Re: [RFC v3 1/9] sysrq: Implement __handle_sysrq_nolock to avoid recursive locking in kdb
On Tue, Apr 29, 2014 at 1:59 AM, Daniel Thompson wrote: > On 28/04/14 18:44, Colin Cross wrote: Is that case documented somewhere in the code comments? >>> >>> Perhaps not near enough to the _nolock but the primary bit of comment is >>> here (and in same file as kdb_sr). >>> --- cut here --- >>> * kdb_main_loop - After initial setup and assignment of the >>> * controlling cpu, all cpus are in this loop. One cpu is in >>> * control and will issue the kdb prompt, the others will spin >>> * until 'go' or cpu switch. >>> --- cut here --- >>> >>> The mechanism kgdb uses to quiesce other CPUs means other CPUs cannot be >>> in irqsave critical sections. >>> >>> >> >> One of the advantages of FIQ debugger is that it can be triggered from >> an FIQ (NMI for those in x86 land), and Jason and I have discussed >> using FIQs for kgdb to allow interrupting cpus stuck in critical >> sections. If that gets implemented the above assumption will no >> longer be correct. > > Reviewing this I realized I missed one of the most critical points in > the above. > > Today kdb, even if triggered by FIQ/NMI, would still be likely to wedge > waiting for the IPI interrupts to be delivered to other processors. > > Did you and Jason discuss getting the active CPU to quiesce the other > processors using FIQ/NMI, or to allow the active CPU to timeout while > waiting for them the stop? > > > Daniel. Yes, all cpus would have to get an FIQ/NMI. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC v3 1/9] sysrq: Implement __handle_sysrq_nolock to avoid recursive locking in kdb
On 28/04/14 18:44, Colin Cross wrote: >>> Is that case documented somewhere in the code comments? >> >> Perhaps not near enough to the _nolock but the primary bit of comment is >> here (and in same file as kdb_sr). >> --- cut here --- >> * kdb_main_loop - After initial setup and assignment of the >> * controlling cpu, all cpus are in this loop. One cpu is in >> * control and will issue the kdb prompt, the others will spin >> * until 'go' or cpu switch. >> --- cut here --- >> >> The mechanism kgdb uses to quiesce other CPUs means other CPUs cannot be >> in irqsave critical sections. >> >> > > One of the advantages of FIQ debugger is that it can be triggered from > an FIQ (NMI for those in x86 land), and Jason and I have discussed > using FIQs for kgdb to allow interrupting cpus stuck in critical > sections. If that gets implemented the above assumption will no > longer be correct. Reviewing this I realized I missed one of the most critical points in the above. Today kdb, even if triggered by FIQ/NMI, would still be likely to wedge waiting for the IPI interrupts to be delivered to other processors. Did you and Jason discuss getting the active CPU to quiesce the other processors using FIQ/NMI, or to allow the active CPU to timeout while waiting for them the stop? Daniel. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC v3 1/9] sysrq: Implement __handle_sysrq_nolock to avoid recursive locking in kdb
On 28/04/14 18:44, Colin Cross wrote: >>> Is that case documented somewhere in the code comments? >> >> Perhaps not near enough to the _nolock but the primary bit of comment is >> here (and in same file as kdb_sr). >> --- cut here --- >> * kdb_main_loop - After initial setup and assignment of the >> * controlling cpu, all cpus are in this loop. One cpu is in >> * control and will issue the kdb prompt, the others will spin >> * until 'go' or cpu switch. >> --- cut here --- >> >> The mechanism kgdb uses to quiesce other CPUs means other CPUs cannot be >> in irqsave critical sections. >> >> > > One of the advantages of FIQ debugger is that it can be triggered from > an FIQ (NMI for those in x86 land), and Jason and I have discussed > using FIQs for kgdb to allow interrupting cpus stuck in critical > sections. If that gets implemented the above assumption will no > longer be correct. Quite so (I've got Anton's old FIQ patches running on latest kernel and am trying to port to a GICv2-without-trustzone qemu model I've written in order to kick the idea about a bit on an ARM multi-arch kernel). This patch has therefore pained me a little bit to not complete cover this case in the patch. As posted I deliberately ignore the problem. In this particular case the SysRq table is so infrequently updated the chances of an badly timed NMI are vanishingly small and, at that point, even if we did actually hit that tiny window its *still* better to have the new behaviour (risk of race) than the old behaviour (guaranteed deadlock). I'd very much welcome other ideas (I have tried out quite a few in my head but none solve the problem of NMI "gratuitiously" hitting critical sections). However when NMI/FIQ finally comes along I'd be tempted to borrow the "bounce to normal interrupt mode" idea from FIQ debugger and ensure commands like "sr" command do not run from the NMI handler. Daniel. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC v3 1/9] sysrq: Implement __handle_sysrq_nolock to avoid recursive locking in kdb
On Mon, Apr 28, 2014 at 3:24 AM, Daniel Thompson wrote: > On 25/04/14 17:45, Steven Rostedt wrote: >> On Fri, 25 Apr 2014 17:29:22 +0100 >> Daniel Thompson wrote: >> >>> If kdb is triggered using SysRq-g then any use of the sr command results >>> in the SysRq key table lock being recursively acquired, killing the debug >>> session. That patch resolves the problem by introducing a _nolock >>> alternative for __handle_sysrq. >>> >>> Strictly speaking this approach risks racing on the key table when kdb is >>> triggered by something other than SysRq-g however in that case any other >>> CPU involved should release the spin lock before kgdb parks the slave >>> CPUs. >> >> Is that case documented somewhere in the code comments? > > Perhaps not near enough to the _nolock but the primary bit of comment is > here (and in same file as kdb_sr). > --- cut here --- > * kdb_main_loop - After initial setup and assignment of the > * controlling cpu, all cpus are in this loop. One cpu is in > * control and will issue the kdb prompt, the others will spin > * until 'go' or cpu switch. > --- cut here --- > > The mechanism kgdb uses to quiesce other CPUs means other CPUs cannot be > in irqsave critical sections. > > One of the advantages of FIQ debugger is that it can be triggered from an FIQ (NMI for those in x86 land), and Jason and I have discussed using FIQs for kgdb to allow interrupting cpus stuck in critical sections. If that gets implemented the above assumption will no longer be correct. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC v3 1/9] sysrq: Implement __handle_sysrq_nolock to avoid recursive locking in kdb
On 25/04/14 17:45, Steven Rostedt wrote: > On Fri, 25 Apr 2014 17:29:22 +0100 > Daniel Thompson wrote: > >> If kdb is triggered using SysRq-g then any use of the sr command results >> in the SysRq key table lock being recursively acquired, killing the debug >> session. That patch resolves the problem by introducing a _nolock >> alternative for __handle_sysrq. >> >> Strictly speaking this approach risks racing on the key table when kdb is >> triggered by something other than SysRq-g however in that case any other >> CPU involved should release the spin lock before kgdb parks the slave >> CPUs. > > Is that case documented somewhere in the code comments? Perhaps not near enough to the _nolock but the primary bit of comment is here (and in same file as kdb_sr). --- cut here --- * kdb_main_loop - After initial setup and assignment of the * controlling cpu, all cpus are in this loop. One cpu is in * control and will issue the kdb prompt, the others will spin * until 'go' or cpu switch. --- cut here --- The mechanism kgdb uses to quiesce other CPUs means other CPUs cannot be in irqsave critical sections. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC v3 1/9] sysrq: Implement __handle_sysrq_nolock to avoid recursive locking in kdb
On Fri, 25 Apr 2014 17:29:22 +0100 Daniel Thompson wrote: > If kdb is triggered using SysRq-g then any use of the sr command results > in the SysRq key table lock being recursively acquired, killing the debug > session. That patch resolves the problem by introducing a _nolock > alternative for __handle_sysrq. > > Strictly speaking this approach risks racing on the key table when kdb is > triggered by something other than SysRq-g however in that case any other > CPU involved should release the spin lock before kgdb parks the slave > CPUs. Is that case documented somewhere in the code comments? -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
