[PATCH 4/8] ARM: hw_breakpoint: disable preemption during debug exception handling
From: Will Deacon <hidden>
Date: 2010-11-29 17:34:44
Subsystem:
arm pmu profiling and debugging, arm port, the rest · Maintainers:
Will Deacon, Mark Rutland, Russell King, Linus Torvalds
On ARM, debug exceptions occur in the form of data or prefetch aborts. One difference is that debug exceptions require access to per-cpu banked registers and data structures which are not saved in the low-level exception code. For kernels built with CONFIG_PREEMPT, there is an unlikely scenario that the debug handler ends up running on a different CPU from the one that originally signalled the event, resulting in random data being read from the wrong registers. This patch adds a debug_entry macro to the low-level exception handling code which checks whether the taken exception is a debug exception. If it is, the preempt count for the faulting process is incremented. After the debug handler has finished, the count is decremented. Acked-by: Catalin Marinas <catalin.marinas@arm.com> Signed-off-by: Will Deacon <redacted> --- arch/arm/kernel/entry-armv.S | 4 ++++ arch/arm/kernel/entry-header.S | 19 +++++++++++++++++++ arch/arm/kernel/hw_breakpoint.c | 14 +++++++++----- 3 files changed, 32 insertions(+), 5 deletions(-)
diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index c09e357..34bbef0 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S@@ -198,6 +198,7 @@ __dabt_svc: @ @ set desired IRQ state, then call main handler @ + debug_entry r1 msr cpsr_c, r9 mov r2, sp bl do_DataAbort
@@ -324,6 +325,7 @@ __pabt_svc: #else bl CPU_PABORT_HANDLER #endif + debug_entry r1 msr cpsr_c, r9 @ Maybe enable interrupts mov r2, sp @ regs bl do_PrefetchAbort @ call abort handler
@@ -439,6 +441,7 @@ __dabt_usr: @ @ IRQs on, then call the main handler @ + debug_entry r1 enable_irq mov r2, sp adr lr, BSYM(ret_from_exception)
@@ -703,6 +706,7 @@ __pabt_usr: #else bl CPU_PABORT_HANDLER #endif + debug_entry r1 enable_irq @ Enable interrupts mov r2, sp @ regs bl do_PrefetchAbort @ call abort handler
diff --git a/arch/arm/kernel/entry-header.S b/arch/arm/kernel/entry-header.S
index d93f976..ae94649 100644
--- a/arch/arm/kernel/entry-header.S
+++ b/arch/arm/kernel/entry-header.S@@ -165,6 +165,25 @@ .endm #endif /* !CONFIG_THUMB2_KERNEL */ + @ + @ Debug exceptions are taken as prefetch or data aborts. + @ We must disable preemption during the handler so that + @ we can access the debug registers safely. + @ + .macro debug_entry, fsr +#if defined(CONFIG_HAVE_HW_BREAKPOINT) && defined(CONFIG_PREEMPT) + ldr r4, =0x40f @ mask out fsr.fs + and r5, r4, \fsr + cmp r5, #2 @ debug exception + bne 1f + get_thread_info r10 + ldr r6, [r10, #TI_PREEMPT] @ get preempt count + add r11, r6, #1 @ increment it + str r11, [r10, #TI_PREEMPT] +1: +#endif + .endm + /* * These are the registers used in the syscall handler, and allow us to * have in theory up to 7 arguments to a function - r0 to r6.
diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c
index 550d949..61eeb41 100644
--- a/arch/arm/kernel/hw_breakpoint.c
+++ b/arch/arm/kernel/hw_breakpoint.c@@ -737,12 +737,12 @@ unlock: /* * Called from either the Data Abort Handler [watchpoint] or the - * Prefetch Abort Handler [breakpoint]. + * Prefetch Abort Handler [breakpoint] with preemption disabled. */ static int hw_breakpoint_pending(unsigned long addr, unsigned int fsr, struct pt_regs *regs) { - int ret = 1; /* Unhandled fault. */ + int ret = 0; u32 dscr; /* We only handle watchpoints and hardware breakpoints. */
@@ -759,11 +759,15 @@ static int hw_breakpoint_pending(unsigned long addr, unsigned int fsr, watchpoint_handler(addr, regs); break; default: - goto out; + ret = 1; /* Unhandled fault. */ } - ret = 0; -out: + /* + * Re-enable preemption after it was disabled in the + * low-level exception handling code. + */ + preempt_enable(); + return ret; }
--
1.7.0.4