A bug (1647683) was reported showing a crash when removing breakpoints. The reproducer was bisected to 3359baad when tb_flush was finally made thread safe. While in MTTCG the locking in breakpoint_invalidate would have prevented any problems, but currently tb_lock() is a NOP for system emulation.
The race is between a tb_flush from the gdbstub and the tb_invalidate_phys_addr() in breakpoint_invalidate(). Ideally we'd have actual locking here; for the moment the simple fix is to do a full tb_flush() for a bp invalidate, since that is thread-safe even if no lock is taken. Reported-by: Julian Brown <jul...@codesourcery.com> Signed-off-by: Peter Maydell <peter.mayd...@linaro.org> --- This is quite similar to Alex's patch http://patchwork.ozlabs.org/patch/703188/ ("exec.c: simplify the breakpoint invalidation logic"). The difference is that this patch doesn't drop the breakpoint_invalidate() function entirely. I think this is better both for a future "correct fix" and as a minimal "just fix this for 2.8 release" change. --- exec.c | 25 ++++++------------------- 1 file changed, 6 insertions(+), 19 deletions(-) diff --git a/exec.c b/exec.c index 3d867f1..08c558e 100644 --- a/exec.c +++ b/exec.c @@ -684,28 +684,15 @@ void cpu_exec_realizefn(CPUState *cpu, Error **errp) #endif } -#if defined(CONFIG_USER_ONLY) -static void breakpoint_invalidate(CPUState *cpu, target_ulong pc) -{ - mmap_lock(); - tb_lock(); - tb_invalidate_phys_page_range(pc, pc + 1, 0); - tb_unlock(); - mmap_unlock(); -} -#else static void breakpoint_invalidate(CPUState *cpu, target_ulong pc) { - MemTxAttrs attrs; - hwaddr phys = cpu_get_phys_page_attrs_debug(cpu, pc, &attrs); - int asidx = cpu_asidx_from_attrs(cpu, attrs); - if (phys != -1) { - /* Locks grabbed by tb_invalidate_phys_addr */ - tb_invalidate_phys_addr(cpu->cpu_ases[asidx].as, - phys | (pc & ~TARGET_PAGE_MASK)); - } + /* Flush the whole TB as this will not have race conditions + * even if we don't have proper locking yet. + * Ideally we would just invalidate the TBs for the + * specified PC. + */ + tb_flush(cpu); } -#endif #if defined(CONFIG_USER_ONLY) void cpu_watchpoint_remove_all(CPUState *cpu, int mask) -- 2.7.4