| Message ID | 20201204034302.2123841-1-jonathan.lemon@gmail.com (mailing list archive) |
|---|---|
| State | Changes Requested |
| Delegated to: | BPF |
| Headers | show |
| Series | [v2,bpf-next] bpf: increment and use correct thread iterator | expand |
| Context | Check | Description |
|---|---|---|
| netdev/cover_letter | success | Link |
| netdev/fixes_present | success | Link |
| netdev/patch_count | success | Link |
| netdev/tree_selection | success | Clearly marked for bpf-next |
| netdev/subject_prefix | success | Link |
| netdev/source_inline | success | Was 0 now: 0 |
| netdev/verify_signedoff | success | Link |
| netdev/module_param | success | Was 0 now: 0 |
| netdev/build_32bit | success | Errors and warnings before: 1 this patch: 1 |
| netdev/kdoc | success | Errors and warnings before: 0 this patch: 0 |
| netdev/verify_fixes | success | Link |
| netdev/checkpatch | warning | WARNING: From:/Signed-off-by: email address mismatch: 'From: Jonathan Lemon <bsd@fb.com>' != 'Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>' |
| netdev/build_allmodconfig_warn | success | Errors and warnings before: 1 this patch: 1 |
| netdev/header_inline | success | Link |
| netdev/stable | success | Stable not CCed |
On 12/3/20 7:43 PM, Jonathan Lemon wrote: > From: Jonathan Lemon <bsd@fb.com> Could you explain in the commit log what problem this patch tries to solve? What bad things could happen without this patch? > > If unable to obtain the file structure for the current task, > proceed to the next task number after the one returned from > task_seq_get_next(), instead of the next task number from the > original iterator. This seems a correct change. The current code should still work but it may do some redundant/unnecessary work in kernel. This only happens when a task does not have any file, no sure whether this is the culprit for the problem this patch tries to address. > > Use thread_group_leader() instead of comparing tgid vs pid, which > might may be racy. I see static inline bool thread_group_leader(struct task_struct *p) { return p->exit_signal >= 0; } I am not sure whether thread_group_leader(task) is equivalent to task->tgid == task->pid or not. Any documentation or explanation? Could you explain why task->tgid != task->pid in the original code could be racy? > > Only obtain the task reference count at the end of the RCU section > instead of repeatedly obtaining/releasing it when iterathing though > a thread group. I think this is an optimization and not about the correctness. > > Fixes: eaaacd23910f ("bpf: Add task and task/file iterator targets") > Fixes: 203d7b054fc7 ("bpf: Avoid iterating duplicated files for task_file iterator") > > Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com> > --- > kernel/bpf/task_iter.c | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c > index 0458a40edf10..66a52fcf589a 100644 > --- a/kernel/bpf/task_iter.c > +++ b/kernel/bpf/task_iter.c > @@ -33,17 +33,17 @@ static struct task_struct *task_seq_get_next(struct pid_namespace *ns, > pid = find_ge_pid(*tid, ns); > if (pid) { > *tid = pid_nr_ns(pid, ns); > - task = get_pid_task(pid, PIDTYPE_PID); > + task = pid_task(pid, PIDTYPE_PID); > if (!task) { > ++*tid; > goto retry; > - } else if (skip_if_dup_files && task->tgid != task->pid && > + } else if (skip_if_dup_files && !thread_group_leader(task) && > task->files == task->group_leader->files) { > - put_task_struct(task); > task = NULL; > ++*tid; > goto retry; > } > + get_task_struct(task); > } > rcu_read_unlock(); > > @@ -164,7 +164,7 @@ task_file_seq_get_next(struct bpf_iter_seq_task_file_info *info) > curr_files = get_files_struct(curr_task); > if (!curr_files) { > put_task_struct(curr_task); > - curr_tid = ++(info->tid); > + curr_tid = curr_tid + 1; > info->fd = 0; > goto again; > } >
On Fri, Dec 04, 2020 at 12:01:53AM -0800, Yonghong Song wrote: > > > On 12/3/20 7:43 PM, Jonathan Lemon wrote: > > From: Jonathan Lemon <bsd@fb.com> > > Could you explain in the commit log what problem this patch > tries to solve? What bad things could happen without this patch? Without the patch, on a particular set of systems, RCU will repeatedly generate stall warnings similar to the trace below. The common factor for all the traces seems to be using task_file_seq_next(). With the patch, all the warnings go away. rcu: INFO: rcu_sched self-detected stall on CPU rcu: \x0910-....: (20666 ticks this GP) idle=4b6/1/0x4000000000000002 softirq=14346773/14346773 fqs=5064 \x09(t=21013 jiffies g=25395133 q=154147) NMI backtrace for cpu 10 #1 Hardware name: Quanta Leopard ORv2-DDR4/Leopard ORv2-DDR4, BIOS F06_3B17 03/16/2018 Call Trace: <IRQ> dump_stack+0x50/0x70 nmi_cpu_backtrace.cold.6+0x13/0x50 ? lapic_can_unplug_cpu.cold.30+0x40/0x40 nmi_trigger_cpumask_backtrace+0xba/0xca rcu_dump_cpu_stacks+0x99/0xc7 rcu_sched_clock_irq.cold.90+0x1b4/0x3aa ? tick_sched_do_timer+0x60/0x60 update_process_times+0x24/0x50 tick_sched_timer+0x37/0x70 __hrtimer_run_queues+0xfe/0x270 hrtimer_interrupt+0xf4/0x210 smp_apic_timer_interrupt+0x5e/0x120 apic_timer_interrupt+0xf/0x20 </IRQ> RIP: 0010:find_ge_pid_upd+0x5/0x20 Code: 80 00 00 00 00 0f 1f 44 00 00 48 83 ec 08 89 7c 24 04 48 8d 7e 08 48 8d 74 24 04 e8 d5 d3 9a 00 48 83 c4 08 c3 0f 1f 44 00 00 <48> 89 f8 48 8d 7e 08 48 89 c6 e9 bc d3 9a 00 cc cc cc cc cc cc cc RSP: 0018:ffffc9002b7abdb8 EFLAGS: 00000297 ORIG_RAX: ffffffffffffff13 RAX: 00000000002ca5cd RBX: ffff889c44c0ba00 RCX: 0000000000000000 RDX: 0000000000000002 RSI: ffffffff8284eb80 RDI: ffffc9002b7abdc4 RBP: ffffc9002b7abe0c R08: ffff8895afe93a00 R09: ffff8891388abb50 R10: 000000000000000c R11: 00000000002ca600 R12: 000000000000003f R13: ffffffff8284eb80 R14: 0000000000000001 R15: 00000000ffffffff task_seq_get_next+0x53/0x180 task_file_seq_get_next+0x159/0x220 task_file_seq_next+0x4f/0xa0 bpf_seq_read+0x159/0x390 vfs_read+0x8a/0x140 ksys_read+0x59/0xd0 do_syscall_64+0x42/0x110 entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > If unable to obtain the file structure for the current task, > > proceed to the next task number after the one returned from > > task_seq_get_next(), instead of the next task number from the > > original iterator. > This seems a correct change. The current code should still work > but it may do some redundant/unnecessary work in kernel. > This only happens when a task does not have any file, > no sure whether this is the culprit for the problem this > patch tries to address. > > > > > Use thread_group_leader() instead of comparing tgid vs pid, which > > might may be racy. > > I see > > static inline bool thread_group_leader(struct task_struct *p) > { > return p->exit_signal >= 0; > } > > I am not sure whether thread_group_leader(task) is equivalent > to task->tgid == task->pid or not. Any documentation or explanation? > > Could you explain why task->tgid != task->pid in the original > code could be racy? My understanding is that anything which uses pid_t for comparision in the kernel is incorrect. Looking at de_thread(), there is a section which swaps the pid and tids around, but doesn't seem to change tgid directly. There's also this comment in linux/pid.h: /* * Both old and new leaders may be attached to * the same pid in the middle of de_thread(). */ So the safest thing to do is use the explicit thread_group_leader() macro rather than trying to open code things. > > Only obtain the task reference count at the end of the RCU section > > instead of repeatedly obtaining/releasing it when iterathing though > > a thread group. > > I think this is an optimization and not about the correctness. Yes, but the loop in question can be executed thousands of times, and there isn't much point in doing this needless work. It's unclear whether this is a significant time contribution to the RCU stall, but reducing the amount of refcounting isn't a bad thing.
On 12/4/20 9:14 AM, Jonathan Lemon wrote: > On Fri, Dec 04, 2020 at 12:01:53AM -0800, Yonghong Song wrote: >> >> >> On 12/3/20 7:43 PM, Jonathan Lemon wrote: >>> From: Jonathan Lemon <bsd@fb.com> >> >> Could you explain in the commit log what problem this patch >> tries to solve? What bad things could happen without this patch? > > Without the patch, on a particular set of systems, RCU will repeatedly > generate stall warnings similar to the trace below. The common factor > for all the traces seems to be using task_file_seq_next(). With the > patch, all the warnings go away. > > rcu: INFO: rcu_sched self-detected stall on CPU > rcu: \x0910-....: (20666 ticks this GP) idle=4b6/1/0x4000000000000002 softirq=14346773/14346773 fqs=5064 > \x09(t=21013 jiffies g=25395133 q=154147) > NMI backtrace for cpu 10 > #1 > Hardware name: Quanta Leopard ORv2-DDR4/Leopard ORv2-DDR4, BIOS F06_3B17 03/16/2018 > Call Trace: > <IRQ> > dump_stack+0x50/0x70 > nmi_cpu_backtrace.cold.6+0x13/0x50 > ? lapic_can_unplug_cpu.cold.30+0x40/0x40 > nmi_trigger_cpumask_backtrace+0xba/0xca > rcu_dump_cpu_stacks+0x99/0xc7 > rcu_sched_clock_irq.cold.90+0x1b4/0x3aa > ? tick_sched_do_timer+0x60/0x60 > update_process_times+0x24/0x50 > tick_sched_timer+0x37/0x70 > __hrtimer_run_queues+0xfe/0x270 > hrtimer_interrupt+0xf4/0x210 > smp_apic_timer_interrupt+0x5e/0x120 > apic_timer_interrupt+0xf/0x20 > </IRQ> > RIP: 0010:find_ge_pid_upd+0x5/0x20 > Code: 80 00 00 00 00 0f 1f 44 00 00 48 83 ec 08 89 7c 24 04 48 8d 7e 08 48 8d 74 24 04 e8 d5 d3 9a 00 48 83 c4 08 c3 0f 1f 44 00 00 <48> 89 f8 48 8d 7e 08 48 89 c6 e9 bc d3 9a 00 cc cc cc cc cc cc cc > RSP: 0018:ffffc9002b7abdb8 EFLAGS: 00000297 ORIG_RAX: ffffffffffffff13 > RAX: 00000000002ca5cd RBX: ffff889c44c0ba00 RCX: 0000000000000000 > RDX: 0000000000000002 RSI: ffffffff8284eb80 RDI: ffffc9002b7abdc4 > RBP: ffffc9002b7abe0c R08: ffff8895afe93a00 R09: ffff8891388abb50 > R10: 000000000000000c R11: 00000000002ca600 R12: 000000000000003f > R13: ffffffff8284eb80 R14: 0000000000000001 R15: 00000000ffffffff > task_seq_get_next+0x53/0x180 > task_file_seq_get_next+0x159/0x220 > task_file_seq_next+0x4f/0xa0 > bpf_seq_read+0x159/0x390 > vfs_read+0x8a/0x140 > ksys_read+0x59/0xd0 > do_syscall_64+0x42/0x110 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > >>> If unable to obtain the file structure for the current task, >>> proceed to the next task number after the one returned from >>> task_seq_get_next(), instead of the next task number from the >>> original iterator. >> This seems a correct change. The current code should still work >> but it may do some redundant/unnecessary work in kernel. >> This only happens when a task does not have any file, >> no sure whether this is the culprit for the problem this >> patch tries to address. >> >>> >>> Use thread_group_leader() instead of comparing tgid vs pid, which >>> might may be racy. >> >> I see >> >> static inline bool thread_group_leader(struct task_struct *p) >> { >> return p->exit_signal >= 0; >> } >> >> I am not sure whether thread_group_leader(task) is equivalent >> to task->tgid == task->pid or not. Any documentation or explanation? >> >> Could you explain why task->tgid != task->pid in the original >> code could be racy? > > My understanding is that anything which uses pid_t for comparision > in the kernel is incorrect. Looking at de_thread(), there is a > section which swaps the pid and tids around, but doesn't seem to > change tgid directly. > > There's also this comment in linux/pid.h: > /* > * Both old and new leaders may be attached to > * the same pid in the middle of de_thread(). > */ > > So the safest thing to do is use the explicit thread_group_leader() > macro rather than trying to open code things. in de_thread(), I see p->exit_signal (used in thread_group_leader()) has race as well. ... exchange_tids(tsk, leader); transfer_pid(leader, tsk, PIDTYPE_TGID); transfer_pid(leader, tsk, PIDTYPE_PGID); transfer_pid(leader, tsk, PIDTYPE_SID); list_replace_rcu(&leader->tasks, &tsk->tasks); list_replace_init(&leader->sibling, &tsk->sibling); tsk->group_leader = tsk; leader->group_leader = tsk; tsk->exit_signal = SIGCHLD; leader->exit_signal = -1; BUG_ON(leader->exit_state != EXIT_ZOMBIE); leader->exit_state = EXIT_DEAD; ... But I am not sure how this race or pid race could impact task traversal differently. > > >>> Only obtain the task reference count at the end of the RCU section >>> instead of repeatedly obtaining/releasing it when iterathing though >>> a thread group. >> >> I think this is an optimization and not about the correctness. > > Yes, but the loop in question can be executed thousands of times, and > there isn't much point in doing this needless work. It's unclear > whether this is a significant time contribution to the RCU stall, > but reducing the amount of refcounting isn't a bad thing. Sure. Totally agree.
On 12/4/20 9:14 AM, Jonathan Lemon wrote: > On Fri, Dec 04, 2020 at 12:01:53AM -0800, Yonghong Song wrote: >> >> >> On 12/3/20 7:43 PM, Jonathan Lemon wrote: >>> From: Jonathan Lemon <bsd@fb.com> >> >> Could you explain in the commit log what problem this patch >> tries to solve? What bad things could happen without this patch? > > Without the patch, on a particular set of systems, RCU will repeatedly > generate stall warnings similar to the trace below. The common factor > for all the traces seems to be using task_file_seq_next(). With the > patch, all the warnings go away. > > rcu: INFO: rcu_sched self-detected stall on CPU > rcu: \x0910-....: (20666 ticks this GP) idle=4b6/1/0x4000000000000002 softirq=14346773/14346773 fqs=5064 > \x09(t=21013 jiffies g=25395133 q=154147) > NMI backtrace for cpu 10 > #1 > Hardware name: Quanta Leopard ORv2-DDR4/Leopard ORv2-DDR4, BIOS F06_3B17 03/16/2018 > Call Trace: > <IRQ> > dump_stack+0x50/0x70 > nmi_cpu_backtrace.cold.6+0x13/0x50 > ? lapic_can_unplug_cpu.cold.30+0x40/0x40 > nmi_trigger_cpumask_backtrace+0xba/0xca > rcu_dump_cpu_stacks+0x99/0xc7 > rcu_sched_clock_irq.cold.90+0x1b4/0x3aa > ? tick_sched_do_timer+0x60/0x60 > update_process_times+0x24/0x50 > tick_sched_timer+0x37/0x70 > __hrtimer_run_queues+0xfe/0x270 > hrtimer_interrupt+0xf4/0x210 > smp_apic_timer_interrupt+0x5e/0x120 > apic_timer_interrupt+0xf/0x20 > </IRQ> > RIP: 0010:find_ge_pid_upd+0x5/0x20 > Code: 80 00 00 00 00 0f 1f 44 00 00 48 83 ec 08 89 7c 24 04 48 8d 7e 08 48 8d 74 24 04 e8 d5 d3 9a 00 48 83 c4 08 c3 0f 1f 44 00 00 <48> 89 f8 48 8d 7e 08 48 89 c6 e9 bc d3 9a 00 cc cc cc cc cc cc cc > RSP: 0018:ffffc9002b7abdb8 EFLAGS: 00000297 ORIG_RAX: ffffffffffffff13 > RAX: 00000000002ca5cd RBX: ffff889c44c0ba00 RCX: 0000000000000000 > RDX: 0000000000000002 RSI: ffffffff8284eb80 RDI: ffffc9002b7abdc4 > RBP: ffffc9002b7abe0c R08: ffff8895afe93a00 R09: ffff8891388abb50 > R10: 000000000000000c R11: 00000000002ca600 R12: 000000000000003f > R13: ffffffff8284eb80 R14: 0000000000000001 R15: 00000000ffffffff > task_seq_get_next+0x53/0x180 > task_file_seq_get_next+0x159/0x220 > task_file_seq_next+0x4f/0xa0 > bpf_seq_read+0x159/0x390 > vfs_read+0x8a/0x140 > ksys_read+0x59/0xd0 > do_syscall_64+0x42/0x110 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 Maybe you can post v3 of the patch with the above information in the commit description so people can better understand what the problem you are trying to solve here? Also, could you also send to bpf@vger.kernel.org? > > >>> If unable to obtain the file structure for the current task, >>> proceed to the next task number after the one returned from >>> task_seq_get_next(), instead of the next task number from the >>> original iterator. >> This seems a correct change. The current code should still work >> but it may do some redundant/unnecessary work in kernel. >> This only happens when a task does not have any file, >> no sure whether this is the culprit for the problem this >> patch tries to address. >> >>> >>> Use thread_group_leader() instead of comparing tgid vs pid, which >>> might may be racy. >> >> I see >> >> static inline bool thread_group_leader(struct task_struct *p) >> { >> return p->exit_signal >= 0; >> } >> >> I am not sure whether thread_group_leader(task) is equivalent >> to task->tgid == task->pid or not. Any documentation or explanation? >> >> Could you explain why task->tgid != task->pid in the original >> code could be racy? > > My understanding is that anything which uses pid_t for comparision > in the kernel is incorrect. Looking at de_thread(), there is a > section which swaps the pid and tids around, but doesn't seem to > change tgid directly. > > There's also this comment in linux/pid.h: > /* > * Both old and new leaders may be attached to > * the same pid in the middle of de_thread(). > */ > > So the safest thing to do is use the explicit thread_group_leader() > macro rather than trying to open code things. I did some limited experiments and did not trigger a case where task->tgid != task->pid not agreeing with !thread_group_leader(). Will need more tests in the environment to reproduce the warning to confirm whether this is the culprit or not. > > >>> Only obtain the task reference count at the end of the RCU section >>> instead of repeatedly obtaining/releasing it when iterathing though >>> a thread group. >> >> I think this is an optimization and not about the correctness. > > Yes, but the loop in question can be executed thousands of times, and > there isn't much point in doing this needless work. It's unclear > whether this is a significant time contribution to the RCU stall, > but reducing the amount of refcounting isn't a bad thing. >
On Wed, Dec 09, 2020 at 11:02:54AM -0800, Yonghong Song wrote: > > > Maybe you can post v3 of the patch with the above information in the > commit description so people can better understand what the problem > you are trying to solve here? > > Also, could you also send to bpf@vger.kernel.org? Sure, I can do that. > > > > If unable to obtain the file structure for the current task, > > > > proceed to the next task number after the one returned from > > > > task_seq_get_next(), instead of the next task number from the > > > > original iterator. > > > This seems a correct change. The current code should still work > > > but it may do some redundant/unnecessary work in kernel. > > > This only happens when a task does not have any file, > > > no sure whether this is the culprit for the problem this > > > patch tries to address. > > > > > > > > > > > Use thread_group_leader() instead of comparing tgid vs pid, which > > > > might may be racy. > > > > > > I see > > > > > > static inline bool thread_group_leader(struct task_struct *p) > > > { > > > return p->exit_signal >= 0; > > > } > > > > > > I am not sure whether thread_group_leader(task) is equivalent > > > to task->tgid == task->pid or not. Any documentation or explanation? > > > > > > Could you explain why task->tgid != task->pid in the original > > > code could be racy? > > > > My understanding is that anything which uses pid_t for comparision > > in the kernel is incorrect. Looking at de_thread(), there is a > > section which swaps the pid and tids around, but doesn't seem to > > change tgid directly. > > > > There's also this comment in linux/pid.h: > > /* > > * Both old and new leaders may be attached to > > * the same pid in the middle of de_thread(). > > */ > > > > So the safest thing to do is use the explicit thread_group_leader() > > macro rather than trying to open code things. > > I did some limited experiments and did not trigger a case where > task->tgid != task->pid not agreeing with !thread_group_leader(). > Will need more tests in the environment to reproduce the warning > to confirm whether this is the culprit or not. Perhaps, but on the other hand, the splats disappear with this patch, so it's doing something right. If your debug code hasn't detected any cases where thread_group_leader() isn't making a difference, then there shouldn't be any objections in making the replacement, right? It does make the code easier to understand and matches the rest of the kernel.
On 12/11/20 8:30 AM, Jonathan Lemon wrote: > On Wed, Dec 09, 2020 at 11:02:54AM -0800, Yonghong Song wrote: >> >> >> Maybe you can post v3 of the patch with the above information in the >> commit description so people can better understand what the problem >> you are trying to solve here? >> >> Also, could you also send to bpf@vger.kernel.org? > > Sure, I can do that. > >>>>> If unable to obtain the file structure for the current task, >>>>> proceed to the next task number after the one returned from >>>>> task_seq_get_next(), instead of the next task number from the >>>>> original iterator. >>>> This seems a correct change. The current code should still work >>>> but it may do some redundant/unnecessary work in kernel. >>>> This only happens when a task does not have any file, >>>> no sure whether this is the culprit for the problem this >>>> patch tries to address. >>>> >>>>> >>>>> Use thread_group_leader() instead of comparing tgid vs pid, which >>>>> might may be racy. >>>> >>>> I see >>>> >>>> static inline bool thread_group_leader(struct task_struct *p) >>>> { >>>> return p->exit_signal >= 0; >>>> } >>>> >>>> I am not sure whether thread_group_leader(task) is equivalent >>>> to task->tgid == task->pid or not. Any documentation or explanation? >>>> >>>> Could you explain why task->tgid != task->pid in the original >>>> code could be racy? >>> >>> My understanding is that anything which uses pid_t for comparision >>> in the kernel is incorrect. Looking at de_thread(), there is a >>> section which swaps the pid and tids around, but doesn't seem to >>> change tgid directly. >>> >>> There's also this comment in linux/pid.h: >>> /* >>> * Both old and new leaders may be attached to >>> * the same pid in the middle of de_thread(). >>> */ >>> >>> So the safest thing to do is use the explicit thread_group_leader() >>> macro rather than trying to open code things. >> >> I did some limited experiments and did not trigger a case where >> task->tgid != task->pid not agreeing with !thread_group_leader(). >> Will need more tests in the environment to reproduce the warning >> to confirm whether this is the culprit or not. > > Perhaps, but on the other hand, the splats disappear with this > patch, so it's doing something right. If your debug code hasn't > detected any cases where thread_group_leader() isn't making a > difference, then there shouldn't be any objections in making the > replacement, right? It does make the code easier to understand > and matches the rest of the kernel. Agree. Let me do a little more experiments to double check we did not miss anything with this particular change and will report back later.
diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c index 0458a40edf10..66a52fcf589a 100644 --- a/kernel/bpf/task_iter.c +++ b/kernel/bpf/task_iter.c @@ -33,17 +33,17 @@ static struct task_struct *task_seq_get_next(struct pid_namespace *ns, pid = find_ge_pid(*tid, ns); if (pid) { *tid = pid_nr_ns(pid, ns); - task = get_pid_task(pid, PIDTYPE_PID); + task = pid_task(pid, PIDTYPE_PID); if (!task) { ++*tid; goto retry; - } else if (skip_if_dup_files && task->tgid != task->pid && + } else if (skip_if_dup_files && !thread_group_leader(task) && task->files == task->group_leader->files) { - put_task_struct(task); task = NULL; ++*tid; goto retry; } + get_task_struct(task); } rcu_read_unlock(); @@ -164,7 +164,7 @@ task_file_seq_get_next(struct bpf_iter_seq_task_file_info *info) curr_files = get_files_struct(curr_task); if (!curr_files) { put_task_struct(curr_task); - curr_tid = ++(info->tid); + curr_tid = curr_tid + 1; info->fd = 0; goto again; }