Hello,
[adding linux-api, linux-man]
On Thu, Nov 13, 2014 at 07:29:53AM +0000, Chanho Min wrote:
> Since commit 28256d612726 ("ARM: cacheflush: split user cache-flushing
> into interruptible chunks"), cacheflush can be interrupted by signal.
>
> But, cacheflush doesn't resume from where we left off if process has
> user-defined signal handlers. It returns -EINTR then cacheflush
> should be re-invoked from the start of address until cache-flushing
> of whole address ranges is completed (restart_syscall isn't available
> in userspace). It may cause regression. So I suggest to disallow
> pending signals during cacheflush.
>
> This partially reverts commit 28256d612726a28a8b9d3c49f2b74198c4423d6a.
Whilst I don't think this is the correct solution, I agree that there's
a potential issue here. We could change the restart return value to
-ERESTARTNOINTR instead, but I can imagine something like a periodic
SIGALRM which could prevent a large cacheflush from ever completing.
Do we actually care about making forward progress in such a scenario?
It is interesting to note that this change has been in mainline since
May last year without any reported issues. That could be down to a number
of reasons:
(1) People are using old kernels on ARM
(2) Code doesn't check the return value from the cacheflush system call,
because it historically always returned 0
(3) People are getting lucky with timing, as this is likely difficult
to hit
Related to (2) is that a `man cacheflush' invocation returns something
about the MIPs system call, that doesn't match what we do for ARM. The
(relatively recent) history of the system call on ARM is:
< v3.5 [*]
- Always returns 0
- Restricts virtual address range to a single VMA
- Page-aligns the region limits (over flushing for smaller ranges)
- Terminates on the first fault
- Flags are ignored but must "ALWAYS be passed as ZERO"
v3.5 - v3.12
- Returns -EINVAL if flags is set or if end < start
- Returns -EINVAL if we couldn't find a vma
- Terminates on the first fault and returns -EFAULT
v3.12 - HEAD
- No longer page-aligns region
- Removes VMA checking as this had a deadlock bug with mmap_sem
and we could handle faults by this point anyway
- Returns -EINVAL if !access_ok for the range
- Splits the range into PAGE_SIZE chunks, checking for reschedule
and pending signals to avoid DoSing the system (the hardware can
only clean by cacheline). This is where the -ERESTART_RESTARTBLOCK
behaviour came in, potentially returning -EINTR to userspace.
This leaves me with the following questions:
- Has this change been shown to break anything in practice?
- Can we change the internal return value to -ERESTARTNOINTR?
- What do we do about kernels that *do* return -EINTR? (>=3.12?)
- Can we get a manpage put together to describe this mess?
Cheers,
Will
[*] rmk may have some more ancient history kicking around, if you like!
> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
> index abd2fc0..275e086 100644
> --- a/arch/arm/kernel/traps.c
> +++ b/arch/arm/kernel/traps.c
> @@ -521,25 +521,6 @@ __do_cache_op(unsigned long start, unsigned long end)
> do {
> unsigned long chunk = min(PAGE_SIZE, end - start);
>
> - if (signal_pending(current)) {
> - struct thread_info *ti = current_thread_info();
> -
> - ti->restart_block = (struct restart_block) {
> - .fn = do_cache_op_restart,
> - };
> -
> - ti->arm_restart_block = (struct arm_restart_block) {
> - {
> - .cache = {
> - .start = start,
> - .end = end,
> - },
> - },
> - };
> -
> - return -ERESTART_RESTARTBLOCK;
> - }
> -
> ret = flush_cache_user_range(start, start + chunk);
> if (ret)
> return ret;
> --
> 1.7.9.5
>
>
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