On 3/1/26 22:02, Jens Axboe wrote:
> On 3/1/26 5:22 PM, Chaitanya Kulkarni wrote:
>> Fixes: 7ffbd48d5cab ("tracing: Cache comms only after an event occurred")
> I don't understand, this dates back to 2012?
>
> --
> Jens Axboe
The commit c71a896154119 ("blktrace: add ftrace plugin") added
tracing_record_cmdline() definition first.
Then commit 7ffbd48d5cab ("tracing: Cache comms only after an event occurred")
updated tracing_record_cmdline() function with __trace_cpu_read() and
__trace_cpu_write().
Above added __trace_cpu_read() when used in process context in the call
chain from starting blk_add_trace() is resulting in the splat :-
run blktests blktrace/002 at 2026-02-25 22:24:33
null_blk: disk nullb1 created
_BUG: using __this_cpu_read() in preemptible [00000000] code: dd/2516_
caller is tracing_record_cmdline+0x10/0x40
CPU: 16 UID: 0 PID: 2516 Comm: dd Tainted: G N 7.0.0-rc1lblk+
#84 PREEMPT(full)
Tainted: [N]=TEST
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
Call Trace:
<TASK>
dump_stack_lvl+0x8d/0xb0
check_preemption_disabled+0xce/0xe0
tracing_record_cmdline+0x10/0x40
__blk_add_trace+0x307/0x5d0
? lock_acquire+0xe0/0x300
? iov_iter_extract_pages+0x101/0xa30
blk_add_trace_bio+0x106/0x
[...]
Hence when __trace_cpu_read() is added in the path from blk_add_trace()
this bug was introduced 7ffbd48d5cab ?
I totally failed to understand why this bug is appearing right now
than before.
-ck
Other reference commits :-
The commit 2cc621fd2e9b8
("tracing: Move saved_cmdline code into trace_sched_switch.c")
moved the lockdep to new file.
The commit c0a581d7126c0
("tracing: Disable interrupt or preemption before acquiring arch_spinlock_t")
Added lockdep_assert_preemption_disabled() in trace_save_cmdline().
