On Fri, Jan 25, 2019 at 5:12 AM Paul E. McKenney <paul...@linux.ibm.com> wrote: > On Fri, Jan 25, 2019 at 02:46:55AM +0100, Jann Horn wrote: > > On Fri, Jan 25, 2019 at 2:22 AM Paul E. McKenney <paul...@linux.ibm.com> > > wrote: > > > On Thu, Jan 24, 2019 at 04:05:16PM -0800, Alexei Starovoitov wrote: > > > > On Thu, Jan 24, 2019 at 03:42:32PM -0800, Paul E. McKenney wrote: > > > > > On Thu, Jan 24, 2019 at 07:56:52PM +0100, Peter Zijlstra wrote: > > > > > > On Thu, Jan 24, 2019 at 07:01:09PM +0100, Peter Zijlstra wrote: > > > > > > > > > > > > > > Thanks for having kernel/locking people on Cc... > > > > > > > > > > > > > > On Wed, Jan 23, 2019 at 08:13:55PM -0800, Alexei Starovoitov > > > > > > > wrote: > > > > > > > > > > > > > > > Implementation details: > > > > > > > > - on !SMP bpf_spin_lock() becomes nop > > > > > > > > > > > > > > Because no BPF program is preemptible? I don't see any assertions > > > > > > > or > > > > > > > even a comment that says this code is non-preemptible. > > > > > > > > > > > > > > AFAICT some of the BPF_RUN_PROG things are under rcu_read_lock() > > > > > > > only, > > > > > > > which is not sufficient. > > > > > > > > > > > > > > > - on architectures that don't support queued_spin_lock trivial > > > > > > > > lock is used. > > > > > > > > Note that arch_spin_lock cannot be used, since not all archs > > > > > > > > agree that > > > > > > > > zero == unlocked and sizeof(arch_spinlock_t) != sizeof(__u32). > > > > > > > > > > > > > > I really don't much like direct usage of qspinlock; esp. not as a > > > > > > > surprise. > > > > > > > > > > Substituting the lightweight-reader SRCU as discussed earlier would > > > > > allow > > > > > use of a more generic locking primitive, for example, one that allowed > > > > > blocking, at least in cases were the context allowed this. > > > > > > > > > > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git > > > > > branch srcu-lr.2019.01.16a. > > > > > > > > > > One advantage of a more generic locking primitive would be keeping BPF > > > > > programs independent of internal changes to spinlock primitives. > > > > > > > > Let's keep "srcu in bpf" discussion separate from bpf_spin_lock > > > > discussion. > > > > bpf is not switching to srcu any time soon. > > > > If/when it happens it will be only for certain prog+map types > > > > like bpf syscall probes that need to be able to do copy_from_user > > > > from bpf prog. > > > > > > Hmmm... What prevents BPF programs from looping infinitely within an > > > RCU reader, and as you noted, preemption disabled? > > > > > > If BPF programs are in fact allowed to loop infinitely, it would be > > > very good for the health of the kernel to have preemption enabled. > > > And to be within an SRCU read-side critical section instead of an RCU > > > read-side critical section. > > > > The BPF verifier prevents loops; this is in push_insn() in > > kernel/bpf/verifier.c, which errors out with -EINVAL when a back edge > > is encountered. For non-root programs, that limits the maximum number > > of instructions per eBPF engine execution to > > BPF_MAXINSNS*MAX_TAIL_CALL_CNT==4096*32==131072 (but that includes > > call instructions, which can cause relatively expensive operations > > like hash table lookups). For programs created with CAP_SYS_ADMIN, > > things get more tricky because you can create your own functions and > > call them repeatedly; I'm not sure whether the pessimal runtime there > > becomes exponential, or whether there is some check that catches this. > > Whew!!! ;-) > > So no more than (say) 100 milliseconds?
Depends on RLIMIT_MEMLOCK and on how hard userspace is trying to make things slow, I guess - if userspace manages to create a hashtable, with a few dozen megabytes in size, with worst-case assignment of elements to buckets (everything in a single bucket), every lookup call on that bucket becomes a linked list traversal through a list that must be stored in main memory because it's too big for the CPU caches. I don't know into how much time that translates.
