| Message ID | 1481047629-7763-1-git-send-email-peter.maydell@linaro.org |
|---|---|
| State | Accepted |
| Commit | a9353fe897ca2687e5b3385ed39e3db3927a90e0 |
| Headers | show |
Peter Maydell <peter.maydell@linaro.org> writes: > 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 <julian@codesourcery.com> > Signed-off-by: Peter Maydell <peter.maydell@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) Reviewed-by: Alex Bennée <alex.bennee@linaro.org> Acked-by: Alex Bennée <alex.bennee@linaro.org> -- Alex Bennée
On Tue, Dec 06, 2016 at 06:07:09PM +0000, Peter Maydell wrote: > 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 <julian@codesourcery.com> > Signed-off-by: Peter Maydell <peter.maydell@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(-) Thanks, applied to my staging tree: https://github.com/stefanha/qemu/commits/staging Stefan
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)
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 <julian@codesourcery.com> Signed-off-by: Peter Maydell <peter.maydell@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(-) -- 2.7.4