Messages in this thread | | | Date | Wed, 9 Dec 2020 13:42:33 +0100 | From | Peter Zijlstra <> | Subject | Re: [patch V2 4/9] softirq: Make softirq control and processing RT aware |
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On Wed, Dec 09, 2020 at 01:36:54PM +0100, Thomas Gleixner wrote: > On Wed, Dec 09 2020 at 11:11, Peter Zijlstra wrote: > > On Fri, Dec 04, 2020 at 06:01:55PM +0100, Thomas Gleixner wrote: > >> From: Thomas Gleixner <tglx@linutronix.de> > >> + /* First entry of a task into a BH disabled section? */ > >> + if (!current->softirq_disable_cnt) { > >> + if (preemptible()) { > >> + local_lock(&softirq_ctrl.lock); > > > > AFAICT this significantly changes the locking rules. > > > > Where previously we could do: > > > > spin_lock(&ponies) > > spin_lock_bh(&foo); > > > > vs > > > > spin_lock_bh(&bar); > > spin_lock(&ponies) > > > > provided the rest of the code observed: bar -> ponies -> foo > > and never takes ponies from in-softirq. > > > > This is now a genuine deadlock on RT. > > I know, but making this work is trying to square the circle.
:-)
> Any approach we tried before going this way had worse problems than > this particular limitation.
OK, but that would've been very good Changelog material methinks.
Also, then we should probably make sure PREEMPT_RT=n builds start suffering the same problem by adding the local_lock unconditionally, otherwise this keeps being a PREEMPT_RT special and we'll keep having to fix it up.
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