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A recent discussion started talking about the locking on the pstore fs and how it relates to the kmsg infrastructure. We noticed it was possible for userspace to r/w to the pstore fs (grabbing the locks in the process) and block the panic path from r/w to the same fs. The reason was the cpu with the lock could be doing work while the crashing cpu is panic'ing. Busting those spinlocks might cause those cpus to step on each other's data. Fine, fair enough. It was suggested it would be nice to serialize the panic path (ie stop the other cpus) and have only one cpu running. This would allow us to bust the spinlocks and not worry about another cpu stepping on the data. Of course, smp_send_stop() does this in the panic case. kmsg_dump() would have to be moved to be called after it. Easy enough. The only problem is on x86 the smp_send_stop() function calls the REBOOT_VECTOR. Any cpu with irqs disabled (which pstore and its backend ERST would do), block this IPI and thus do not stop. This makes it difficult to reliably log data to the pstore fs. The patch below switches from the REBOOT_VECTOR to NMI (and mimics what kdump does). Switching to NMI allows us to deliver the IPI when irqs are disabled, increasing the reliability of this function. However, Andi carefully noted that on some machines this approach does not work because of broken BIOSes or whatever. To help accomodate this, the next couple of patches will run a selftest and provide a knob to disable. V2: uses atomic ops to serialize the cpu that shuts everyone down V3: comment cleanup Signed-off-by: Don Zickus <dzickus@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Robert Richter <robert.richter@amd.com> Cc: seiji.aguchi@hds.com Cc: vgoyal@redhat.com Cc: mjg@redhat.com Cc: tony.luck@intel.com Cc: gong.chen@intel.com Cc: satoru.moriya@hds.com Cc: avi@redhat.com Cc: Andi Kleen <andi@firstfloor.org> Link: http://lkml.kernel.org/r/1318533267-18880-2-git-send-email-dzickus@redhat.com Signed-off-by: Ingo Molnar <mingo@elte.hu> |
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