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732f3ca7d4
Mathieu Desnoyers revealed a bug in the original code. The nop that is used to relpace the mcount caller can be a two part nop. This runs the risk where a process can be preempted after executing the first nop, but before the second part of the nop. The ftrace code calls kstop_machine to keep multiple CPUs from executing code that is being modified, but it does not protect against a task preempting in the middle of a two part nop. If the above preemption happens and the tracer is enabled, after the kstop_machine runs, all those nops will be calls to the trace function. If the preempted process that was preempted between the two nops is executed again, it will execute half of the call to the trace function, and this might crash the system. This patch instead uses what both the latest Intel and AMD spec suggests. That is the P6_NOP5 sequence of "0x0f 0x1f 0x44 0x00 0x00". Note, some older CPUs and QEMU might fault on this nop, so this nop is executed with fault handling first. If it detects a fault, it will then use the code "0x66 0x66 0x66 0x66 0x90". If that faults, it will then default to a simple "jmp 1f; .byte 0x00 0x00 0x00; 1:". The jmp is not optimal but will do if the first two can not be executed. TODO: Examine the cpuid to determine the nop to use. Signed-off-by: Steven Rostedt <srostedt@redhat.com> Signed-off-by: Ingo Molnar <mingo@elte.hu> |
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