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![Rafael J. Wysocki](/assets/img/avatar_default.png)
There is a race condition between the freezer and request_firmware() such that if request_firmware() is run on one CPU and freeze_processes() is run on another CPU and usermodehelper_disable() called by it succeeds to grab umhelper_sem for writing before usermodehelper_read_trylock() called from request_firmware() acquires it for reading, the request_firmware() will fail and trigger a WARN_ON() complaining that it was called at a wrong time. However, in fact, it wasn't called at a wrong time and freeze_processes() simply happened to be executed simultaneously. To avoid this race, at least in some cases, modify usermodehelper_read_trylock() so that it doesn't fail if the freezing of tasks has just started and hasn't been completed yet. Instead, during the freezing of tasks, it will try to freeze the task that has called it so that it can wait until user space is thawed without triggering the scary warning. For this purpose, change usermodehelper_disabled so that it can take three different values, UMH_ENABLED (0), UMH_FREEZING and UMH_DISABLED. The first one means that usermode helpers are enabled, the last one means "hard disable" (i.e. the system is not ready for usermode helpers to be used) and the second one is reserved for the freezer. Namely, when freeze_processes() is started, it sets usermodehelper_disabled to UMH_FREEZING which tells usermodehelper_read_trylock() that it shouldn't fail just yet and should call try_to_freeze() if woken up and cannot return immediately. This way all freezable tasks that happen to call request_firmware() right before freeze_processes() is started and lose the race for umhelper_sem with it will be frozen and will sleep until thaw_processes() unsets usermodehelper_disabled. [For the non-freezable callers of request_firmware() the race for umhelper_sem against freeze_processes() is unfortunately unavoidable.] Reported-by: Stephen Boyd <sboyd@codeaurora.org> Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl> Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: stable@vger.kernel.org
220 lines
4.7 KiB
C
220 lines
4.7 KiB
C
/*
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* drivers/power/process.c - Functions for starting/stopping processes on
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* suspend transitions.
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*
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* Originally from swsusp.
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*/
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#undef DEBUG
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#include <linux/interrupt.h>
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#include <linux/oom.h>
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#include <linux/suspend.h>
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#include <linux/module.h>
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#include <linux/syscalls.h>
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#include <linux/freezer.h>
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#include <linux/delay.h>
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#include <linux/workqueue.h>
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#include <linux/kmod.h>
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/*
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* Timeout for stopping processes
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*/
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#define TIMEOUT (20 * HZ)
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static int try_to_freeze_tasks(bool user_only)
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{
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struct task_struct *g, *p;
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unsigned long end_time;
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unsigned int todo;
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bool wq_busy = false;
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struct timeval start, end;
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u64 elapsed_csecs64;
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unsigned int elapsed_csecs;
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bool wakeup = false;
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do_gettimeofday(&start);
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end_time = jiffies + TIMEOUT;
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if (!user_only)
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freeze_workqueues_begin();
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while (true) {
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todo = 0;
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read_lock(&tasklist_lock);
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do_each_thread(g, p) {
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if (p == current || !freeze_task(p))
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continue;
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/*
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* Now that we've done set_freeze_flag, don't
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* perturb a task in TASK_STOPPED or TASK_TRACED.
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* It is "frozen enough". If the task does wake
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* up, it will immediately call try_to_freeze.
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*
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* Because freeze_task() goes through p's scheduler lock, it's
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* guaranteed that TASK_STOPPED/TRACED -> TASK_RUNNING
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* transition can't race with task state testing here.
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*/
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if (!task_is_stopped_or_traced(p) &&
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!freezer_should_skip(p))
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todo++;
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} while_each_thread(g, p);
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read_unlock(&tasklist_lock);
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if (!user_only) {
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wq_busy = freeze_workqueues_busy();
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todo += wq_busy;
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}
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if (!todo || time_after(jiffies, end_time))
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break;
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if (pm_wakeup_pending()) {
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wakeup = true;
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break;
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}
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/*
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* We need to retry, but first give the freezing tasks some
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* time to enter the regrigerator.
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*/
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msleep(10);
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}
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do_gettimeofday(&end);
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elapsed_csecs64 = timeval_to_ns(&end) - timeval_to_ns(&start);
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do_div(elapsed_csecs64, NSEC_PER_SEC / 100);
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elapsed_csecs = elapsed_csecs64;
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if (todo) {
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printk("\n");
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printk(KERN_ERR "Freezing of tasks %s after %d.%02d seconds "
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"(%d tasks refusing to freeze, wq_busy=%d):\n",
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wakeup ? "aborted" : "failed",
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elapsed_csecs / 100, elapsed_csecs % 100,
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todo - wq_busy, wq_busy);
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if (!wakeup) {
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read_lock(&tasklist_lock);
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do_each_thread(g, p) {
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if (p != current && !freezer_should_skip(p)
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&& freezing(p) && !frozen(p))
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sched_show_task(p);
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} while_each_thread(g, p);
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read_unlock(&tasklist_lock);
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}
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} else {
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printk("(elapsed %d.%02d seconds) ", elapsed_csecs / 100,
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elapsed_csecs % 100);
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}
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return todo ? -EBUSY : 0;
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}
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/**
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* freeze_processes - Signal user space processes to enter the refrigerator.
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*
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* On success, returns 0. On failure, -errno and system is fully thawed.
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*/
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int freeze_processes(void)
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{
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int error;
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error = __usermodehelper_disable(UMH_FREEZING);
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if (error)
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return error;
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if (!pm_freezing)
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atomic_inc(&system_freezing_cnt);
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printk("Freezing user space processes ... ");
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pm_freezing = true;
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error = try_to_freeze_tasks(true);
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if (!error) {
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printk("done.");
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__usermodehelper_set_disable_depth(UMH_DISABLED);
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oom_killer_disable();
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}
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printk("\n");
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BUG_ON(in_atomic());
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if (error)
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thaw_processes();
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return error;
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}
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/**
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* freeze_kernel_threads - Make freezable kernel threads go to the refrigerator.
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*
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* On success, returns 0. On failure, -errno and only the kernel threads are
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* thawed, so as to give a chance to the caller to do additional cleanups
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* (if any) before thawing the userspace tasks. So, it is the responsibility
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* of the caller to thaw the userspace tasks, when the time is right.
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*/
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int freeze_kernel_threads(void)
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{
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int error;
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printk("Freezing remaining freezable tasks ... ");
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pm_nosig_freezing = true;
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error = try_to_freeze_tasks(false);
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if (!error)
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printk("done.");
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printk("\n");
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BUG_ON(in_atomic());
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if (error)
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thaw_kernel_threads();
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return error;
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}
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void thaw_processes(void)
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{
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struct task_struct *g, *p;
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if (pm_freezing)
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atomic_dec(&system_freezing_cnt);
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pm_freezing = false;
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pm_nosig_freezing = false;
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oom_killer_enable();
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printk("Restarting tasks ... ");
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thaw_workqueues();
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read_lock(&tasklist_lock);
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do_each_thread(g, p) {
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__thaw_task(p);
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} while_each_thread(g, p);
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read_unlock(&tasklist_lock);
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usermodehelper_enable();
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schedule();
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printk("done.\n");
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}
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void thaw_kernel_threads(void)
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{
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struct task_struct *g, *p;
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pm_nosig_freezing = false;
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printk("Restarting kernel threads ... ");
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thaw_workqueues();
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read_lock(&tasklist_lock);
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do_each_thread(g, p) {
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if (p->flags & (PF_KTHREAD | PF_WQ_WORKER))
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__thaw_task(p);
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} while_each_thread(g, p);
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read_unlock(&tasklist_lock);
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schedule();
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printk("done.\n");
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}
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