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New race in x86_pmu_stop() was introduced by replacing the atomic __test_and_clear_bit() of cpuc->active_mask by separate test_bit() and __clear_bit() calls in the following commit:3966c3feca("x86/perf/amd: Remove need to check "running" bit in NMI handler") The race causes panic for PEBS events with enabled callchains: BUG: unable to handle kernel NULL pointer dereference at 0000000000000000 ... RIP: 0010:perf_prepare_sample+0x8c/0x530 Call Trace: <NMI> perf_event_output_forward+0x2a/0x80 __perf_event_overflow+0x51/0xe0 handle_pmi_common+0x19e/0x240 intel_pmu_handle_irq+0xad/0x170 perf_event_nmi_handler+0x2e/0x50 nmi_handle+0x69/0x110 default_do_nmi+0x3e/0x100 do_nmi+0x11a/0x180 end_repeat_nmi+0x16/0x1a RIP: 0010:native_write_msr+0x6/0x20 ... </NMI> intel_pmu_disable_event+0x98/0xf0 x86_pmu_stop+0x6e/0xb0 x86_pmu_del+0x46/0x140 event_sched_out.isra.97+0x7e/0x160 ... The event is configured to make samples from PEBS drain code, but when it's disabled, we'll go through NMI path instead, where data->callchain will not get allocated and we'll crash: x86_pmu_stop test_bit(hwc->idx, cpuc->active_mask) intel_pmu_disable_event(event) { ... intel_pmu_pebs_disable(event); ... EVENT OVERFLOW -> <NMI> intel_pmu_handle_irq handle_pmi_common TEST PASSES -> test_bit(bit, cpuc->active_mask)) perf_event_overflow perf_prepare_sample { ... if (!(sample_type & __PERF_SAMPLE_CALLCHAIN_EARLY)) data->callchain = perf_callchain(event, regs); CRASH -> size += data->callchain->nr; } </NMI> ... x86_pmu_disable_event(event) } __clear_bit(hwc->idx, cpuc->active_mask); Fixing this by disabling the event itself before setting off the PEBS bit. Signed-off-by: Jiri Olsa <jolsa@kernel.org> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com> Cc: Arnaldo Carvalho de Melo <acme@redhat.com> Cc: David Arcari <darcari@redhat.com> Cc: Jiri Olsa <jolsa@redhat.com> Cc: Lendacky Thomas <Thomas.Lendacky@amd.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Stephane Eranian <eranian@google.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Vince Weaver <vincent.weaver@maine.edu> Fixes:3966c3feca("x86/perf/amd: Remove need to check "running" bit in NMI handler") Link: http://lkml.kernel.org/r/20190504151556.31031-1-jolsa@kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.