Re: [PATCH] pid_namespace: make zap_pid_ns_processes() freezable

[Rafael J. Wysocki (Intel)]

On Thu, Oct 1, 2026 at 2:13 PM Aviv Vaknin <vaknins33@xxxxxxxxx> wrote:
>
> When the init of a pid namespace exits, zap_pid_ns_processes() waits
> until every other pid in the namespace has been freed. That wait has no
> upper bound: a tracer outside the namespace that doesn't reap its traced
> zombies keeps their pids allocated (see the comment above the final
> loop). While a task waits there, every suspend attempt fails:
>
>   Freezing user space processes failed after 20.006 seconds (2 tasks refusing to freeze, wq_busy=0):
>   task:pidns-freeze-re state:S stack:14200 pid:104   tgid:104   ppid:99     task_flags:0x40014c flags:0x00080000
>   task:pidns-freeze-re state:S stack:14160 pid:109   tgid:105   ppid:104    task_flags:0x40044c flags:0x00080802
>
> and keeps failing until the tracer reaps or exits, because neither wait
> can be frozen:
>
> - The first loop sleeps in kernel_wait4(). The freezer's fake signal
>   ends that wait with -ERESTARTSYS, then the loop clears TIF_SIGPENDING
>   and waits again without ever calling try_to_freeze().
>
> - The final loop sleeps in TASK_INTERRUPTIBLE without TASK_FREEZABLE.
>   Worse, nothing clears the TIF_SIGPENDING that the fake signal set, so
>   from the first suspend attempt on schedule() returns at once and the
>   task spins at 100% CPU for as long as the wait lasts.
>
> Neither loop has ever been freezable. The spin came with
> commit b9a985db9896 ("pid_ns: Sleep in TASK_INTERRUPTIBLE in zap_pid_ns_processes"),
> which replaced TASK_UNINTERRUPTIBLE to keep such long waits from
> triggering the hung task detector.
>
> This happens in practice with Chromium-based browsers, whose renderers
> are pid namespace inits. If a renderer crashes while the browser shuts
> down, crashpad (outside the namespace) ptrace-attaches its threads to
> dump it. When the renderer is SIGKILLed mid-dump, crashpad keeps the
> dead threads as unreaped traced zombies, and it can then block in
> waitpid() on the renderer's last thread, which is itself waiting in
> zap_pid_ns_processes() for those zombies. One such renderer blocks
> suspend system-wide until crashpad is killed or the machine reboots.
> The userspace side is being reported to Chromium separately.
>
> Call try_to_freeze() after kernel_wait4() in the first loop, and sleep
> in TASK_IDLE | TASK_FREEZABLE in the final loop, as coredump_task_exit()
> already does on the same exit path. Like TASK_INTERRUPTIBLE, TASK_IDLE
> keeps the hung task detector quiet and adds no load, but it is not
> woken by signals, so a pending signal can no longer turn the wait into
> a busy loop. TASK_FREEZABLE lets the freezer freeze the task in place;
> a free_pid() wakeup that arrives while it is frozen is kept in
> ->saved_state and acted on at thaw.
>
> Tested on v7.3-rc5 in QEMU with PROVE_LOCKING, DEBUG_ATOMIC_SLEEP and
> DETECT_HUNG_TASK (10 s timeout), using a reproducer in which a tracer
> PTRACE_SEIZEs two threads of a nested pid namespace init and doesn't
> reap them (it works unprivileged too, from a user namespace), and using
> the real browser and crashpad trigger:
>
> - before: freezing fails after 20 s, and afterwards the final loop
>   spins (500 ticks of system time per 5 s).
> - after: freezing completes in 0.001 s, the waiting task sits in state
>   I without using CPU, no hung task or lockdep report appears over
>   30-45 s, and the task exits as soon as the zombies are reaped.
>
> Fixes: 3eb07c8c8adb ("pid namespaces: destroy pid namespace on init's death")
> Fixes: b9a985db9896 ("pid_ns: Sleep in TASK_INTERRUPTIBLE in zap_pid_ns_processes")
> Cc: stable@xxxxxxxxxxxxxxx # 6.1+
> Assisted-by: Claude:claude-opus-5-5
> Signed-off-by: Aviv Vaknin <vaknins33@xxxxxxxxx>

Acked-by: Rafael J. Wysocki (Intel) <rafael@xxxxxxxxxx>

> ---
> Based on vfs kernel-7.4.signal (also in next-20260930), on top of
> 5c0985dd3c32 ("pid_namespace: prevent TIF_NOTIFY_SIGNAL from
> interrupting the reaper"). That commit keeps TIF_NOTIFY_SIGNAL from
> waking the reaper; this one is about the freezer, whose fake signal
> sets TIF_SIGPENDING, which the guard doesn't cover. Retested on this
> base with the reproducer below (same debug config): freezing completes
> in 0.000 s, no spin, no warnings. A version for v7.3-rc5 is available
> if wanted.
>
> Reproducer (run as root in a VM, then follow its instructions):
>
>   // SPDX-License-Identifier: GPL-2.0
>   /*
>    * pidns-freeze-repro: a task waiting in zap_pid_ns_processes() blocks suspend.
>    *
>    *   main (tracer, root pid ns)
>    *    `- A: pid 1 of pid ns A, single-threaded
>    *       `- B: pid 1 of nested pid ns B, multithreaded
>    *
>    * main PTRACE_SEIZEs non-leader threads of B, then A exits.  zap(A) kills B;
>    * the seized threads stay traced zombies until main reaps them, keeping ns B's
>    * pids allocated.  B's last thread waits in zap's final loop, A in zap's first
>    * loop (kernel_wait4() for B).  While "stuck" is shown, run as root:
>    *   echo freezer > /sys/power/pm_test; echo mem > /sys/power/state
>    * Unpatched, freezing fails after 20 s ("2 tasks refusing to freeze") and B's
>    * thread then busy-loops in zap.  At the end main reaps, ending the hang.
>    *
>    * Build: gcc -O2 -Wall -pthread -o pidns-freeze-repro pidns-freeze-repro.c
>    * Usage: ./pidns-freeze-repro [hold_seconds]  (root, in a VM; default: Enter)
>    * With -DUSERNS, A also gets a new user ns, so no privilege is needed.
>    */
>   #define _GNU_SOURCE
>   #include <dirent.h>
>   #include <pthread.h>
>   #include <sched.h>
>   #include <signal.h>
>   #include <stdarg.h>
>   #include <stdio.h>
>   #include <stdlib.h>
>   #include <string.h>
>   #include <sys/ptrace.h>
>   #include <sys/wait.h>
>   #include <unistd.h>
>
>   #ifdef USERNS
>   #define NS_FLAGS (CLONE_NEWUSER | CLONE_NEWPID)
>   #else
>   #define NS_FLAGS CLONE_NEWPID
>   #endif
>   #define NTHREADS 4    /* B's threads besides its leader */
>   #define NSEIZE   2    /* how many of them main seizes */
>
>   static int ready[2], go[2];   /* B -> main: threads are up; main -> A: exit */
>   static char stack_a[256 << 10], stack_b[256 << 10];
>
>   static void *idle_thread(void *arg) { for (;;) pause(); return arg; }
>
>   static int init_b(void *arg)          /* pid 1 of ns B */
>   {
>         pthread_t t;
>
>         for (int i = 0; i < NTHREADS; i++)
>                 pthread_create(&t, NULL, idle_thread, arg);
>         if (write(ready[1], "r", 1) != 1)
>                 _exit(1);
>         for (;;) pause();
>   }
>
>   static int init_a(void *arg)          /* pid 1 of ns A */
>   {
>         close(go[1]);                   /* only main may hold the write end */
>         if (clone(init_b, stack_b + sizeof(stack_b), CLONE_NEWPID | SIGCHLD, arg) < 0)
>                 _exit(1);
>         while (read(go[0], &arg, 1) > 0)        /* EOF: main has seized */
>                 ;
>         _exit(0);                       /* -> zap_pid_ns_processes(A) */
>   }
>
>   /* Read a small /proc file into buf ("" if it is gone). */
>   static char *slurp(char *buf, size_t len, const char *fmt, ...)
>   {
>         char path[300];
>         va_list ap;
>         FILE *f;
>
>         va_start(ap, fmt);
>         vsnprintf(path, sizeof(path), fmt, ap);
>         va_end(ap);
>         buf[0] = 0;
>         if ((f = fopen(path, "r"))) {
>                 buf[fread(buf, 1, len - 1, f)] = 0;
>                 fclose(f);
>         }
>         return buf;
>   }
>
>   /* Global pid of A's child B: the process whose PPid is A. */
>   static pid_t child_of(pid_t parent)
>   {
>         DIR *d = opendir("/proc");
>         struct dirent *e;
>         char buf[512], *p;
>         int ppid;
>
>         while (d && (e = readdir(d))) {
>                 p = strrchr(slurp(buf, sizeof(buf), "/proc/%s/stat", e->d_name), ')');
>                 if (p && sscanf(p + 2, "%*c %d", &ppid) == 1 && ppid == parent)
>                         return atoi(e->d_name); /* (leaks d; fine here) */
>         }
>         return -1;
>   }
>
>   /* Print a task's state and wchan; return whether it is exiting (PF_EXITING). */
>   static int show(const char *what, pid_t pid, pid_t tid)
>   {
>         char st[512], wchan[64], *p, c = '?';
>         unsigned int flags = 0;
>
>         p = strrchr(slurp(st, sizeof(st), "/proc/%d/task/%d/stat", pid, tid), ')');
>         if (p)
>                 sscanf(p + 2, "%c %*d %*d %*d %*d %*d %u", &c, &flags);
>         slurp(wchan, sizeof(wchan), "/proc/%d/task/%d/wchan", pid, tid);
>         printf("  %s tid %d: state %c, flags %#x, wchan %s\n", what, tid, c, flags, wchan);
>         return flags & 0x4;
>   }
>
>   int main(int argc, char **argv)
>   {
>         pid_t a, b, tid, seized[NSEIZE];
>         int n = 0, status;
>         struct dirent *e;
>         char buf[64];
>         DIR *d;
>
>         if (pipe(ready) || pipe(go))
>                 return perror("pipe"), 1;
>         if ((a = clone(init_a, stack_a + sizeof(stack_a), NS_FLAGS | SIGCHLD, NULL)) < 0)
>                 return perror("clone (run as root)"), 1;
>         if (read(ready[0], buf, 1) != 1 || (b = child_of(a)) < 0)
>                 return fprintf(stderr, "setup failed\n"), 1;
>
>         /* From outside both namespaces, seize non-leader threads of B. */
>         snprintf(buf, sizeof(buf), "/proc/%d/task", b);
>         for (d = opendir(buf); d && (e = readdir(d)) && n < NSEIZE; )
>                 if ((tid = atoi(e->d_name)) > 0 && tid != b && !ptrace(PTRACE_SEIZE, tid, 0, 0))
>                         seized[n++] = tid;
>         printf("A=%d B=%d: seized %d threads of B\n", a, b, n);
>
>         close(go[1]);                           /* A exits now */
>         sleep(2);
>         if (!n || waitpid(a, &status, WNOHANG) != 0 || !show("A", a, a))
>                 return printf("not reproduced\n"), 1;
>         for (rewinddir(d); (e = readdir(d)); )
>                 if ((tid = atoi(e->d_name)) > 0)
>                         show("B", b, tid);
>         printf("stuck: A and B can't finish exiting until main reaps its %d traced threads\n", n);
>         printf("now run: echo freezer > /sys/power/pm_test; echo mem > /sys/power/state\n");
>         fflush(stdout);
>         if (argc > 1)
>                 sleep(atoi(argv[1]));
>         else
>                 getchar();
>
>         /* Reap the traced zombies: ns B empties, then B and A finish exiting. */
>         for (int i = 0, left = n; left && i < 500; i++, usleep(10000))
>                 for (int k = 0; k < n; k++)
>                         if (seized[k] && waitpid(seized[k], &status, __WALL | WNOHANG) == seized[k])
>                                 seized[k] = 0, left--;
>         for (int i = 0; i < 500; i++, usleep(10000))
>                 if (waitpid(a, &status, WNOHANG) == a)
>                         return printf("reaped the zombies: hang resolved\n"), 0;
>         printf("still stuck after reaping\n");
>         return 2;
>   }
>
>  kernel/pid_namespace.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
> index 8bc9edb40..abdc05462 100644
> --- a/kernel/pid_namespace.c
> +++ b/kernel/pid_namespace.c
> @@ -23,6 +23,7 @@
>  #include <linux/sched/task.h>
>  #include <linux/sched/signal.h>
>  #include <linux/idr.h>
> +#include <linux/freezer.h>
>  #include <linux/nstree.h>
>  #include <uapi/linux/wait.h>
>  #include "pid_sysctl.h"
> @@ -243,6 +244,11 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
>         do {
>                 clear_thread_flag(TIF_SIGPENDING);
>                 rc = kernel_wait4(-1, NULL, __WALL, NULL);
> +               /*
> +                * The freezer's fake signal ends the wait with -ERESTARTSYS;
> +                * freeze here, or one stuck pid namespace blocks suspend.
> +                */
> +               try_to_freeze();
>         } while (rc != -ECHILD);
>
>         /*
> @@ -269,7 +275,13 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
>          * free_pid() will awaken this task.
>          */
>         for (;;) {
> -               set_current_state(TASK_INTERRUPTIBLE);
> +               /*
> +                * TASK_IDLE: no hung task warning or load for a wait that can
> +                * last as long as a tracer keeps a zombie, and a pending signal
> +                * (e.g. the freezer's fake one) can't turn it into a busy loop.
> +                * TASK_FREEZABLE: let the freezer freeze us while we wait.
> +                */
> +               set_current_state(TASK_IDLE | TASK_FREEZABLE);
>                 if (pid_ns->pid_allocated == init_pids)
>                         break;
>                 schedule();
>
> base-commit: ed14a591175bb5f56c2936b082cffb9e4b935e6d
> --
> 2.55.0
>