| Message ID | 1558430404-4840-1-git-send-email-tengfeif@codeaurora.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | arm64: break while loop if task had been rescheduled | expand |
On 05/21/2019 02:50 PM, Tengfei Fan wrote: > While printing a task's backtrace and this task isn't > current task, it is possible that task's fp and fp+8 > have the same value, so cannot break the while loop. > This can break while loop if this task had been > rescheduled during print this task's backtrace. This is very confusing. IIUC it suggests that while printing the backtrace for non-current tasks the do/while loop does not exit because fp and fp+8 might have the same value ? When would this happen ? Even in that case the commit message here does not properly match the change in this patch. This patch tries to stop printing the stack for non-current tasks if their state change while there is one dump_backtrace() trying to print back trace. Dont we have any lock preventing a task in this situation (while dumping it's backtrace) from running again or changing state. > > Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org> > --- > arch/arm64/kernel/traps.c | 23 +++++++++++++++++++++++ > 1 file changed, 23 insertions(+) > > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c > index 2975598..9df6e02 100644 > --- a/arch/arm64/kernel/traps.c > +++ b/arch/arm64/kernel/traps.c > @@ -103,6 +103,9 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) > { > struct stackframe frame; > int skip = 0; > + long cur_state = 0; > + unsigned long cur_sp = 0; > + unsigned long cur_fp = 0; > > pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk); > > @@ -127,6 +130,9 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) > */ > frame.fp = thread_saved_fp(tsk); > frame.pc = thread_saved_pc(tsk); > + cur_state = tsk->state; > + cur_sp = thread_saved_sp(tsk); > + cur_fp = frame.fp; Should 'saved_state|sp|fp' instead as its applicable to non-current tasks only. > } > #ifdef CONFIG_FUNCTION_GRAPH_TRACER > frame.graph = 0; > @@ -134,6 +140,23 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) > > printk("Call trace:\n"); > do { > + if (tsk != current && (cur_state != tsk->state > + /* > + * We would not be printing backtrace for the task > + * that has changed state from uninterruptible to > + * running before hitting the do-while loop but after > + * saving the current state. If task is in running This does not check any explicit task states like 'un-interruptible' or 'running' but instead tracks change from any previously 'saved' state. > + * state before saving the state, then we may print > + * wrong call trace or end up in infinite while loop > + * if *(fp) and *(fp+8) are same. While the situation Then dump_backtrace() must detect it, should not save it and just abort. > + * will stop print when that task schedule out. Thats not a reliable solution. AFICS we should not proceed further if there is a chance of an wrong trace or an infinite loop. Hoping that the printing will stop when task gets scheduled out does not seem right. > + */ > + || cur_sp != thread_saved_sp(tsk) > + || cur_fp != thread_saved_fp(tsk))) { Why does any of these three mismatches detect the problematic transition not just the state ?
On Tue, May 21, 2019 at 05:20:04PM +0800, Tengfei Fan wrote: > While printing a task's backtrace and this task isn't > current task, it is possible that task's fp and fp+8 > have the same value, so cannot break the while loop. > This can break while loop if this task had been > rescheduled during print this task's backtrace. There are a few cases where backtracing can get stuck in an infinite loop. I'd attempted to address that more generally in my arm64/robust-stacktrace branch [1]. Looking at tsk->state here is inherently racy, and doesn't solve the general case, so I'd prefer to avoid that. Do my patches help you here? If so, I'm happy to rebase those to v5.2-rc1 and repost. Thanks, Mark. [1] https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/robust-stacktrace > > Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org> > --- > arch/arm64/kernel/traps.c | 23 +++++++++++++++++++++++ > 1 file changed, 23 insertions(+) > > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c > index 2975598..9df6e02 100644 > --- a/arch/arm64/kernel/traps.c > +++ b/arch/arm64/kernel/traps.c > @@ -103,6 +103,9 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) > { > struct stackframe frame; > int skip = 0; > + long cur_state = 0; > + unsigned long cur_sp = 0; > + unsigned long cur_fp = 0; > > pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk); > > @@ -127,6 +130,9 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) > */ > frame.fp = thread_saved_fp(tsk); > frame.pc = thread_saved_pc(tsk); > + cur_state = tsk->state; > + cur_sp = thread_saved_sp(tsk); > + cur_fp = frame.fp; > } > #ifdef CONFIG_FUNCTION_GRAPH_TRACER > frame.graph = 0; > @@ -134,6 +140,23 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) > > printk("Call trace:\n"); > do { > + if (tsk != current && (cur_state != tsk->state > + /* > + * We would not be printing backtrace for the task > + * that has changed state from uninterruptible to > + * running before hitting the do-while loop but after > + * saving the current state. If task is in running > + * state before saving the state, then we may print > + * wrong call trace or end up in infinite while loop > + * if *(fp) and *(fp+8) are same. While the situation > + * will stop print when that task schedule out. > + */ > + || cur_sp != thread_saved_sp(tsk) > + || cur_fp != thread_saved_fp(tsk))) { > + printk("The task:%s had been rescheduled!\n", > + tsk->comm); > + break; > + } > /* skip until specified stack frame */ > if (!skip) { > dump_backtrace_entry(frame.pc); > -- > The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, > a Linux Foundation Collaborative Project >
On 2019-05-22 17:04, Anshuman Khandual wrote: > On 05/21/2019 02:50 PM, Tengfei Fan wrote: >> While printing a task's backtrace and this task isn't >> current task, it is possible that task's fp and fp+8 >> have the same value, so cannot break the while loop. >> This can break while loop if this task had been >> rescheduled during print this task's backtrace. > > This is very confusing. IIUC it suggests that while printing > the backtrace for non-current tasks the do/while loop does not > exit because fp and fp+8 might have the same value ? When would > this happen ? Even in that case the commit message here does not > properly match the change in this patch. In our issue, we got fp=pc=0xFFFFFF8025A13BA0, so cannot exit while loop in dump_basktrace(). After analyze our issue's dump, we found one task(such as: task A) is exiting via invoke do_exit() during another task is showing task A's dumptask. In kernel code, do_exit() and exit_notify are defined as follows: void noreturn do_exit(long code) { ...... exit_notify(tsk, group_dead); ...... } static void exit_notify(struct task_struct *tsk, int group_dead) { ...... } Because of exit_notify() is a static function, so it is inlined to do_exit() when compile kernel, so we can get partial assembly code of do_exit() as follows: …… { bool autoreap; struct task_struct *p, *n; LIST_HEAD(dead); write_lock_irq(&tasklist_lock); c10: 90000000 adrp x0, 0 <tasklist_lock> c14: 910003e8 mov x8, sp c18: 91000000 add x0, x0, #0x0 */ static void exit_notify(struct task_struct *tsk, int group_dead) { bool autoreap; struct task_struct *p, *n; LIST_HEAD(dead); c1c: a90023e8 stp x8, x8, [sp] write_lock_irq(&tasklist_lock); c20: 94000000 bl 0 <_raw_write_lock_irq> c24: f9435268 ldr x8, [x19,#1696] …… From the code "c14:" and "c1c:", we will find sp's addr value is stored in sp and sp+8, so sp's vaule equal (sp+8)'s value. In our issue, there is a chance of fp point sp, so there will be fp=pc=fp's addr value,so code cannot break from while loop in dump_backtrace(). > > This patch tries to stop printing the stack for non-current tasks > if their state change while there is one dump_backtrace() trying > to print back trace. Dont we have any lock preventing a task in > this situation (while dumping it's backtrace) from running again > or changing state. I haven't found any lock preventing a task in this situation, and I think we shouldn't prevent task running if this task is scheduled. > >> >> Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org> >> --- >> arch/arm64/kernel/traps.c | 23 +++++++++++++++++++++++ >> 1 file changed, 23 insertions(+) >> >> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c >> index 2975598..9df6e02 100644 >> --- a/arch/arm64/kernel/traps.c >> +++ b/arch/arm64/kernel/traps.c >> @@ -103,6 +103,9 @@ void dump_backtrace(struct pt_regs *regs, struct >> task_struct *tsk) >> { >> struct stackframe frame; >> int skip = 0; >> + long cur_state = 0; >> + unsigned long cur_sp = 0; >> + unsigned long cur_fp = 0; >> >> pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk); >> >> @@ -127,6 +130,9 @@ void dump_backtrace(struct pt_regs *regs, struct >> task_struct *tsk) >> */ >> frame.fp = thread_saved_fp(tsk); >> frame.pc = thread_saved_pc(tsk); >> + cur_state = tsk->state; >> + cur_sp = thread_saved_sp(tsk); >> + cur_fp = frame.fp; > > Should 'saved_state|sp|fp' instead as its applicable to non-current > tasks only. 'saved_state|sp|fp' only applies to non-current tasks. > >> } >> #ifdef CONFIG_FUNCTION_GRAPH_TRACER >> frame.graph = 0; >> @@ -134,6 +140,23 @@ void dump_backtrace(struct pt_regs *regs, struct >> task_struct *tsk) >> >> printk("Call trace:\n"); >> do { >> + if (tsk != current && (cur_state != tsk->state >> + /* >> + * We would not be printing backtrace for the task >> + * that has changed state from "saved" state to other >> + * state before hitting the do-while loop but after >> + * saving the current state. If task's current state > > This does not check any explicit task states like 'un-interruptible' or > 'running' but instead tracks change from any previously 'saved' state. have updated comments. > > >> + * not equal the "saved" state, then we may print >> + * wrong call trace or end up in infinite while loop >> + * if *(fp) and *(fp+8) are same. While the situation > > Then dump_backtrace() must detect it, should not save it and just > abort. have updatd commentes. > > >> + * should be stoped once we found the task's state + * is changed, so we detect the task's current state, + * sp and fp in each while. > > Thats not a reliable solution. AFICS we should not proceed further if > there is a chance of an wrong trace or an infinite loop. Hoping that > the printing will stop when task gets scheduled out does not seem > right. In this patch, it will break while loop and stop to print backtrace if we find the task's state change or there is a chance of an infinite loop. > >> + */ >> + || cur_sp != thread_saved_sp(tsk) >> + || cur_fp != thread_saved_fp(tsk))) { > > Why does any of these three mismatches detect the problematic > transition > not just the state ? 1. we can use "cur_state != tsk->state" prevent printing backtrace if the task's state is changed after "saved" task's state. 2. we can use "cur_sp != thread_saved_sp(tsk)" and "cur_fp != thread_saved_fp(tsk)" prevent printing backtrace if the task's state is changed before "saved" task's state. Because the value of "thread_saved_sp(tsk)" and "thread_saved_fp(tsk)" will not equal "saved" sp(cur_sp) and fp(cur_fp).
On 2019-05-24 18:41, Mark Rutland wrote: > On Tue, May 21, 2019 at 05:20:04PM +0800, Tengfei Fan wrote: >> While printing a task's backtrace and this task isn't >> current task, it is possible that task's fp and fp+8 >> have the same value, so cannot break the while loop. >> This can break while loop if this task had been >> rescheduled during print this task's backtrace. > > There are a few cases where backtracing can get stuck in an infinite > loop. I'd attempted to address that more generally in my > arm64/robust-stacktrace branch [1]. > > Looking at tsk->state here is inherently racy, and doesn't solve the > general case, so I'd prefer to avoid that. > > Do my patches help you here? If so, I'm happy to rebase those to > v5.2-rc1 and repost. I think your arm64/robust-stacktrace branch [1] can cover my issue, please rebase and reposet Thanks, Tengfei Fan > > Thanks, > Mark. > > [1] > https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/robust-stacktrace > >> >> Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org> >> --- >> arch/arm64/kernel/traps.c | 23 +++++++++++++++++++++++ >> 1 file changed, 23 insertions(+) >> >> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c >> index 2975598..9df6e02 100644 >> --- a/arch/arm64/kernel/traps.c >> +++ b/arch/arm64/kernel/traps.c >> @@ -103,6 +103,9 @@ void dump_backtrace(struct pt_regs *regs, struct >> task_struct *tsk) >> { >> struct stackframe frame; >> int skip = 0; >> + long cur_state = 0; >> + unsigned long cur_sp = 0; >> + unsigned long cur_fp = 0; >> >> pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk); >> >> @@ -127,6 +130,9 @@ void dump_backtrace(struct pt_regs *regs, struct >> task_struct *tsk) >> */ >> frame.fp = thread_saved_fp(tsk); >> frame.pc = thread_saved_pc(tsk); >> + cur_state = tsk->state; >> + cur_sp = thread_saved_sp(tsk); >> + cur_fp = frame.fp; >> } >> #ifdef CONFIG_FUNCTION_GRAPH_TRACER >> frame.graph = 0; >> @@ -134,6 +140,23 @@ void dump_backtrace(struct pt_regs *regs, struct >> task_struct *tsk) >> >> printk("Call trace:\n"); >> do { >> + if (tsk != current && (cur_state != tsk->state >> + /* >> + * We would not be printing backtrace for the task >> + * that has changed state from uninterruptible to >> + * running before hitting the do-while loop but after >> + * saving the current state. If task is in running >> + * state before saving the state, then we may print >> + * wrong call trace or end up in infinite while loop >> + * if *(fp) and *(fp+8) are same. While the situation >> + * will stop print when that task schedule out. >> + */ >> + || cur_sp != thread_saved_sp(tsk) >> + || cur_fp != thread_saved_fp(tsk))) { >> + printk("The task:%s had been rescheduled!\n", >> + tsk->comm); >> + break; >> + } >> /* skip until specified stack frame */ >> if (!skip) { >> dump_backtrace_entry(frame.pc); >> -- >> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora >> Forum, >> a Linux Foundation Collaborative Project >>
diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c index 2975598..9df6e02 100644 --- a/arch/arm64/kernel/traps.c +++ b/arch/arm64/kernel/traps.c @@ -103,6 +103,9 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) { struct stackframe frame; int skip = 0; + long cur_state = 0; + unsigned long cur_sp = 0; + unsigned long cur_fp = 0; pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk); @@ -127,6 +130,9 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) */ frame.fp = thread_saved_fp(tsk); frame.pc = thread_saved_pc(tsk); + cur_state = tsk->state; + cur_sp = thread_saved_sp(tsk); + cur_fp = frame.fp; } #ifdef CONFIG_FUNCTION_GRAPH_TRACER frame.graph = 0; @@ -134,6 +140,23 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) printk("Call trace:\n"); do { + if (tsk != current && (cur_state != tsk->state + /* + * We would not be printing backtrace for the task + * that has changed state from uninterruptible to + * running before hitting the do-while loop but after + * saving the current state. If task is in running + * state before saving the state, then we may print + * wrong call trace or end up in infinite while loop + * if *(fp) and *(fp+8) are same. While the situation + * will stop print when that task schedule out. + */ + || cur_sp != thread_saved_sp(tsk) + || cur_fp != thread_saved_fp(tsk))) { + printk("The task:%s had been rescheduled!\n", + tsk->comm); + break; + } /* skip until specified stack frame */ if (!skip) { dump_backtrace_entry(frame.pc);
While printing a task's backtrace and this task isn't current task, it is possible that task's fp and fp+8 have the same value, so cannot break the while loop. This can break while loop if this task had been rescheduled during print this task's backtrace. Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org> --- arch/arm64/kernel/traps.c | 23 +++++++++++++++++++++++ 1 file changed, 23 insertions(+)