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From: Stefan Metzmacher <[email protected]>
To: Jens Axboe <[email protected]>, [email protected]
Cc: [email protected], [email protected],
	[email protected]
Subject: Re: [PATCH 09/18] io-wq: fork worker threads from original task
Date: Thu, 4 Mar 2021 17:13:56 +0100	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>


Am 04.03.21 um 14:19 schrieb Stefan Metzmacher:
> Hi Jens,
> 
>>> Can you please explain why CLONE_SIGHAND is used here?
>>
>> We can't have CLONE_THREAD without CLONE_SIGHAND... The io-wq workers
>> don't really care about signals, we don't use them internally.
> 
> I'm 100% sure, but I heard rumors that in some situations signals get
> randomly delivered to any thread of a userspace process.

Ok, from task_struct:

        /* Signal handlers: */
        struct signal_struct            *signal;
        struct sighand_struct __rcu             *sighand;
        sigset_t                        blocked;
        sigset_t                        real_blocked;
        /* Restored if set_restore_sigmask() was used: */
        sigset_t                        saved_sigmask;
        struct sigpending               pending;

The signal handlers are shared, but 'blocked' is per thread/task.

> My fear was that the related logic may select a kernel thread if they
> share the same signal handlers.

I found the related logic in the interaction between
complete_signal() and wants_signal().

static inline bool wants_signal(int sig, struct task_struct *p)
{
        if (sigismember(&p->blocked, sig))
                return false;

...

Would it make sense to set up task->blocked to block all signals?

Something like this:

--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -611,15 +611,15 @@ pid_t io_wq_fork_thread(int (*fn)(void *), void *arg)
 {
        unsigned long flags = CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|
                                CLONE_IO|SIGCHLD;
-       struct kernel_clone_args args = {
-               .flags          = ((lower_32_bits(flags) | CLONE_VM |
-                                   CLONE_UNTRACED) & ~CSIGNAL),
-               .exit_signal    = (lower_32_bits(flags) & CSIGNAL),
-               .stack          = (unsigned long)fn,
-               .stack_size     = (unsigned long)arg,
-       };
+       sigset_t mask, oldmask;
+       pid_t pid;

-       return kernel_clone(&args);
+       sigfillset(&mask);
+       sigprocmask(SIG_BLOCK, &mask, &oldmask);
+       pid = kernel_thread(fn, arg, flags);
+       sigprocmask(SIG_SETMASK, &oldmask, NULL);
+
+       return ret;
 }

I think using kernel_thread() would be a good simplification anyway.

sig_task_ignored() has some PF_IO_WORKER logic.

Or is there any PF_IO_WORKER related logic that prevents
an io_wq thread to be excluded in complete_signal().

Or PF_IO_WORKER would teach kernel_clone to ignore CLONE_SIGHAND
and create a fresh handler and alter the copy_signal() and copy_sighand()
checks...

metze

  reply	other threads:[~2021-03-04 16:15 UTC|newest]

Thread overview: 49+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-19 17:09 [PATCHSET RFC 0/18] Remove kthread usage from io_uring Jens Axboe
2021-02-19 17:09 ` [PATCH 01/18] io_uring: remove the need for relying on an io-wq fallback worker Jens Axboe
2021-02-19 20:25   ` Eric W. Biederman
2021-02-19 20:37     ` Jens Axboe
2021-02-22 13:46   ` Pavel Begunkov
2021-02-19 17:09 ` [PATCH 02/18] io-wq: don't create any IO workers upfront Jens Axboe
2021-02-19 17:09 ` [PATCH 03/18] io_uring: disable io-wq attaching Jens Axboe
2021-02-19 17:09 ` [PATCH 04/18] io-wq: get rid of wq->use_refs Jens Axboe
2021-02-19 17:09 ` [PATCH 05/18] io_uring: tie async worker side to the task context Jens Axboe
2021-02-20  8:11   ` Hao Xu
2021-02-20 14:38     ` Jens Axboe
2021-02-21  9:16       ` Hao Xu
2021-02-19 17:09 ` [PATCH 06/18] io-wq: don't pass 'wqe' needlessly around Jens Axboe
2021-02-19 17:09 ` [PATCH 07/18] arch: setup PF_IO_WORKER threads like PF_KTHREAD Jens Axboe
2021-02-19 22:21   ` Eric W. Biederman
2021-02-19 23:26     ` Jens Axboe
2021-02-19 17:10 ` [PATCH 08/18] kernel: treat PF_IO_WORKER like PF_KTHREAD for ptrace/signals Jens Axboe
2021-02-19 17:10 ` [PATCH 09/18] io-wq: fork worker threads from original task Jens Axboe
2021-03-04 12:23   ` Stefan Metzmacher
2021-03-04 13:05     ` Jens Axboe
2021-03-04 13:19       ` Stefan Metzmacher
2021-03-04 16:13         ` Stefan Metzmacher [this message]
2021-03-04 16:42           ` Jens Axboe
2021-03-04 17:09             ` Stefan Metzmacher
2021-03-04 17:32               ` Jens Axboe
2021-03-04 18:19                 ` Jens Axboe
2021-03-04 18:56                   ` Linus Torvalds
2021-03-04 19:19                     ` Jens Axboe
2021-03-04 19:46                       ` Linus Torvalds
2021-03-04 19:54                         ` Jens Axboe
2021-03-04 20:00                           ` Jens Axboe
2021-03-04 20:23                             ` Jens Axboe
2021-03-04 20:50                           ` Linus Torvalds
2021-03-04 20:54                             ` Jens Axboe
2021-03-05 19:16           ` Eric W. Biederman
2021-03-05 19:00       ` Eric W. Biederman
2021-02-19 17:10 ` [PATCH 10/18] io-wq: worker idling always returns false Jens Axboe
2021-02-19 17:10 ` [PATCH 11/18] io_uring: remove any grabbing of context Jens Axboe
2021-02-19 17:10 ` [PATCH 12/18] io_uring: remove io_identity Jens Axboe
2021-02-19 17:10 ` [PATCH 13/18] io-wq: only remove worker from free_list, if it was there Jens Axboe
2021-02-19 17:10 ` [PATCH 14/18] io-wq: make io_wq_fork_thread() available to other users Jens Axboe
2021-02-19 17:10 ` [PATCH 15/18] io_uring: move SQPOLL thread io-wq forked worker Jens Axboe
2021-02-19 17:10 ` [PATCH 16/18] Revert "proc: don't allow async path resolution of /proc/thread-self components" Jens Axboe
2021-02-19 17:10 ` [PATCH 17/18] Revert "proc: don't allow async path resolution of /proc/self components" Jens Axboe
2021-02-19 17:10 ` [PATCH 18/18] net: remove cmsg restriction from io_uring based send/recvmsg calls Jens Axboe
2021-02-19 23:44 ` [PATCHSET RFC 0/18] Remove kthread usage from io_uring Stefan Metzmacher
2021-02-19 23:51   ` Jens Axboe
2021-02-21  5:04 ` Linus Torvalds
2021-02-21 21:22   ` Jens Axboe

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