Interrupt is allowed during exception handling.
There are warning messages if the kernel enables the configuration
'CONFIG_DEBUG_ATOMIC_SLEEP=y'.

BUG: sleeping function called from invalid context at kernel/locking/rwsem.c:23
in_atomic(): 0, irqs_disabled(): 1, pid: 43, name: ash
CPU: 0 PID: 43 Comm: ash Tainted:  G	 W	 4.15.0-rc8-00089-g89ffdae-dirty #17
Call Trace:
[<000000009abb1587>] walk_stackframe+0x0/0x7a
[<00000000d4f3d088>] ___might_sleep+0x102/0x11a
[<00000000b1fd792a>] down_read+0x18/0x28
[<000000000289ec01>] do_page_fault+0x86/0x2f6
[<00000000012441f6>] _do_fork+0x1b4/0x1e0
[<00000000f46c3e3b>] ret_from_syscall+0xa/0xe
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NZong Li <zong@andestech.com>
Signed-off-by: NPalmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>

The SUM bit is enabled at the beginning of the copy_{to,from}_user and
{get,put}_user routines, and cleared before they return.  But these user
copy helper can be interrupted by exceptions, in which case the SUM bit
will remain set, which leads to elevated privileges for the code running
in exception context, as that can now access userspace address space
unconditionally.  This frequently happens when the user copy routines
access freshly allocated user memory that hasn't been faulted in, and a
pagefault needs to be taken before the user copy routines can continue.

Fix this by unconditionally clearing SUM when the exception handler is
called - the restore code will automatically restore it based on the
saved value.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>

Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>

This patch contains the implementation of tasks on RISC-V, most of which
is involved in task switching.
Signed-off-by: NPalmer Dabbelt <palmer@dabbelt.com>