherwise) > * interruptible by signals, fails with [ERESTART] on stops (avoids > holding resources across a stop) (new) > > The new kind of sleep would fail with [ERESTART] only for stops, since > [EINTR] should only be returned if a signal handler was called. There > cannot be a signal handler since a SIGTSTP/SIGTTIN/SIGTTOU signal with a > handler does not stop the process. > And where would this new kind of sleep used ? The advlock sleep is the one place. Does fifo sleep for reader or writer on open require this kind of handling (IMO no) ? I think this can be relatively easily implemented with either a flag for XXXsleep(9) (my older style of PBDRY) or using only the thread flag (jhb' newer TDF_SBDRY approach). Probably the later should be used, for consistency and easier marking of larger blocks of code.
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