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Since commit:1fd7e41699("perf/core: Remove perf_cpu_context::unique_pmu") ... when a PMU is unregistered then its associated ->pmu_cpu_context is unconditionally freed. Whilst this is fine for dynamically allocated context types (i.e. those registered using perf_invalid_context), this causes a problem for sharing of static contexts such as perf_{sw,hw}_context, which are used by multiple built-in PMUs and effectively have a global lifetime. Whilst testing the ARM SPE driver, which must use perf_sw_context to support per-task AUX tracing, unregistering the driver as a result of a module unload resulted in: Unable to handle kernel NULL pointer dereference at virtual address 00000038 Internal error: Oops: 96000004 [#1] PREEMPT SMP Modules linked in: [last unloaded: arm_spe_pmu] PC is at ctx_resched+0x38/0xe8 LR is at perf_event_exec+0x20c/0x278 [...] ctx_resched+0x38/0xe8 perf_event_exec+0x20c/0x278 setup_new_exec+0x88/0x118 load_elf_binary+0x26c/0x109c search_binary_handler+0x90/0x298 do_execveat_common.isra.14+0x540/0x618 SyS_execve+0x38/0x48 since the software context has been freed and the ctx.pmu->pmu_disable_count field has been set to NULL. This patch fixes the problem by avoiding the freeing of static PMU contexts altogether. Whilst the sharing of dynamic contexts is questionable, this actually requires the caller to share their context pointer explicitly and so the burden is on them to manage the object lifetime. Reported-by: Kim Phillips <kim.phillips@arm.com> Signed-off-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Mark Rutland <mark.rutland@arm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Fixes:1fd7e41699("perf/core: Remove perf_cpu_context::unique_pmu") Link: http://lkml.kernel.org/r/1507040450-7730-1-git-send-email-will.deacon@arm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.