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.
