powerpc/watchpoints: Disable preemption in thread_change_pc()

thread_change_pc() uses CPU local data, so must be protected from
swapping CPUs while it is reading the breakpoint struct.

The error is more noticeable after 1e60f3564bad ("powerpc/watchpoints:
Track perf single step directly on the breakpoint"), which added an
unconditional __this_cpu_read() call in thread_change_pc(). However the
existing __this_cpu_read() that runs if a breakpoint does need to be
re-inserted has the same issue.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230829063457.54157-2-bgray@linux.ibm.com

diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
index b8513dc..2854376 100644 (file)
--- a/arch/powerpc/kernel/hw_breakpoint.c
+++ b/arch/powerpc/kernel/hw_breakpoint.c
@@ -230,13 +230,15 @@ void thread_change_pc(struct task_struct *tsk, struct pt_regs *regs)
        struct arch_hw_breakpoint *info;
        int i;
 
+       preempt_disable();
+
        for (i = 0; i < nr_wp_slots(); i++) {
                struct perf_event *bp = __this_cpu_read(bp_per_reg[i]);
 
                if (unlikely(bp && counter_arch_bp(bp)->perf_single_step))
                        goto reset;
        }
-       return;
+       goto out;
 
 reset:
        regs_set_return_msr(regs, regs->msr & ~MSR_SE);
@@ -245,6 +247,9 @@ reset:
                __set_breakpoint(i, info);
                info->perf_single_step = false;
        }
+
+out:
+       preempt_enable();
 }
 
 static bool is_larx_stcx_instr(int type)