Messages in this thread Patch in this message | | | Date | Mon, 11 Nov 2019 11:44:03 +0100 | From | Sebastian Andrzej Siewior <> | Subject | Re: [PATCH v2] percpu-refcount: Use normal instead of RCU-sched" |
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On 2019-11-08 18:17:47 [+0000], Christopher Lameter wrote: > On Fri, 8 Nov 2019, Sebastian Andrzej Siewior wrote: > > > diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h > > index 7aef0abc194a2..390031e816dcd 100644 > > --- a/include/linux/percpu-refcount.h > > +++ b/include/linux/percpu-refcount.h > > @@ -186,14 +186,14 @@ static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr) > > { > > unsigned long __percpu *percpu_count; > > > > - rcu_read_lock_sched(); > > + rcu_read_lock(); > > > > if (__ref_is_percpu(ref, &percpu_count)) > > this_cpu_add(*percpu_count, nr); > > You can use > > __this_cpu_add() > > instead since rcu_read_lock implies preempt disable.
Paul may correct me but rcu_read_lock() does not imply preempt_disable(). rcu_read_lock() does preempt_disable() on preemption models other than "Low-Latency Desktop". You can be preempted in a RCU-read section.
> This will not change the code for x86 but other platforms that do not use > atomic operation here will be able to avoid including to code to disable > preempt for the per cpu operations.
x86 triggers this warning with the suggested change:
| BUG: using __this_cpu_add() in preemptible [00000000] code: login/2370 | caller is blk_mq_get_request+0x74/0x4c0 | CPU: 0 PID: 2370 Comm: login Not tainted 5.4.0-rc7+ #82 | Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.12.0-1 04/01/2014 | Call Trace: | dump_stack+0x7a/0xaa | __this_cpu_preempt_check.cold+0x49/0x4e | blk_mq_get_request+0x74/0x4c0 | blk_mq_make_request+0x111/0x890 | generic_make_request+0xd3/0x3f0 | submit_bio+0x42/0x140 | submit_bh_wbc.isra.0+0x13f/0x170 | ll_rw_block+0xa0/0xb0 | __breadahead+0x3f/0x70 | __ext4_get_inode_loc+0x40a/0x520 | __ext4_iget+0x10a/0xcf0 | ext4_lookup+0x106/0x1f0 | lookup_open+0x267/0x8e0 | path_openat+0x7c8/0xcb0 | do_filp_open+0x8c/0x100 | do_sys_open+0x17a/0x230 | __x64_sys_openat+0x1b/0x20 | do_syscall_64+0x5a/0x230 | entry_SYSCALL_64_after_hwframe+0x49/0xbe
> Same is valid for all other per cpu operations in the patch.
Sebastian
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