Date: Wed, 8 Jun 2016 16:56:35 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Mark Johnston <markj@FreeBSD.org> Cc: Jilles Tjoelker <jilles@stack.nl>, freebsd-current@FreeBSD.org, cem@FreeBSD.org Subject: Re: thread suspension when dumping core Message-ID: <20160608135635.GY38613@kib.kiev.ua> In-Reply-To: <20160608133508.GA93263@charmander> References: <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> <20160607160155.GP38613@kib.kiev.ua> <20160607211919.GA49961@stack.nl> <20160608043055.GV38613@kib.kiev.ua> <20160608133508.GA93263@charmander>
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On Wed, Jun 08, 2016 at 06:35:08AM -0700, Mark Johnston wrote: > On Wed, Jun 08, 2016 at 07:30:55AM +0300, Konstantin Belousov wrote: > > On Tue, Jun 07, 2016 at 11:19:19PM +0200, Jilles Tjoelker wrote: > > > I also wonder whether we may be overengineering things here. Perhaps > > > the advlock sleep can simply turn off TDF_SBDRY. > > Well, this was the very first patch suggested. I would be fine with that, > > but again, out-of-tree code seems to be not quite fine with that local > > solution. > > In our particular case, we could possibly use a similar approach. In > general, it seems incorrect to clear TDF_SBDRY if the thread calling > sx_sleep() has any locks held. It is easy to verify that all callers of > lf_advlock() are safe in this respect, but this kind of auditing is > generally hard. In fact, I believe the sx_sleep that led to the problem > described in D2612 is the same as the one in my case. That is, the > sleeping thread may or may not hold a vnode lock depending on context. I do not think that in-tree code sleeps with a vnode lock held in the lf_advlock(). Otherwise, system would hang in lock cascade by an attempt to obtain an advisory lock. I think we can even assert this with witness. There is another sleep, which Jilles mentioned, in lf_purgelocks(), called from vgone(). This sleep indeed occurs under the vnode lock, and as such must be non-suspendable. The sleep waits until other threads leave the lf_advlock() for the reclaimed vnode, and they should leave in deterministic time due to issued wakeups. So this sleep is exempt from the considerations, and TDF_SBDRY there is correct. I am fine with either the braces around sx_sleep() in lf_advlock() to clear TDF_SBDRY (sigdeferstsop()), or with the latest patch I sent, which adds temporal override for TDF_SBDRY with TDF_SRESTART. My understanding is that you prefer the later. If I do not mis-represent your position, I understand why you do prefer that.
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