[PATCH] riscv: trace: fix snapshot deadlock with sbi ecall

[Paul Walmsley]

On Tue, 23 Dec 2025, Martin Kaiser wrote:

> If sbi_ecall.c's functions are traceable,
> 
> echo "__sbi_ecall:snapshot" > /sys/kernel/tracing/set_ftrace_filter
> 
> may get the kernel into a deadlock.
> 
> (Functions in sbi_ecall.c are excluded from tracing if
> CONFIG_RISCV_ALTERNATIVE_EARLY is set.)
> 
> __sbi_ecall triggers a snapshot of the ringbuffer. The snapshot code
> raises an IPI interrupt, which results in another call to __sbi_ecall
> and another snapshot...
> 
> All it takes to get into this endless loop is one initial __sbi_ecall.
> On RISC-V systems without SSTC extension, the clock events in
> timer-riscv.c issue periodic sbi ecalls, making the problem easy to
> trigger.
> 
> Always exclude the sbi_ecall.c functions from tracing to fix the
> potential deadlock.
> 
> sbi ecalls can easiliy be logged via trace events, excluding ecall
> functions from function tracing is not a big limitation.
> 
> Signed-off-by: Martin Kaiser <martin at kaiser.cx>

Thanks, queued for v6.19-rc.

- Paul