Messages in this thread | | | Date | Wed, 23 May 2018 17:42:34 +0100 | From | Will Deacon <> | Subject | Re: [PATCH] perf/ring_buffer: ensure atomicity and order of updates |
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On Thu, May 10, 2018 at 02:06:32PM +0100, Mark Rutland wrote: > Userspace can read/write the user page at any point in time, and in > perf_output_put_handle() we're very careful to use memory barriers to > ensure ordering between updates to data and the user page. > > We don't use barriers when updating aux_head, where similar ordering > constraints apply. This could result in userspace seeing stale data, or > data being overwritten while userspace was still consuming it. > > Further, we update data_head and aux_head with plain assignments, which > the compiler can tear, potentially resulting in userspace seeing > erroneous values. > > We can solve both of these problems by using smp_store_release to update > data_head and aux_head, so let's do so. > > Signed-off-by: Mark Rutland <mark.rutland@arm.com> > Cc: Ingo Molnar <mingo@redhat.com> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Will Deacon <will.deacon@arm.com> > --- > kernel/events/ring_buffer.c | 13 ++++++------- > 1 file changed, 6 insertions(+), 7 deletions(-) > > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c > index 6c6b3c48db71..839b207e4c77 100644 > --- a/kernel/events/ring_buffer.c > +++ b/kernel/events/ring_buffer.c > @@ -63,10 +63,10 @@ static void perf_output_put_handle(struct perf_output_handle *handle) > * kernel user > * > * if (LOAD ->data_tail) { LOAD ->data_head > - * (A) smp_rmb() (C) > + * (A) smp_rmb() (C) > * STORE $data LOAD $data > - * smp_wmb() (B) smp_mb() (D) > - * STORE ->data_head STORE ->data_tail > + * smp_mb() (D) > + * RELEASE ->data_head (B) STORE ->data_tail > * }
One thing to be aware of here is that the choice of ordering primitive (e.g. using fences vs acquire/release operations) has the potential to create ABI with userspace. I don't know of any architectures which currently care, but if were were to merge a non multi-copy atomic architecture with native acquire/release instructions, you could see issues if e.g. userspace used smp_rmb(); READ_ONCE but the kernel used a RELEASE store.
Anyway, that's currently theoretical, but I think it's an argument for putting these accessors in a uapi header.
Will
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