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Re: [Qemu-devel] [PATCH v0 3/7] threads: add infrastructure to process s
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH v0 3/7] threads: add infrastructure to process sigsegv |
Date: |
Thu, 12 Jul 2018 10:53:27 +0100 |
User-agent: |
Mutt/1.10.0 (2018-05-17) |
* Denis Plotnikov (address@hidden) wrote:
> Allows to define sigsegv handler temporary for all threads.
> This is useful to implement copy-on-write logic while
> linux usefaultfd doesn't support write-protected faults.
> In the future, switch to using WP userfaultfd when it's
> available.
>
> It's going to be used on background snapshotting.
I'll leave the details of signal handling to someone else
(anyone knows how this would interact with qemu-user; or the
signalfd's in util/main-loop.c ? )
But also, I'd still like to understand how this works when the
kernel makes guest accesses for things like vhost.
> Signed-off-by: Denis Plotnikov <address@hidden>
> ---
> include/qemu/thread.h | 5 ++++
> util/qemu-thread-posix.c | 50 ++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 55 insertions(+)
>
> diff --git a/include/qemu/thread.h b/include/qemu/thread.h
> index 9910f49b3a..886985d289 100644
> --- a/include/qemu/thread.h
> +++ b/include/qemu/thread.h
> @@ -210,4 +210,9 @@ void qemu_lockcnt_inc_and_unlock(QemuLockCnt *lockcnt);
> */
> unsigned qemu_lockcnt_count(QemuLockCnt *lockcnt);
>
> +
> +typedef void (*sigsegv_handler)(int v0, siginfo_t *v1, void *v2);
> +void sigsegv_user_handler_set(sigsegv_handler handler);
> +void sigsegv_user_handler_reset(void);
> +
> #endif
> diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
> index 7306475899..e51abc9275 100644
> --- a/util/qemu-thread-posix.c
> +++ b/util/qemu-thread-posix.c
> @@ -489,6 +489,45 @@ static void qemu_thread_set_name(QemuThread *thread,
> const char *name)
> #endif
> }
>
> +static sigsegv_handler sigsegv_user_handler;
> +
> +void sigsegv_user_handler_set(sigsegv_handler handler)
> +{
> + assert(handler);
> + atomic_set(&sigsegv_user_handler, handler);
> +}
> +
> +static sigsegv_handler sigsegv_user_handler_get(void)
> +{
> + return atomic_read(&sigsegv_user_handler);
> +}
> +
> +void sigsegv_user_handler_reset(void)
> +{
> + atomic_set(&sigsegv_user_handler, NULL);
> +}
> +
> +static void sigsegv_default_handler(int v0, siginfo_t *v1, void *v2)
v0/v1/v2 aren't great names for the parameters.
> +{
> + sigsegv_handler handler = sigsegv_user_handler_get();
> +
> + if (!handler) {
> + // remove the sigsegv handler if it's not set by user
> + // this will lead to re-raising the error without a handler
> + // and exiting from the program with "Sigmentation fault"
Style guide doesn't allow C99 comments.
(And typo: Sig->Seg)
Dave
> + int err;
> + struct sigaction act;
> + memset(&act, 0, sizeof(act));
> + act.sa_flags = SA_RESETHAND;
> + err = sigaction(SIGSEGV, &act, NULL);
> + if (err) {
> + error_exit(err, __func__);
> + }
> + } else {
> + handler(v0, v1, v2);
> + }
> +}
> +
> void qemu_thread_create(QemuThread *thread, const char *name,
> void *(*start_routine)(void*),
> void *arg, int mode)
> @@ -496,14 +535,25 @@ void qemu_thread_create(QemuThread *thread, const char
> *name,
> sigset_t set, oldset;
> int err;
> pthread_attr_t attr;
> + struct sigaction act;
>
> err = pthread_attr_init(&attr);
> if (err) {
> error_exit(err, __func__);
> }
>
> + memset(&act, 0, sizeof(act));
> + act.sa_flags = SA_SIGINFO;
> + act.sa_sigaction = sigsegv_default_handler;
> + err = sigaction(SIGSEGV, &act, NULL);
> + if (err) {
> + error_exit(err, __func__);
> + }
> +
> /* Leave signal handling to the iothread. */
> sigfillset(&set);
> + // ...all but SIGSEGV
> + sigdelset(&set, SIGSEGV);
> pthread_sigmask(SIG_SETMASK, &set, &oldset);
> err = pthread_create(&thread->thread, &attr, start_routine, arg);
> if (err)
> --
> 2.17.0
>
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK
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