The following commit has been merged into the core/entry branch of tip: Commit-ID: 179a9cf79212bb3b96fb69a314583189cd863c5b Gitweb: https://git.kernel.org/tip/179a9cf79212bb3b96fb69a314583189cd863c5b Author: Frederic Weisbecker <frederic@xxxxxxxxxx> AuthorDate: Tue, 17 Nov 2020 16:16:34 +01:00 Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx> CommitterDate: Thu, 19 Nov 2020 11:25:42 +01:00 context_tracking: Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK The typical steps with context tracking are: 1) Task runs in userspace 2) Task enters the kernel (syscall/exception/IRQ) 3) Task switches from context tracking state CONTEXT_USER to CONTEXT_KERNEL (user_exit()) 4) Task does stuff in kernel 5) Task switches from context tracking state CONTEXT_KERNEL to CONTEXT_USER (user_enter()) 6) Task exits the kernel If an exception fires between 5) and 6), the pt_regs and the context tracking disagree on the context of the faulted/trapped instruction. CONTEXT_KERNEL must be set before the exception handler, that's unconditional for those handlers that want to be able to call into schedule(), but CONTEXT_USER must be restored when the exception exits whereas pt_regs tells that we are resuming to kernel space. This can't be fixed with storing the context tracking state in a per-cpu or per-task variable since another exception may fire onto the current one and overwrite the saved state. Also the task can schedule. So it has to be stored in a per task stack. This is how exception_enter()/exception_exit() paper over the problem: 5) Task switches from context tracking state CONTEXT_KERNEL to CONTEXT_USER (user_enter()) 5.1) Exception fires 5.2) prev_state = exception_enter() // save CONTEXT_USER to prev_state // and set CONTEXT_KERNEL 5.3) Exception handler 5.4) exception_enter(prev_state) // restore CONTEXT_USER 5.5) Exception resumes 6) Task exits the kernel The condition to live without exception_enter()/exception_exit() is to forbid exceptions and IRQs between 2) and 3) and between 5) and 6), or if any is allowed to trigger, it won't call into context tracking, eg: NMIs, and it won't schedule. These requirements are met by architectures supporting CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK and those can therefore afford not to implement this hack. Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Link: https://lkml.kernel.org/r/20201117151637.259084-3-frederic@xxxxxxxxxx --- include/linux/context_tracking.h | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h index d53cd33..bceb064 100644 --- a/include/linux/context_tracking.h +++ b/include/linux/context_tracking.h @@ -51,7 +51,8 @@ static inline enum ctx_state exception_enter(void) { enum ctx_state prev_ctx; - if (!context_tracking_enabled()) + if (IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) || + !context_tracking_enabled()) return 0; prev_ctx = this_cpu_read(context_tracking.state); @@ -63,7 +64,8 @@ static inline enum ctx_state exception_enter(void) static inline void exception_exit(enum ctx_state prev_ctx) { - if (context_tracking_enabled()) { + if (!IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) && + context_tracking_enabled()) { if (prev_ctx != CONTEXT_KERNEL) context_tracking_enter(prev_ctx); }
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