On Fri, May 21, 2021 at 11:21:17AM +0000, Hans Petter Selasky wrote:
> The branch main has been updated by hselasky:
>
> URL:
> https://cgit.FreeBSD.org/src/commit/?id=b764a426534f2f5f86d6625288c74dafdbc94d2b
>
> commit b764a426534f2f5f86d6625288c74dafdbc94d2b
> Author: Hans Petter Selasky <[email protected]>
> AuthorDate: 2021-05-21 11:17:42 +0000
> Commit: Hans Petter Selasky <[email protected]>
> CommitDate: 2021-05-21 11:18:41 +0000
>
> There is a window where threads are removed from the process list and
> where
> the thread destructor is invoked. Catch that window by waiting for all
> task_struct allocations to be returned before freeing the UMA zone in the
> LinuxKPI. Else UMA may fail to release the zone due to concurrent access
> and panic:
>
> panic() - Bad link element prev->next != elm
> zone_release()
> bucket_drain()
> bucket_free()
> zone_dtor()
> zone_free_item()
> uma_zdestroy()
> linux_current_uninit()
>
> This failure can be triggered by loading and unloading the LinuxKPI module
> in a loop:
>
> while true
> do
> kldload linuxkpi
> kldunload linuxkpi
> done
>
> Discussed with: kib@
No, it was not discussed, with me.
It contains parts of my half-done patches.
And I disagree with what the global counting you added there, both on
principle and on implementation.
> MFC after: 1 week
> Sponsored by: Mellanox Technologies // NVIDIA Networking
> ---
> sys/compat/linuxkpi/common/src/linux_current.c | 35
> ++++++++++++++++++++++----
> 1 file changed, 30 insertions(+), 5 deletions(-)
>
> diff --git a/sys/compat/linuxkpi/common/src/linux_current.c
> b/sys/compat/linuxkpi/common/src/linux_current.c
> index 9bae7ee92e49..51e396081c04 100644
> --- a/sys/compat/linuxkpi/common/src/linux_current.c
> +++ b/sys/compat/linuxkpi/common/src/linux_current.c
> @@ -45,6 +45,7 @@ extern u_int first_msi_irq, num_msi_irqs;
>
> static eventhandler_tag linuxkpi_thread_dtor_tag;
>
> +static atomic_t linux_current_allocs;
> static uma_zone_t linux_current_zone;
> static uma_zone_t linux_mm_zone;
>
> @@ -146,6 +147,10 @@ linux_alloc_current(struct thread *td, int flags)
> /* free mm_struct pointer, if any */
> uma_zfree(linux_mm_zone, mm);
>
> + /* keep track of number of allocations */
> + if (atomic_add_return(1, &linux_current_allocs) == INT_MAX)
> + panic("linux_alloc_current: Refcount too high!");
> +
> return (0);
> }
>
> @@ -173,6 +178,10 @@ linux_free_current(struct task_struct *ts)
> {
> mmput(ts->mm);
> uma_zfree(linux_current_zone, ts);
> +
> + /* keep track of number of allocations */
> + if (atomic_sub_return(1, &linux_current_allocs) < 0)
> + panic("linux_free_current: Negative refcount!");
> }
>
> static void
> @@ -271,10 +280,6 @@ SYSCTL_INT(_compat_linuxkpi, OID_AUTO,
> task_struct_reserve,
> static void
> linux_current_init(void *arg __unused)
> {
> - lkpi_alloc_current = linux_alloc_current;
> - linuxkpi_thread_dtor_tag = EVENTHANDLER_REGISTER(thread_dtor,
> - linuxkpi_thread_dtor, NULL, EVENTHANDLER_PRI_ANY);
> -
> TUNABLE_INT_FETCH("compat.linuxkpi.task_struct_reserve",
> &lkpi_task_resrv);
> if (lkpi_task_resrv == 0) {
> @@ -298,6 +303,12 @@ linux_current_init(void *arg __unused)
> UMA_ALIGN_PTR, 0);
> uma_zone_reserve(linux_mm_zone, lkpi_task_resrv);
> uma_prealloc(linux_mm_zone, lkpi_task_resrv);
> +
> + atomic_thread_fence_seq_cst();
> +
> + lkpi_alloc_current = linux_alloc_current;
> + linuxkpi_thread_dtor_tag = EVENTHANDLER_REGISTER(thread_dtor,
> + linuxkpi_thread_dtor, NULL, EVENTHANDLER_PRI_ANY);
> }
> SYSINIT(linux_current, SI_SUB_EVENTHANDLER, SI_ORDER_SECOND,
> linux_current_init, NULL);
> @@ -309,6 +320,10 @@ linux_current_uninit(void *arg __unused)
> struct task_struct *ts;
> struct thread *td;
>
> + lkpi_alloc_current = linux_alloc_current_noop;
> +
> + atomic_thread_fence_seq_cst();
> +
> sx_slock(&allproc_lock);
> FOREACH_PROC_IN_SYSTEM(p) {
> PROC_LOCK(p);
> @@ -321,8 +336,18 @@ linux_current_uninit(void *arg __unused)
> PROC_UNLOCK(p);
> }
> sx_sunlock(&allproc_lock);
> +
> + /*
> + * There is a window where threads are removed from the
> + * process list and where the thread destructor is invoked.
> + * Catch that window by waiting for all task_struct
> + * allocations to be returned before freeing the UMA zone.
> + */
> + while (atomic_read(&linux_current_allocs) != 0)
> + pause("W", 1);
> +
> EVENTHANDLER_DEREGISTER(thread_dtor, linuxkpi_thread_dtor_tag);
> - lkpi_alloc_current = linux_alloc_current_noop;
> +
> uma_zdestroy(linux_current_zone);
> uma_zdestroy(linux_mm_zone);
> }
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